Secondary battery

Abstract

Provided is a secondary battery with which excellent battery characteristics can be obtained. This secondary battery is provided with: a positive electrode that includes a positive electrode active material layer; a negative electrode; and an electrolyte solution that contains an aqueous solvent. The positive electrode active material layer contains: a positive electrode active material containing a lithium-containing compound that absorbs and releases lithium ions; and a positive electrode binder containing a polymer compound. The polymer compound contains at least one of a repeating unit represented by formula (1) and a repeating unit represented by formula (2), and the solubility of the polymer compound in 100g of water is 1g or less. The ratio of the volume density of the positive electrode active material layer to the true density of the positive electrode active material is 30%-70%.

Description

secondary battery

[0001] The present technology relates to a secondary battery.

[0002] Due to the widespread use of various electronic devices such as mobile phones, secondary batteries have been developed as power sources that are small, lightweight, and have high energy density. These secondary batteries include a positive electrode, a negative electrode, and an aqueous electrolyte solution, which contains an aqueous solvent. Various studies have been conducted on the configuration of secondary batteries that include an aqueous electrolyte solution.

[0003] Specifically, acetal resin or acrylic resin is used as the material for the electrodes (see, for example, Patent Documents 1 to 5). In this case, the solubility parameter and the porosity of the electrodes are specified.

[0004] Japanese Patent Application Laid-Open No. 2020-047576 Japanese Patent Application Laid-Open No. 2018-206675 International Publication No. 2016 / 171028 Japanese Patent Application Laid-Open No. 2014-238944 Japanese Patent Application Laid-Open No. 2015-106489

[0005] Various studies have been conducted on the configuration of secondary batteries containing aqueous electrolyte solutions, but the battery characteristics of these secondary batteries are still insufficient and there is room for improvement.

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

[0007] According to one embodiment of the present disclosure, there is provided a secondary battery including a positive electrode including a positive electrode active material layer, a negative electrode, and an electrolyte solution including an aqueous solvent. The positive electrode active material layer includes a positive electrode active material including a lithium-containing compound that absorbs and releases lithium ions, and a positive electrode binder including a polymer compound. The polymer compound includes at least one of a repeating unit represented by formula (1) and a repeating unit represented by formula (2), and the solubility of the polymer compound in 100 g of water is 1 g or less. The ratio of the volume density of the positive electrode active material layer to the true density of the positive electrode active material is 30% or more and 70% or less.

[0008] (R1 is either a hydrocarbon group or an oxygen-containing hydrocarbon group, R2 is either a hydrogen group or a hydrocarbon group, R3 is either a hydrocarbon group or an oxygen-containing hydrocarbon group, n1 and n2 are each an integer of 2 or greater. An asterisk (*) represents a dangling bond.)

[0009] According to a secondary battery of one embodiment of the present technology, the positive electrode active material layer contains a positive electrode active material and a positive electrode binder, the electrolyte contains an aqueous solvent, the positive electrode active material contains a lithium-containing compound that absorbs and releases lithium ions, the positive electrode binder contains a polymer compound, and the polymer compound contains at least one of a repeating unit shown in formula (1) and a repeating unit shown in formula (2), the solubility of the polymer compound in 100 g of water is 1 g or less, and the ratio of the volume density of the positive electrode active material layer to the true density of the positive electrode active material is 30% or more and 70% or less, and therefore excellent battery characteristics can be obtained.

[0010] 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.

[0011] Fig. 1 is a cross-sectional view showing a configuration of a secondary battery according to a first embodiment of the present technology. Fig. 2 is a cross-sectional view showing a configuration of a secondary battery according to a second embodiment of the present technology. Fig. 3 is a cross-sectional view showing a configuration of a secondary battery according to a first modified example. Fig. 4 is a cross-sectional view showing a configuration of a secondary battery according to a second modified example.

[0012] 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. Secondary battery (first embodiment) 1-1. Overall configuration 1-2. Detailed configuration of positive electrode active material layer 1-3. Operation 1-4. Manufacturing method 1-5. Actions and effects 2. Secondary battery (second embodiment) 2-1. Configuration 2-2. Operation 2-3. Manufacturing method 2-4. Actions and effects 3. Modified example 4. Uses of secondary battery

[0013] 1. Secondary Battery (First Embodiment) First, a secondary battery according to a first embodiment of the present technology will be described.

[0014] The secondary battery described here is a so-called lithium secondary battery, in which charge-discharge reactions proceed by utilizing the absorption and release of lithium ions in a positive electrode. This secondary battery includes a positive electrode, a negative electrode, and an aqueous electrolyte. The aqueous electrolyte is a liquid electrolyte, and more specifically, as described above, is an electrolyte containing an aqueous solvent.

[0015] <1-1. Overall Configuration> Fig. 1 shows a cross-sectional configuration of a secondary battery in the first embodiment. As shown in Fig. 1, this secondary battery includes an outer casing 10, a positive electrode 20, a negative electrode 30, and an electrolyte solution 40. In Fig. 1, the electrolyte solution 40 is lightly shaded.

[0016] The secondary battery of the first embodiment includes one type of aqueous electrolyte (electrolyte 40), and is therefore a so-called single-liquid type secondary battery.

[0017] [Exterior Body] As shown in FIG. 1, the exterior body 10 is a substantially box-shaped exterior member that houses the positive electrode 20, the negative electrode 30, the electrolyte solution 40, and the like, and has an internal space S.

[0018] This exterior body 10 includes one or more of a metal material, a glass material, a polymer compound, etc. Note that the exterior body 10 may be a rigid metal can, a glass case, a plastic case, etc., or a flexible (or pliable) metal foil, a polymer film, etc.

[0019] 1, the positive electrode 20 is disposed in the internal space S and absorbs and releases lithium ions. Here, the positive electrode 20 includes a positive electrode current collector 20A and a positive electrode active material layer 20B.

[0020] The positive electrode current collector 20A may be omitted. That is, the positive electrode 20 does not include the positive electrode current collector 20A, and may include only the positive electrode active material layer 20B.

[0021] (Positive Electrode Current Collector) The positive electrode current collector 20A is a conductive support member that supports the positive electrode active material layer 20B and has a pair of surfaces that support the positive electrode active material layer 20B. This positive electrode current collector 20A contains one or more conductive materials such as metal materials, carbon materials, and conductive ceramic materials. Specific examples of metal materials include titanium, nickel, aluminum, and alloys of these. Specific examples of conductive ceramic materials include indium tin oxide (ITO).

[0022] The material forming the positive electrode current collector 20A is preferably insoluble or hardly soluble in the electrolyte solution 40 and corrosion-resistant, and also has low reactivity with the positive electrode active material described below. For this reason, the positive electrode current collector 20A preferably contains a metal material, because this suppresses deterioration of the positive electrode current collector 20A during charge and discharge.

[0023] Among these, it is preferable that the positive electrode current collector 20A contains titanium as a constituent element, and more specifically, it is more preferable that the positive electrode current collector 20A contains titanium, because this further suppresses deterioration of the positive electrode current collector 20A.

[0024] The positive electrode current collector 20A may be a conductor whose surface is plated with the above-mentioned conductive material. The material for forming the conductor is not particularly limited and can be selected arbitrarily.

[0025] Here, the positive electrode current collector 20A includes a connection terminal portion 20AT on which the positive electrode active material layer 20B is not provided, and the connection terminal portion 20AT is led out from the inside (internal space S) of the outer casing 10 to the outside.

[0026] (Positive Electrode Active Material Layer) The positive electrode active material layer 20B is a layer that absorbs and releases lithium ions. Here, the positive electrode active material layer 20B is provided on both sides of the positive electrode current collector 20A. However, the positive electrode active material layer 20B may be provided on only one side of the positive electrode current collector 20A, on the side where the positive electrode 20 faces the negative electrode 30.

[0027] The positive electrode active material layer 20B contains a positive electrode active material and a positive electrode binder. However, the positive electrode active material layer 20B may further contain one or more of a positive electrode conductive agent, etc. The detailed configuration of the positive electrode active material layer 20B will be described later.

[0028] 1, the negative electrode 30 is disposed in the internal space S and absorbs and releases cations such as lithium ions. Here, the negative electrode 30 includes a negative electrode current collector 30A and a negative electrode active material layer 30B.

[0029] The negative electrode current collector 30A may be omitted. That is, the negative electrode 30 does not include the negative electrode current collector 30A, and may include only the negative electrode active material layer 30B.

[0030] (Negative electrode current collector) The negative electrode current collector 30A is a conductive support member that supports the negative electrode active material layer 30B, and has a pair of surfaces that support the negative electrode active material layer 30B. The negative electrode current collector 30A contains one or more conductive materials such as a metal material, a carbon material, and a conductive ceramic material.

[0031] Specific examples of metal materials include stainless steel (SUS), titanium, zinc, tin, lead, and alloys thereof. The stainless steel may be a highly corrosion-resistant stainless steel to which one or more of niobium, molybdenum, or other additive elements have been added. Specifically, the stainless steel may be SUS444 to which molybdenum has been added as an additive element. Details regarding the conductive ceramic material are as described above.

[0032] The material forming the negative electrode current collector 30A is preferably insoluble or hardly soluble in the electrolyte solution 40 and corrosion-resistant, and also has low reactivity with the negative electrode active material described below. For this reason, the negative electrode current collector 30A preferably contains a metal material, because this suppresses deterioration of the negative electrode current collector 30A during charge and discharge.

[0033] The negative electrode current collector 30A may be a conductor whose surface is plated with the above-mentioned conductive material. The material for forming the conductor is not particularly limited and can be selected arbitrarily.

[0034] Here, the negative electrode current collector 30A includes a connection terminal portion 30AT on which the negative electrode active material layer 30B is not provided, and the connection terminal portion 30AT is led out from the inside (internal space S) of the outer casing 10 to the outside.

[0035] (Negative Electrode Active Material Layer) The negative electrode active material layer 30B is a layer that absorbs and releases cations such as lithium ions. Here, the negative electrode active material layer 30B is provided on both sides of the negative electrode current collector 30A. However, the negative electrode active material layer 30B may be provided on only one side of the negative electrode current collector 30A, on the side where the negative electrode 30 faces the positive electrode 20.

[0036] The negative electrode active material layer 30B may contain one or more types of negative electrode active materials that absorb and release lithium ions, but may also contain one or more types of a negative electrode binder, a negative electrode conductive agent, and the like.

[0037] Specifically, negative electrode active materials that absorb and release lithium ions include titanium oxide, carbon materials, and metal-based materials. Specific examples of titanium oxide include anatase-type titanium oxide, rutile-type titanium oxide, and brookite-type titanium oxide. Specific examples of carbon materials include graphite. Metal-based materials are materials that contain, as constituent elements, one or more of metal elements and semimetal elements that can form an alloy with lithium.

[0038] Alternatively, the negative electrode active material layer 30B may contain, instead of the negative electrode active material that absorbs and releases lithium ions, one or more types of other negative electrode active materials that absorb and release cations other than lithium ions. The types of cations other than lithium ions may be one type or two or more types.

[0039] Specifically, other negative electrode active materials that absorb and release cations other than lithium ions include titanium-containing compounds, niobium-containing compounds, vanadium-containing compounds, iron-containing compounds, molybdenum-containing compounds, and zinc.

[0040] The titanium-containing compound is an alkali metal titanium composite oxide, a titanium phosphate, etc. The alkali metal titanium composite oxide is a potassium titanium composite oxide, etc., and a specific example of the potassium titanium composite oxide is K 2　 Ti 3　 O 7　 and K. 4　 Ti 5　 O 12 Specific examples of titanium phosphate include TiP 2　 O 7　 and NaTi 2　 (P.O. 4　 ) 3　 And so on.

[0041] Niobium-containing compounds include hydrogen niobium compounds and titanium-niobium composite oxides. Specific examples of hydrogen niobium compounds include H 4　 Nb 6　 O 17 Specific examples of titanium-niobium composite oxides include TiNb 2　 O 7　 and Ti 2　 Nb 10 O 29 And so on.

[0042] The vanadium-containing compound is vanadium oxide. A specific example of the vanadium oxide is vanadium dioxide (VO 2　 ) etc.

[0043] The iron-containing compound is an iron hydroxide, etc. A specific example of the iron hydroxide is iron oxyhydroxide (FeOOH).

[0044] The molybdenum-containing compound includes molybdenum oxide and cobalt-molybdenum composite oxide. Specific examples of molybdenum oxide include molybdenum dioxide (MoO 2　 ) and the like. A specific example of the cobalt-molybdenum composite oxide is CoMoO 4　 And so on.

[0045] Of course, the negative electrode active material layer 30B may contain both one or more types of negative electrode active materials that absorb and release lithium ions and one or more types of other negative electrode active materials that absorb and release cations other than lithium ions.

[0046] [Electrolyte] The electrolyte 40 is contained in the internal space S and is an aqueous electrolyte, as described above. That is, the electrolyte 40 is a solution in which an ionic substance that can be ionized in an aqueous solvent is dissolved or dispersed.

[0047] The electrolyte solution 40 contains an aqueous solvent and one or more ionic substances that can be ionized in the aqueous solvent. More specifically, the electrolyte solution 40 used in the secondary battery contains lithium ions that are absorbed and released in the positive electrode 20.

[0048] The type of aqueous solvent is not particularly limited, but specifically includes pure water, etc. The type of ionic substance is not particularly limited, but specifically includes one or more of acids, bases, and electrolyte salts, etc. Specific examples of acids include carbonic acid, oxalic acid, nitric acid, sulfuric acid, hydrochloric acid, acetic acid, and citric acid.

[0049] The electrolyte salt is a salt containing a cation and an anion. Specifically, the electrolyte salt contains one or more types of lithium salts, and the lithium salt contains lithium ions as cations.

[0050] The type of anion is not particularly limited. Among them, the anion preferably includes one or more of sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, dihydrogen phosphate ions, and carboxylate ions. This is because fluctuations in the pH of the electrolyte solution 40 are sufficiently suppressed. Specific examples of carboxylate ions include formate ions, acetate ions, propionate ions, tartrate ions, and citrate ions.

[0051] In particular, the pH of the electrolyte solution 40 used in a single-liquid secondary battery is preferably less than 11. Specifically, the pH of the electrolyte solution 40 is preferably 3 to 8, more preferably 4 to 8, and even more preferably 4 to 6. This is because the charge / discharge reaction in the positive electrode 20 is more likely to proceed. In addition, the components of the secondary battery, such as the exterior body 10 and the positive electrode current collector 20A, are less susceptible to corrosion, thereby improving the electrochemical durability (stability) of the secondary battery.

[0052] The content of the ionic substance, i.e., the concentration (mol / kg) of the electrolyte solution 40 is not particularly limited and can be set arbitrarily. Specifically, the concentration of the electrolyte solution 40 is preferably 0.01 mol / kg to 10 mol / kg, and more preferably 0.2 mol / kg to 4 mol / kg. This is because the charge / discharge reaction in the positive electrode 20 in the electrolyte solution 40 tends to proceed stably.

[0053] The electrolyte salt may further contain one or more of the other metal salts in addition to the lithium salt. The type of the other metal salt is not particularly limited, but specific examples include alkali metal salts (excluding lithium salts), alkaline earth metal salts, and transition metal salts. Specific examples of alkali metal salts include sodium salts and potassium salts, and specific examples of alkaline earth metal salts include calcium salts and magnesium salts.

[0054] Furthermore, the electrolytic solution 40 is preferably a saturated solution of an electrolyte salt, because this facilitates stable absorption and desorption of lithium ions during charging and discharging, thereby facilitating stable progress of the charge and discharge reaction.

[0055] To confirm whether the electrolyte solution 40 is a saturated solution of electrolyte salt, the secondary battery is disassembled and the internal space S is visually observed to determine whether the electrolyte salt has precipitated. In this case, the electrolyte solution 40, the surface of the positive electrode 20, and the inner wall surface of the exterior body 10 are visually observed. If the electrolyte salt has precipitated and the electrolyte solution 40 (liquid) and the electrolyte salt precipitate (solid) coexist, the electrolyte solution 40 is considered to be a saturated solution of electrolyte salt. Note that, to determine the composition of the precipitate, a surface analysis method such as X-ray photoelectron spectroscopy (XPS) or a composition analysis method such as inductively coupled plasma (ICP) optical emission spectroscopy may be used.

[0056] The electrolytic solution 40 may also be a pH buffer solution. This pH buffer solution may be an aqueous solution in which a weak acid and its conjugate base are mixed, or an aqueous solution in which a weak base and its conjugate acid are mixed. This is because fluctuations in the pH of the electrolytic solution 40 are sufficiently suppressed.

[0057] The electrolyte solution 40 may further contain one or more buffers, such as trishydroxymethylaminomethane and ethylenediaminetetraacetic acid.

[0058] <1-2. Detailed Configuration of Positive Electrode Active Material Layer> As described above, the positive electrode active material layer 20B contains a positive electrode active material and a positive electrode binder, and may further contain a positive electrode conductive agent.

[0059] [Positive Electrode Active Material] The positive electrode active material is a substance that absorbs and releases lithium ions, and more specifically, contains one or more lithium-containing compounds that absorb and release lithium ions. The lithium-containing compounds are a general term for compounds that contain lithium as a constituent element.

[0060] The type of lithium-containing compound is not particularly limited, but specific examples include lithium composite oxides and lithium phosphate compounds. Lithium composite oxides are oxides containing lithium and one or more transition metal elements as constituent elements, and have a layered rock salt or spinel crystal structure. Lithium phosphate compounds are phosphate compounds containing lithium and one or more transition metal elements as constituent elements, and have an olivine crystal structure. The type of transition metal element is not particularly limited, but specific examples include nickel, cobalt, manganese, and iron.

[0061] A specific example of a lithium composite oxide having a layered rock salt type crystal structure is LiNiO 2　 , LiCoO 2　 , LiCo 0.98 Al 0.01 Mg 0.01 O 2　 , LiNi 0.5　 Co 0.2　 Mn 0.3　 O 2　 , LiNi 0.8　 Co 0.15 Al 0.05 O 2　 , LiNi 0.33 Co 0.33 Mn 0.33 O 2　 , Li 1.2　 Mn 0.52 Co 0.175　 Ni 0.1　 O 2　 and Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 ) O 2　 Specific examples of lithium composite oxides having a spinel-type crystal structure include LiMn 2　 O 4　 A specific example of a lithium phosphate compound having an olivine-type crystal structure is LiFePO 4　 , LiMnPO 4　 , LiMn 0.5　 Fe 0.5　 P.O. 4　 , LiMn 0.7　 Fe 0.3　 P.O. 4　 and LiMn0.75 Fe 0.25 P.O. 4　 And so on.

[0062] Among them, the lithium-containing compound preferably contains a lithium composite oxide, which has a spinel-type crystal structure and contains manganese as a constituent element. This is because the charge efficiency and discharge efficiency are improved. Specific examples of the lithium composite oxide described here are, as described above, LiMn 2　 O 4　 And so on.

[0063] Alternatively, the lithium-containing compound preferably contains a lithium phosphate compound having an olivine-type crystal structure and containing iron as a constituent element, because this improves both the charge efficiency and the discharge efficiency. Specific examples of the lithium phosphate compound described here include, as described above, LiFePO 4　 , LiMn 0.5　 Fe 0.5　 P.O. 4　 , LiMn 0.7　 Fe 0.3　 P.O. 4　 and LiMn 0.75 Fe 0.25 P.O. 4　 And so on.

[0064] [Positive Electrode Binder] The positive electrode binder is a material that binds the positive electrode active material, etc., and contains one or more polymer compounds that have polarity but little water solubility. Hereinafter, polymer compounds that have polarity but little water solubility are referred to as "water-insoluble polar polymer compounds."

[0065] Specifically, the water-insoluble polar polymer compound contains one or both of a repeating unit represented by formula (1) and a repeating unit represented by formula (2).

[0066] (R1 is either a hydrocarbon group or an oxygen-containing hydrocarbon group, R2 is either a hydrogen group or a hydrocarbon group, R3 is either a hydrocarbon group or an oxygen-containing hydrocarbon group, n1 and n2 are each an integer of 2 or greater. An asterisk (*) represents a dangling bond.)

[0067] The reasons why the positive electrode binder contains a water-insoluble polar polymer compound are as follows. First, the binding properties of the positive electrode active material and the like are ensured using the positive electrode binder, which makes it easier to maintain the structure of the positive electrode active material layer 20B during charge and discharge. Second, even when the electrolyte solution 40, which is an aqueous electrolyte solution, is used, the ability to absorb and release lithium ions in the positive electrode active material layer 20B is ensured, which makes it easier for the charge and discharge reaction to proceed stably in the positive electrode active material layer 20B.

[0068] Hereinafter, the repeating unit shown in formula (1) will be referred to as the “first repeating unit,” and the repeating unit shown in formula (2) will be referred to as the “second repeating unit.” As described above, the water-insoluble polar polymer compound may contain only the first repeating unit, only the second repeating unit, or both the first repeating unit and the second repeating unit.

[0069] (First repeating unit) The first repeating unit has a polyacrylic acid ester type structure as shown in formula (1). As described above, the asterisk (*) represents a dangling bond, and therefore the first repeating unit is a divalent group having two dangling bonds.

[0070] As described above, the type of R1 is not particularly limited as long as it is either a hydrocarbon group or an oxygen-containing hydrocarbon group.

[0071] The hydrocarbon group for R1 is a general term for a monovalent group composed of carbon and hydrogen, and the number of carbon atoms in the hydrocarbon group is not particularly limited. The type of hydrocarbon group is not particularly limited, but specific examples include alkyl groups, alkenyl groups, alkynyl groups, aryl groups, and cycloalkyl groups. However, each of the alkyl groups, alkenyl groups, and alkynyl groups may be linear or branched.

[0072] Specific examples of alkyl groups include methyl, ethyl, propyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, and decyl groups. Specific examples of alkenyl groups include vinyl and allyl groups. Specific examples of alkynyl groups include ethynyl and propargyl groups. Specific examples of aryl groups include phenyl and naphthyl groups. Specific examples of cycloalkyl groups include cyclopropyl, cyclobutyl, cyclopentyl, and cyclohexyl groups.

[0073] The oxygen-containing hydrocarbon group is a generic term for a monovalent group in which one or more ether bonds (—O—) are introduced into the above-mentioned hydrocarbon group, and the position at which the one or more ether bonds are introduced into the hydrocarbon group is not particularly limited.

[0074] For example, the hydrocarbon group may be a linear propyl group (—CH 2　 -CH 2　 -CH 3　 ) Specific examples of the oxygen-containing hydrocarbon group are as described below.

[0075] When the number of ether bonds is one, the oxygen-containing hydrocarbon group is —O—CH 2　 -CH 2　 -CH 3　 , -CH 2　 -O-CH 2　 -CH 3　 and -CH 2　 -CH 2　 -O-CH 3　 When the number of ether bonds is two, the oxygen-containing hydrocarbon group is —O—CH 2　 -O-CH 2　 -CH 3　, —O—CH 2　 -CH 2　 -O-CH 3　 and -CH 2　 -O-CH 2　 -O-CH 3　 When the number of ether bonds is three, the oxygen-containing hydrocarbon group is —O—CH 2　 -O-CH 2　 -O-CH 3　 is.

[0076] Among these, R1 is preferably an alkyl group having 1 to 10 carbon atoms. This is because sufficient solubility and compatibility with the water-insoluble polar polymer compound can be obtained in the step of preparing the positive electrode mixture slurry, which will be described later, and sufficient binding property can be obtained in the positive electrode active material layer 20B formed using the positive electrode mixture slurry.

[0077] As described above, the type of R2 is not particularly limited as long as it is either a hydrogen group or a hydrocarbon group. Details regarding the hydrocarbon group are as described above. Note that the type of R2 and the type of R1 may be the same or different from each other.

[0078] Among these, R2 is preferably an alkyl group having 1 or 2 carbon atoms. This is because sufficient solubility and compatibility with the water-insoluble polar polymer compound can be obtained in the step of preparing a positive electrode mixture slurry, which will be described later, and sufficient binding property can be obtained in the positive electrode active material layer 20B formed using the positive electrode mixture slurry.

[0079] (Second repeating unit) The second repeating unit has a polyvinyl butyral type structure as shown in formula (2). As described above, the asterisk (*) represents a dangling bond, and therefore the second repeating unit is a divalent group having two dangling bonds.

[0080] The type of R3 is not particularly limited as long as it is either a hydrocarbon group or an oxygen-containing hydrocarbon group, as described above. Details regarding the hydrocarbon group and the oxygen-containing hydrocarbon group are as described above.

[0081] Among these, R3 is preferably an alkyl group having 1 to 10 carbon atoms. This is because sufficient solubility and compatibility with the water-insoluble polar polymer compound can be obtained in the step of preparing the positive electrode mixture slurry, which will be described later, and sufficient binding property can be obtained in the positive electrode active material layer 20B formed using the positive electrode mixture slurry.

[0082] (Terminal Group) As described above, each of the first repeating unit and the second repeating unit has two dangling bonds, and therefore the water-insoluble polar polymer compound has dangling bonds at both ends, i.e., contains two dangling bonds.

[0083] Hereinafter, the monovalent groups bonded to the two dangling bonds are referred to as "end groups." Therefore, the water-insoluble polar polymer compound contains two end groups. The type of one end group and the type of the other end group may be the same or different from each other.

[0084] The type of the terminal group is not particularly limited, but specifically may be any of a hydrogen group, a hydrocarbon group, and an oxygen-containing hydrocarbon group. Details regarding the hydrocarbon group and the oxygen-containing hydrocarbon group are as described above.

[0085] (Water solubility) As described above, water-insoluble polar polymer compounds have almost no water solubility. Therefore, the solubility of a water-insoluble polar polymer compound in 100 g of water is sufficiently small. Hereinafter, the solubility of a water-insoluble polar polymer compound in 100 g of water will be referred to as "water solubility." The "water" described here is so-called pure water.

[0086] Specifically, the water solubility is 1 g or less. This is because the binding property of the positive electrode active material using the positive electrode binder is ensured, thereby ensuring the physical strength of the positive electrode active material layer 20B. This makes it easier to maintain the structure of the positive electrode active material layer 20B even after repeated charge and discharge, and therefore makes it easier for the positive electrode active material layer 20B to stably and continuously store and release lithium ions.

[0087] The procedure for measuring the water solubility is as follows: In the following, a case where the positive electrode active material layer 20B contains a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent will be described.

[0088] First, the secondary battery is disassembled to recover the positive electrode 20. Subsequently, the positive electrode 20 is immersed in an organic solvent, and the organic solvent is then stirred.

[0089] The type of organic solvent is not particularly limited as long as it is one or more organic solvents that can dissolve the positive electrode binder, such as N-methyl-2-pyrrolidone, toluene, butyl acetate, ethanol, and terpineol.

[0090] As a result, the positive electrode binder, which is a soluble component, is dissolved in the organic solvent, and the positive electrode active material and positive electrode conductive agent, which are insoluble components, are precipitated.

[0091] Subsequently, the organic solvent is filtered and the filtrate is dried, whereby the positive electrode active material and the positive electrode conductive agent are separated from the positive electrode binder, and the positive electrode binder in a solid state is recovered.

[0092] Finally, a predetermined amount of the positive electrode binder is added to a specified amount of water while stirring the water, and the dissolution state of the positive electrode binder is visually observed by repeating this process. For example, the specified amount is 10 g and the constant amount is 10 mg.

[0093] This allows the water solubility of the water-insoluble polar polymer compound that is the positive electrode binder to be determined based on the amount (g) of positive electrode binder added when a precipitate occurs due to the positive electrode binder no longer dissolving in water.

[0094] (Weight-average molecular weight) The weight-average molecular weight of the water-insoluble polar polymer compound is not particularly limited. In particular, the weight-average molecular weight is preferably 50,000 to 300,000. This is because the binding property of the positive electrode active material using the positive electrode binder is ensured.

[0095] The procedure for measuring the weight-average molecular weight is as follows. First, the positive electrode binder (a water-insoluble polar polymer compound) is recovered using the same procedure as for measuring the water solubility described above. Next, the positive electrode binder is added to an aqueous solvent such as pure water, and the aqueous solvent is stirred. This results in the positive electrode binder being dispersed in the aqueous solvent, yielding a dispersion. Finally, the dispersion is analyzed using static light scattering to measure the molecular weight of the positive electrode binder contained in the dispersion. In this case, a particle size distribution analyzer (such as the Partica LA-960 laser diffraction / scattering particle size distribution analyzer manufactured by Horiba, Ltd.) is used as the analytical device. Furthermore, the scattered light intensity is measured using dispersions with any three different concentrations, and the molecular weight is determined using the Debye plot method. This allows the weight-average molecular weight of the water-insoluble polymer compound to be measured.

[0096] [Density Ratio] In this positive electrode active material layer 20B, the amount of voids present in the positive electrode active material layer 20B is optimized, in other words, the porosity of the positive electrode active material layer 20B is optimized.

[0097] Specifically, the true density (g / cm 3　 ) of the volume density (g / cm 3　 The density ratio, which is the ratio of the total mass of the positive electrode active material layer 20B to the total mass of the positive electrode active material layer 20B, is 30% to 70%. This is because, even when an aqueous electrolyte solution 40 is used, the electrolyte solution 40 is easily impregnated into the inside of the positive electrode active material layer 20B. This makes it easier for lithium ions to be stably and continuously absorbed and released in the positive electrode active material layer 20B even when charge and discharge are repeated.

[0098] This density ratio is a physical property value that affects the porosity of the above-mentioned positive electrode active material layer 20B, and is calculated based on the formula: density ratio=(volume density / true density)×100.

[0099] As will be described later, the volume density varies depending on the compression molding conditions of the positive electrode active material layer 20B in the manufacturing process of the positive electrode 20. The compression molding conditions include the pressing pressure and pressing time. Therefore, by changing the volume density depending on the compression molding conditions, it is possible to control the density ratio to a desired value.

[0100] The procedure for measuring the density ratio is as follows: In the following, a case where the positive electrode active material layer 20B contains a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent will be described.

[0101] First, the secondary battery is disassembled to recover the positive electrode 20. Next, the weight and dimensions (length, width, and thickness) of the positive electrode 20 are measured. Next, the positive electrode 20 is immersed in an organic solvent, and the organic solvent is stirred. Details regarding the organic solvent are as described above. As a result, the positive electrode current collector 20A is separated from the positive electrode active material layer 20B in the organic solvent, and the positive electrode current collector 20A is recovered. Furthermore, in the organic solvent, the positive electrode binder, which is a soluble component, is dissolved, and the positive electrode active material and positive electrode conductive agent, which are insoluble components, are precipitated.

[0102] Next, the weight and dimensions (length, width, and thickness) of the positive electrode current collector 20 A are measured. Next, the weight and dimensions of the positive electrode active material layer 20 B are calculated based on the weight and dimensions of the positive electrode 20 and the weight and dimensions of the positive electrode current collector 20 A, thereby calculating the volume density of the positive electrode active material layer 20 B.

[0103] Next, the organic solvent is filtered, and the filtrate is dried. As a result, a mixture of the positive electrode active material and the positive electrode conductive agent, which are solid components, is recovered. Next, if necessary, the mixture is heated in the air (heating temperature = 300 ° C to 700 ° C) to remove any organic matter remaining in the mixture. Next, the mixture is centrifuged using a centrifuge to recover the positive electrode active material. Next, if necessary, the positive electrode active material is pulverized.

[0104] Next, the positive electrode active material (lithium-containing compound) is analyzed using a powder X-ray diffraction method or the like to identify the crystal structure of the positive electrode active material and measure the ratio of the transition metal element contained in the transition metal site in the positive electrode active material.

[0105] Next, based on the analysis results of the positive electrode active material, the true density of the positive electrode active material is determined as described below.

[0106] The lithium-containing compound has a spinel-type crystal structure. 2　 O 4　 When the positive electrode active material is 3　 The lithium-containing compound has an olivine-type crystal structure, LiFePO 4　 When the positive electrode active material is 3　 The lithium-containing compound is LiMnPO having an olivine-type crystal structure. 4　 When the positive electrode active material is 3　 The lithium-containing compound is LiCoO having a layered rock salt type crystal structure. 2　 When the positive electrode active material is 3　 Let's say.

[0107] The lithium-containing compound has a layered rock salt type crystal structure, LiNiO 2　 When the positive electrode active material is 3　 When the lithium-containing compound is a ternary compound having a layered rock salt crystal structure, the true density of the positive electrode active material is 4.6 g / cm 3　 This ternary compound is a composite oxide containing nickel, manganese, and cobalt as constituent elements, and specifically, Li(Ni 1 / 3　 Mn 1 / 3　 Co 1 / 3　 ) O 2　 And so on.

[0108] More specifically, when the crystal structure is mainly a spinel-type crystal structure and the molar ratio of the transition metal element (manganese) contained in the transition metal site is 50% or more, as described above, the lithium-containing compound is LiMn2　 O 4　 Therefore, the true density of the positive electrode active material is 4.3 g / cm 3　 Even if elements other than manganese are contained in the transition metal sites, if the molar ratio of manganese is 50% or more, the true density is not particularly corrected and is set to 4.3 g / cm 3　 Let's say.

[0109] When the crystal structure is mainly an olivine-type crystal structure and the molar ratio of the transition metal element (iron) contained in the transition metal site is 50% or more, as described above, the lithium-containing compound is LiFePO 4　 Therefore, the true density of the positive electrode active material is 3.6 g / cm 3　 Even if elements other than iron are contained in the transition metal sites, if the molar ratio of iron is 50% or more, the true density is not particularly corrected and is set to 3.6 g / cm 3　 Let's say.

[0110] When the crystal structure is mainly an olivine-type crystal structure, and the molar ratio of the transition metal element (iron) contained in the transition metal site is less than 50%, but the molar ratio of the transition metal element (manganese) contained in the transition metal site is 50% or more, as described above, the lithium-containing compound is LiMnPO 4　 Therefore, the true density of the positive electrode active material is 3.4 g / cm 3　 Even if elements other than manganese are contained in the transition metal sites, if the molar ratio of manganese is 50% or more, the true density is not particularly corrected and is set to 3.4 g / cm. 3　 Let's say.

[0111] When the crystal structure is mainly a layered rock salt type crystal structure and the molar ratio of the transition metal (cobalt) contained in the transition metal site is 50% or more, as described above, the lithium-containing compound is LiCoO 2　 Therefore, the true density of the positive electrode active material is 5.1 g / cm 3　 Even if elements other than cobalt are contained in the transition metal sites, if the molar ratio of cobalt is 50% or more, the true density is not particularly corrected and is set to 5.1 g / cm 3　 Let's say.

[0112] When the crystal structure is mainly a layered rock salt type crystal structure, and the molar ratios of the transition metals (cobalt and manganese) contained in the transition metal sites are both less than 50%, but the molar ratio of nickel contained in the transition metal sites is 50% or more, the lithium-containing compound is LiNiO as described above. 2　 Therefore, the true density of the positive electrode active material is 4.8 g / cm 3　 Even if elements other than nickel are contained in the transition metal sites, if the molar ratio of nickel is 50% or more, the true density is not particularly corrected and is set to 4.8 g / cm 3　 Let's say.

[0113] When the crystal structure is mainly a layered rock salt type crystal structure, the molar ratio of the transition metal (cobalt) contained in the transition metal site is less than 50%, and the transition metal site contains cobalt, nickel, and manganese, as described above, the lithium-containing compound is a ternary compound, and therefore the true density of the positive electrode active material is 4.6 g / cm 3　 Even if elements other than cobalt, nickel, and manganese are contained in the transition metal sites, if cobalt, nickel, and manganese are contained in the transition metal sites, the true density is set to 4.6 g / cm without any special correction. 3　 Let's say.

[0114] In addition, when two or more types of positive electrode active materials are used, the value of true density is the sum (weighted average) of the values ​​obtained by multiplying the true density of each positive electrode active material by the content ratio of each positive electrode active material. As an example, when lithium manganate (content ratio = 50 vol%) and lithium iron phosphate (content ratio = 50 vol%) are used, the true density is 3.9 g / cm 3　 Let's say.

[0115] This allows the true density of the positive electrode active material to be determined.

[0116] Finally, the density ratio is calculated based on the volume density and the true density.

[0117] [Positive Electrode Conductive Agent] The positive electrode conductive agent is a material that improves the conductivity of the positive electrode active material layer 20B and contains one or more types of conductive materials. The type of conductive material is not particularly limited, but specific examples include carbon materials, metal materials, conductive ceramic materials, and conductive polymer compounds.

[0118] In particular, it is preferable that the positive electrode conductive agent contains a carbon material, because this sufficiently improves the conductivity of the positive electrode active material layer 20B. Specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.

[0119] <1-3. Operation> This secondary battery operates as described below.

[0120] During charging, lithium ions are released from the positive electrode 20, and cations such as the lithium ions are absorbed into the negative electrode 30 via the electrolyte 40. During discharging, cations such as lithium ions are released from the negative electrode 30, and the lithium ions are absorbed into the positive electrode 20 via the electrolyte 40.

[0121] <1-4. Manufacturing Method> When manufacturing this secondary battery, the positive electrode 20 and the negative electrode 30 are manufactured and the electrolyte solution 40 is prepared according to the procedure described below as an example, and then the secondary battery is assembled.

[0122] [Fabrication of Positive Electrode] First, a positive electrode active material (lithium-containing compound), a positive electrode binder (a water-insoluble polar polymer compound), and a positive electrode conductive agent are mixed together to form a positive electrode mixture. Next, the positive electrode mixture is poured into a solvent to prepare a paste-like positive electrode mixture slurry. This solvent may be an aqueous solvent or an organic solvent. The positive electrode mixture slurry is then applied to both surfaces of the positive electrode current collector 20A (excluding the connection terminal portion 20AT) to form the positive electrode active material layer 20B. Finally, the positive electrode active material layer 20B is compression-molded using a roll press or the like. Compression molding conditions, such as the pressing pressure and pressing time, can be set as desired. In this case, the positive electrode active material layer 20B may be heated, or the positive electrode active material layer 20B may be compression-molded multiple times. This completes the fabrication of the positive electrode 20.

[0123] [Fabrication of Negative Electrode] The negative electrode 30 is fabricated using the same procedure as the fabrication procedure for the positive electrode 20 described above. Specifically, a negative electrode mixture, in which a negative electrode active material, a negative electrode binder, and a negative cathode conductive agent are mixed together, is poured into a solvent to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to both surfaces (excluding the connection terminal portion 30AT) of the negative electrode current collector 30A to form a negative electrode active material layer 30B, which is then compression-molded. This completes the fabrication of the negative electrode 30.

[0124] [Preparation of Electrolyte Solution] An ionic substance is added to an aqueous solvent. This disperses or dissolves the ionic substance in the aqueous solvent, thereby preparing the electrolyte solution 40. In this case, the pH of the electrolyte solution 40 can be adjusted by adjusting conditions such as the type and concentration (mol / kg) of the ionic substance.

[0125] [Assembly of Secondary Battery] The positive electrode 20 and the negative electrode 30 are housed in the internal space S of the exterior body 10. In this case, the connection terminals 20AT and 30AT are led out from the interior (internal space S) of the exterior body 10 to the outside.

[0126] Thereafter, electrolyte 40 is supplied into internal space S through an injection hole (not shown) provided in exterior body 10, and the injection hole is then sealed.

[0127] As a result, the electrolyte solution 40 is accommodated in the internal space S in which the positive electrode 20 and the negative electrode 30 are respectively arranged, and a single-liquid type secondary battery using one type of aqueous electrolyte solution (electrolyte solution 40) is completed.

[0128] <1-5. Actions and Effects> In the secondary battery of the first embodiment, the positive electrode active material layer 20B contains a positive electrode active material (lithium-containing compound) and a positive electrode binder (a water-insoluble polar polymer compound), and the electrolyte contains an aqueous solvent. In addition, the water solubility of the water-insoluble polar polymer compound is 1 g or less, and the density ratio of the positive electrode active material layer 20B is 30% to 70%.

[0129] In this case, since the positive electrode binder contains a water-insoluble polar polymer compound, as described above, the structure of the positive electrode active material layer 20B is more likely to be maintained during charging and discharging, and the charge-discharge reaction in the positive electrode active material layer 20B is more likely to proceed stably even when the electrolyte solution 40, which is an aqueous electrolyte solution, is used.

[0130] Furthermore, since the water solubility of the water-insoluble polar polymer compound is 1 g or less, the physical strength of the positive electrode active material layer 20B is ensured, as described above, which facilitates stable and continuous absorption and desorption of lithium ions in the positive electrode active material layer 20B even after repeated charge and discharge.

[0131] Furthermore, because the density ratio of the positive electrode active material layer 20B is 30% to 70%, even if an aqueous electrolyte solution 40 is used, the electrolyte solution 40 can be easily impregnated into the positive electrode active material layer 20B, as described above. This makes it easier for lithium ions to be absorbed and released more stably and continuously in the positive electrode active material layer 20B even when charge and discharge are repeated.

[0132] For these reasons, even if charging and discharging are repeated, lithium ions are stably and continuously absorbed and released in the positive electrode active material layer 20B, and the charge and discharge reaction proceeds stably in the positive electrode active material layer 20B, thereby obtaining excellent battery characteristics.

[0133] In particular, in the first repeating unit and the second repeating unit, sufficient solubility and compatibility with the water-insoluble polar polymer compound can be obtained if the hydrocarbon group for each of R1 and R3 is an alkyl group having 1 to 10 carbon atoms and the hydrocarbon group for R2 is an alkyl group having 1 or 2 carbon atoms. Therefore, sufficient binding strength can be obtained in the positive electrode active material layer 20B, and thus even greater effects can be obtained.

[0134] Furthermore, if the positive electrode 20 includes a positive electrode current collector 20A and the positive electrode current collector 20A contains titanium as a constituent element, deterioration of the positive electrode current collector 20A during charging and discharging is suppressed, thereby achieving a greater effect.

[0135] Furthermore, if the lithium-containing compound contains a lithium composite oxide, and the lithium composite oxide has a spinel-type crystal structure and contains manganese as a constituent element, the charging efficiency and the discharging efficiency are improved, thereby achieving a greater effect.

[0136] The effects described here can be similarly obtained even when the lithium-containing compound includes a lithium phosphate compound, and the lithium phosphate compound has an olivine-type crystal structure and contains iron as a constituent element.

[0137] Furthermore, if the positive electrode active material layer 20B contains a positive electrode conductive agent, and the positive electrode conductive agent contains a carbon material, the conductivity of the positive electrode active material layer 20B is improved, and therefore, a greater effect can be obtained.

[0138] Furthermore, if the pH of the electrolytic solution 40 is less than 11, the charge / discharge reaction will proceed more easily, and therefore a greater effect can be obtained.

[0139] 2. Secondary Battery (Second Embodiment) Next, a secondary battery according to a second embodiment of the present technology will be described.

[0140] The secondary battery of the second embodiment is a dual-liquid type secondary battery that uses two types of aqueous electrolytes (a cathode electrolyte 61 and an anode electrolyte 62), unlike the secondary battery of the first embodiment that is a single-liquid type secondary battery that uses one type of aqueous electrolyte (electrolyte 40).

[0141] <2-1. Configuration> Fig. 2 shows a cross-sectional configuration of a secondary battery in the second embodiment, and corresponds to Fig. 1. The secondary battery of the second embodiment described here has the same configuration as the secondary battery of the first embodiment (Fig. 1), except for the points described below.

[0142] 2, this secondary battery additionally includes a partition wall 50, and also includes a positive electrode electrolyte 61 and a negative electrode electrolyte 62 instead of the electrolyte 40. In FIG. 2, the positive electrode electrolyte 61 is lightly shaded, and the negative electrode electrolyte 62 is darkly shaded.

[0143] The exterior body 10 has two spaces (a positive electrode chamber S1 and a negative electrode chamber S2) separated by a partition wall 50.

[0144] The partition wall 50 is disposed between the positive electrode 20 and the negative electrode 30, and separates the internal space S (see FIG. 1 ) into a positive electrode chamber S1 and a negative electrode chamber S2. As a result, the positive electrode 20 and the negative electrode 30 are separated from each other via the partition wall 50, and also face each other via the partition wall 50.

[0145] The partition wall 50 prevents anions from passing between the positive electrode chamber S1 and the negative electrode chamber S2, but allows substances (excluding anions) such as lithium ions (cations) that are absorbed and released in the positive electrode 20 and the negative electrode 30 to pass through. This is to prevent the positive electrode electrolyte 61 and the negative electrode electrolyte 62 from mixing with each other. That is, the partition wall 50 allows lithium ions to pass from the positive electrode chamber S1 to the negative electrode chamber S2, and allows lithium ions to pass from the negative electrode chamber S2 to the positive electrode chamber S1.

[0146] Specifically, the partition wall 50 includes one or both of an ion exchange membrane and a solid electrolyte membrane, because the permeability of lithium ions is improved in the partition wall 50. The ion exchange membrane is a porous membrane (cation exchange membrane) that is permeable to lithium ions, and the solid electrolyte membrane has lithium ion conductivity.

[0147] The solid electrolyte membrane may be a so-called inorganic particle membrane. This inorganic particle membrane is a composite membrane containing inorganic particles with lithium ion conductivity, a binder, and a fibrous material. The inorganic particles include one or more inorganic materials such as oxides, sulfides, hydroxides, carbonates, and sulfates. The binder includes one or more polymer compounds such as polyvinyl formal, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polymethyl methacrylate, and polytetrafluoroethylene. The fibrous material includes one or more fibrous materials such as cellulose fibers, polysaccharides, polyvinyl alcohol, polyacrylic acid, anionic derivatives of polystyrene, and cationic derivatives of polystyrene.

[0148] In particular, it is preferable that the partition wall 50 includes an ion exchange membrane rather than a solid electrolyte membrane. This is because the aqueous solvent in the positive electrode electrolyte 61 and the aqueous solvent in the negative electrode electrolyte 62 can easily permeate into the partition wall 50. This improves lithium ion conductivity inside the partition wall 50.

[0149] The positive electrode 20 is housed inside the positive electrode chamber S1 and absorbs and releases lithium ions, and the negative electrode 30 is housed inside the negative electrode chamber S2 and absorbs and releases lithium ions.

[0150] The positive electrode electrolyte 61 and the negative electrode electrolyte 62 are each an aqueous electrolyte. The positive electrode electrolyte 61 is accommodated inside the positive electrode chamber S1, and the negative electrode electrolyte 62 is accommodated inside the negative electrode chamber S2. Therefore, the positive electrode electrolyte 61 and the negative electrode electrolyte 62 are separated from each other by the partition wall 50 to prevent them from mixing with each other.

[0151] As a result, the positive electrode electrolyte 61 contained inside the positive electrode chamber S1 is not in contact with the negative electrode 30 but is in contact only with the positive electrode 20. On the other hand, the negative electrode electrolyte 62 contained inside the negative electrode chamber S2 is not in contact with the positive electrode 20 but is in contact only with the negative electrode 30.

[0152] The pH of the positive electrode electrolyte 61 and the pH of the negative electrode electrolyte 62 are different from each other. Specifically, the pH of the negative electrode electrolyte 62 is higher than the pH of the positive electrode electrolyte 61. As long as this pH magnitude relationship is satisfied, the compositions of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 (such as the type of aqueous solvent and the type and concentration of the ionic substance) can be set arbitrarily.

[0153] The pH of the negative electrode electrolyte 62 is higher than the pH of the positive electrode electrolyte 61 because the difference in pH between the two electrolytes causes a shift in the decomposition potential of the aqueous solvent. As a result, the decomposition reaction of the aqueous solvent is thermodynamically suppressed during charge and discharge, while the potential window of the aqueous solvent is expanded. Therefore, a high voltage can be obtained, and the charge and discharge reaction utilizing the absorption and release of lithium ions can proceed sufficiently and stably.

[0154] In particular, it is preferable that the composition formula (type of electrolyte salt) of the positive electrode electrolyte 61 and the composition formula (type of electrolyte salt) of the negative electrode electrolyte 62 are different from each other, because this makes it easier to ensure the magnitude relationship regarding the pH described above.

[0155] As long as the above-described magnitude relationship regarding pH is satisfied, the pH values ​​of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 are not particularly limited.

[0156] In particular, the pH of the anode electrolyte 62 in contact with the anode 30 is preferably 11 or higher, more preferably 12 or higher, and even more preferably 13 or higher. This is because the pH of the anode electrolyte 62 becomes sufficiently large, making it easier to ensure the magnitude relationship of the pH described above. Also, this is because the difference between the pH of the cathode electrolyte 61 and the pH of the anode electrolyte 62 becomes sufficiently large, making it easier to maintain the magnitude relationship of the pH between the two.

[0157] Furthermore, the pH of the positive electrode electrolyte 61 in contact with the positive electrode 20 is preferably less than 11. Specifically, the pH of the positive electrode electrolyte 61 is preferably 3 to 8, more preferably 4 to 8, and even more preferably 4 to 6. This is because the pH of the positive electrode electrolyte 61 becomes sufficiently low, making it easier to ensure the above-described pH magnitude relationship and to maintain the pH magnitude relationship. Furthermore, the exterior body 10, the positive electrode current collector 20A, the negative electrode current collector 30A, and the like become less susceptible to corrosion, improving the electrochemical durability (stability) of the secondary battery.

[0158] It is preferable that one or both of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 be saturated solutions of an electrolyte salt (lithium salt), similar to the electrolyte 40 of the first embodiment. This is because charge / discharge reactions tend to proceed stably during charge / discharge. The method for checking whether each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 is a saturated solution of a lithium salt is the same as the method for checking whether the electrolyte 40 is a saturated solution of a lithium salt.

[0159] Furthermore, each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 may be a pH buffer solution. This pH buffer solution may be an aqueous solution in which a weak acid and its conjugate base are mixed, or may be an aqueous solution in which a weak base and its conjugate acid are mixed. This is because the pH of the positive electrode electrolyte 61 and the pH of the negative electrode electrolyte 62 are each easily maintained because the pH fluctuations are sufficiently suppressed.

[0160] In particular, it is preferable that the positive electrode electrolyte 61 contains, as anions, one or more of sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, dihydrogen phosphate ions, and carboxylate ions. This is because fluctuations in the pH of the positive electrode electrolyte 61 are sufficiently suppressed, making it easier to sufficiently maintain the pH of the positive electrode electrolyte 61. Specific examples of carboxylate ions include formate ions, acetate ions, propionate ions, tartrate ions, and citrate ions.

[0161] The pH of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 may each contain, as a buffer, one or more of trishydroxymethylaminomethane, ethylenediaminetetraacetic acid, and the like.

[0162] More specifically, it is preferable that the positive electrode electrolyte 61 contains, as an anion, one or more of sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, and dihydrogen phosphate ions, and the negative electrode electrolyte 62 contains, as an anion, hydroxide ions. This is because the pH of the positive electrode electrolyte 61 can be easily controlled to be sufficiently high, and the pH of the negative electrode electrolyte 62 can be easily controlled to be sufficiently low.

[0163] Here, it is preferable that the positive electrode electrolyte 61 and the negative electrode electrolyte 62 are isotonic solutions that are isotonic with each other. This is because the osmotic pressures of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 are optimized, making it easier to maintain the pH magnitude relationship between the two.

[0164] <2-2. Operation> This secondary battery operates as described below.

[0165] During charging, lithium ions are released from the positive electrode 20 and cations such as the lithium ions are absorbed into the negative electrode 30 via the positive electrode electrolyte 61 , the partition wall 50 and the negative electrode electrolyte 62 .

[0166] During discharge, cations such as lithium ions are released from the negative electrode 30 and the lithium ions are absorbed into the positive electrode 20 via the negative electrode electrolyte 62 , the partition wall 50 and the positive electrode electrolyte 61 .

[0167] 2-3. Manufacturing Method The manufacturing procedure for this secondary battery is the same as that for the secondary battery of the first embodiment, except for the points described below.

[0168] An ionic substance is added to an aqueous solvent when preparing each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62. In this case, the pH of the negative electrode electrolyte 62 is made higher than the pH of the positive electrode electrolyte 61 by adjusting conditions such as the type and concentration (mol / kg) of the ionic substance.

[0169] When assembling a secondary battery, first, an exterior body 10 (cathode chamber S1 and anode chamber S2) to which a partition wall 50 has been attached is prepared. Next, a cathode 20 is housed inside the cathode chamber S1, and a connection terminal 20AT is led out from inside the cathode chamber S1 to the outside. Also, anode 30 is housed inside the anode chamber S2, and a connection terminal 30AT is led out from inside the anode chamber S2 to the outside. Finally, a cathode electrolyte 61 is supplied into the cathode chamber S1 through a cathode injection hole (not shown) provided in the exterior body 10, and anode electrolyte 62 is supplied into the anode chamber S2 through a cathode injection hole (not shown) provided in the exterior body 10. Then, the cathode injection hole and the anode injection hole are each sealed.

[0170] As a result, the positive electrode electrolyte 61 is accommodated inside the positive electrode chamber S1 that accommodates the positive electrode 20, and the negative electrode electrolyte 62 is accommodated inside the negative electrode chamber S2 that accommodates the negative electrode 30. Thus, a dual-liquid type secondary battery using two types of aqueous electrolytes (positive electrode electrolyte 61 and negative electrode electrolyte 62) is completed.

[0171] <2-4. Actions and Effects> The secondary battery of the second embodiment has a configuration similar to that of the secondary battery of the first embodiment, except that it includes cathode electrolyte 61 and anode electrolyte 62 instead of electrolyte 40, and the pH of anode electrolyte 62 is higher than the pH of cathode electrolyte 61. Therefore, for the same reasons as those described for the secondary battery of the first embodiment, excellent operating characteristics can be obtained.

[0172] In this case, in particular, since two types of aqueous electrolytes (positive electrode electrolyte 61 and negative electrode electrolyte 62) with different pH values ​​are used, a high voltage can be obtained, and the charge / discharge reaction utilizing the absorption and release of lithium ions can proceed sufficiently and stably, thereby achieving better battery characteristics.

[0173] Other functions and effects of the secondary battery of the second embodiment are similar to those of the secondary battery of the first embodiment.

[0174] 3. Modifications The configuration of the secondary battery can be modified as appropriate, as described below.

[0175] [Modification 1] In the first embodiment, as shown in Fig. 1, electrolytic solution 40, which is a liquid electrolyte, is used. However, as shown in Fig. 3, which corresponds to Fig. 1, electrolyte layers 81, 82, which are gel electrolytes, may be used instead of electrolytic solution 40. The configuration of the secondary battery shown in Fig. 3 is the same as the configuration of the secondary battery shown in Fig. 1, except for the points described below.

[0176] Here, the secondary battery additionally includes a separator 70 and the above-described electrolyte layers 81 and 82. The separator 70 is disposed between the positive electrode 20 and the negative electrode 30. The electrolyte layer 81 is disposed between the positive electrode 20 and the separator 70, and the electrolyte layer 82 is disposed between the negative electrode 30 and the separator 70. As a result, the electrolyte layer 81 is adjacent to both the positive electrode 20 and the separator 70, and the electrolyte layer 82 is adjacent to both the negative electrode 30 and the separator 70.

[0177] Each of the electrolyte layers 81 and 82 contains a polymer compound together with the electrolytic solution 40, and the electrolytic solution 40 is held by the polymer compound. The type of polymer compound is not particularly limited, but specifically, it may be one or more of polyvinylidene fluoride, polyethylene oxide, etc. In Figure 3, each of the electrolyte layers 81 and 82 is lightly shaded.

[0178] The separator 70 is an insulating porous film that separates the electrolyte layers 81 and 82 from each other and allows lithium ions to pass through, and contains a polymer compound such as polyethylene.

[0179] When forming the electrolyte layer 81, a sol-like precursor solution is prepared by mixing the electrolytic solution 40, the polymer compound, and a dilution solvent together, and then the precursor solution is applied to the surface of the positive electrode 20. The procedure for forming the electrolyte layer 82 is the same as the procedure for forming the electrolyte layer 81, except that the precursor solution is applied to the surface of the negative electrode 30.

[0180] In this case, too, lithium ions can move between the positive electrode 20 and the negative electrode 30 via the electrolyte layers 81 and 82, so that the same effect as in the case shown in Fig. 1 can be obtained. In this case, leakage of the electrolyte can be particularly prevented.

[0181] [Modification 2] In the second embodiment, as shown in Fig. 2, positive electrode electrolyte 61 and negative electrode electrolyte 62, which are liquid electrolytes, are used. However, as shown in Fig. 4, which corresponds to Fig. 2, electrolyte layers 91 and 92, which are gel electrolytes, may be used instead of positive electrode electrolyte 61 and negative electrode electrolyte 62. The configuration of the secondary battery shown in Fig. 4 is the same as the configuration of the secondary battery shown in Fig. 2, except for the points described below.

[0182] Here, the secondary battery additionally includes the above-described electrolyte layers 91 and 92. The electrolyte layer 91 is disposed between the positive electrode 20 and the partition wall 50, and the electrolyte layer 92 is disposed between the negative electrode 30 and the partition wall 50. As a result, the electrolyte layer 91 is adjacent to both the positive electrode 20 and the partition wall 50, and the electrolyte layer 92 is adjacent to both the negative electrode 30 and the partition wall 50.

[0183] The electrolyte layer 91 contains a polymer compound together with the cathode electrolyte 61, and the cathode electrolyte 61 is held by the polymer compound. The electrolyte layer 92 contains a polymer compound together with the anode electrolyte 62, and the anode electrolyte 62 is held by the polymer compound. Details regarding the types of polymer compounds are as described above. In FIG. 4 , the electrolyte layer 91 containing the cathode electrolyte 61 is lightly shaded, and the electrolyte layer 92 containing the anode electrolyte 62 is darkly shaded.

[0184] When forming the electrolyte layer 91, a sol-like precursor solution is prepared by mixing the positive electrode electrolyte 61, the polymer compound, and a dilution solvent together, and the precursor solution is then applied to the surface of the positive electrode 20. When forming the electrolyte layer 92, a sol-like precursor solution is prepared by mixing the negative electrode electrolyte 62, the polymer compound, and a dilution solvent together, and the precursor solution is then applied to the surface of the negative electrode 30.

[0185] In this case, too, lithium ions can move between the positive electrode 20 and the negative electrode 30 via the electrolyte layers 91 and 92, so that the same effect as in the case shown in Fig. 2 can be obtained. In this case, leakage of the electrolyte can be particularly prevented.

[0186] 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.

[0187] 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.

[0188] 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.

[0189] Of course, the secondary battery may be used for purposes other than the series of purposes exemplified here.

[0190] An embodiment of the present technology will be described.

[0191] Examples 1 to 13 and Comparative Examples 1 to 9 As will be described below, secondary batteries were manufactured, and then the battery characteristics of the secondary batteries were evaluated.

[0192] [Fabrication of Electrochemical Measurement Cell] In order to simply evaluate the battery characteristics, an electrochemical measurement cell having a configuration substantially similar to that of the single-liquid secondary battery described in the first embodiment ( FIG. 1 ) was fabricated by the procedure described below.

[0193] (Fabrication of Positive Electrode) First, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent (carbon black, which is a carbon material) were mixed together to prepare a positive electrode mixture.

[0194] The positive electrode active material is a lithium composite oxide (LiMn) having a spinel-type crystal structure and containing manganese as a constituent element. 2　 O 4　 (LMO)) and a lithium phosphate compound (LiFePO) having an olivine-type crystal structure and containing iron as a constituent element. 4　 (LFP)) was used.

[0195] The positive electrode binders used were polymethyl acrylate (PAM), a water-insoluble polar polymer compound containing a first repeating unit (R1 = methyl group and R2 = hydrogen group), and polyvinyl butyral (PVB), a water-insoluble polar polymer compound containing a second repeating unit (R3 = n-propyl group). The water solubility (g) and weight-average molecular weight of the water-insoluble polar polymer compounds are shown in Tables 1 to 3.

[0196] When obtaining a positive electrode mixture, the mixing ratio (mass ratio) was changed as described below. The mixing ratio of the positive electrode active material was changed in the range of 70 parts by mass to 94 parts by mass. The mixing ratio of the positive electrode binder was changed in the range of 3 parts by mass to 20 parts by mass. The mixing ratio of the positive electrode conductive agent was changed in the range of 3 parts by mass to 10 parts by mass.

[0197] Next, the positive electrode mixture was added to a solvent (N-methyl-2-pyrrolidone, an organic solvent), and the organic solvent was stirred to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry was applied to both surfaces of the positive electrode current collector 20A (excluding the connection terminal portion 20AT) using a coating device, and the positive electrode mixture slurry was then dried to form the positive electrode active material layer 20B.

[0198] As the positive electrode current collector 20A, a mesh-shaped titanium foil (Ti) having a thickness of 200 μm and a mesh-shaped nickel foil (Ni) having a thickness of 200 μm were used.

[0199] Finally, the positive electrode active material layer 20B was compression-molded using a press, thereby producing the positive electrodes 20 (Examples 1 to 13 and Comparative Examples 3, 4, and 6 to 8).

[0200] The density percentages (%) are as shown in Tables 1 to 3. In this case, the compression molding conditions (pressing pressure and pressing time) of the positive electrode active material layer 20B were changed to change the density percentage in accordance with the change in volume density.

[0201] For comparison, a positive electrode 20 was fabricated using the same procedure, except that other polymer compounds were used as the positive electrode binder instead of the water-insoluble polar polymer compound. The other polymer compounds used were polyvinylidene fluoride (PVDF), carboxymethyl cellulose (CMC), and polyacrylic acid (PAA) (Comparative Examples 1, 2, 5, and 9). The water solubilities (g) and weight-average molecular weights of the other polymer compounds are shown in Table 3.

[0202] (Fabrication of Negative Electrode) As the negative electrode 30, a mesh-shaped nickel foil having a thickness of 200 μm was used.

[0203] (Preparation of Electrolyte Solution) An ionic substance (lithium sulfate, an electrolyte salt) was added to a solvent (water, an aqueous solvent), and the solvent was then stirred to prepare an aqueous electrolyte solution 40. In this case, the concentration of the electrolyte solution 40 was 2 mol / kg and the pH was 4.

[0204] (Assembly of Electrochemical Measurement Cell) First, the positive electrode 20 and the negative electrode 30 were housed in the internal space S of a glass exterior body 10 (glass beaker). In this case, the connection terminal 20AT was led out from the inside of the exterior body 10 to the outside, and a portion of the negative electrode 30 was also led out. Next, a reference electrode (a silver / silver chloride electrode, not shown) was placed in the internal space S. Finally, the electrolyte 40 was supplied into the internal space S. As a result, the electrolyte 40 was housed in the internal space S, completing the electrochemical measurement cell.

[0205] [Evaluation of Battery Characteristics] The battery characteristics (discharge characteristics and cycle characteristics) were evaluated according to the procedures described below, and the results shown in Tables 1 to 3 were obtained.

[0206] (Discharge Characteristics) The electrochemical measurement cell was charged and discharged in a room temperature environment (temperature = 23°C) to calculate the discharge capacity (mAh / g), which is an index for evaluating the discharge characteristics. This discharge capacity is the discharge capacity (mAh) per weight (g) of the positive electrode active material.

[0207] During charging, the battery was charged at a constant current of 1 C until the voltage reached −1.45 V relative to the reference electrode (silver-silver chloride), and then charged at a constant voltage of −1.45 V until the current reached 0.5 C. During discharging, the battery was discharged at a constant current of 1 C until the voltage reached −1.00 V relative to the reference electrode. 1 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 1 hour, and 0.5 C is the current value at which the battery capacity is fully discharged in 2 hours.

[0208] (Cycle Characteristics) First, the discharge capacity (discharge capacity at the first cycle) of the secondary battery was measured by charging and discharging the secondary battery in a room temperature environment under the same charge and discharge conditions as those used to evaluate the discharge characteristics.

[0209] Subsequently, the secondary battery was repeatedly charged and discharged in the same environment until the total number of cycles reached 20, and the discharge capacity (discharge capacity at the 20th cycle) was measured under the same charge and discharge conditions as those used to evaluate the discharge characteristics described above.

[0210] 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 20th cycle / discharge capacity at 1st cycle)×100.

[0211] As shown in Table 3, the capacity retention rate was not calculated in cases where charging and discharging were not possible (Comparative Examples 3 and 6) and in cases where charging and discharging were possible but the discharge capacity was significantly reduced (Comparative Examples 4 and 7).

[0212]

[0213]

[0214]

[0215] [Discussion] As shown in Tables 1 to 3, in the electrochemical measurement cell using the aqueous electrolyte (electrolyte 40), the discharge capacity and capacity retention rate each varied depending on the configuration of the positive electrode 20.

[0216] Specifically, when the two conditions that the positive electrode binder contained a water-insoluble polar polymer compound (water solubility ≦1 g) and the density ratio was 30% to 70% were not satisfied (Comparative Examples 1 to 9), charging and discharging was not possible at all, or even if charging and discharging was possible, one or both of the discharge capacity and the capacity retention rate decreased.

[0217] In contrast, when the positive electrode binder contained a water-insoluble polar polymer compound (water solubility ≦1 g) and the density ratio was 30% to 70%, the discharge capacity increased and the capacity retention rate also increased (Examples 1 to 13).

[0218] In this case (Examples 1 to 13), the following tendency was particularly observed.

[0219] First, even when the weight-average molecular weight of the water-insoluble polar polymer compound was changed, a high discharge capacity and a high capacity retention rate were obtained.

[0220] Secondly, when the positive electrode current collector 20A contained titanium as a constituent element, the discharge capacity was further increased and the capacity retention rate was also further increased.

[0221] Third, when the positive electrode active material contains a lithium composite oxide (LMO) or a lithium phosphate compound (LFP), a high discharge capacity and a high capacity retention rate are obtained.

[0222] Fourth, when the positive electrode active material layer 20B contains a positive electrode conductive agent (carbon material), a high discharge capacity and a high capacity retention rate were obtained.

[0223] Fifth, when the pH of the electrolytic solution 40 was less than 11, a high discharge capacity was obtained, and a high capacity retention rate was also obtained.

[0224] Note that this study has specifically examined only the single-liquid secondary battery (electrochemical measurement cell) described in the first embodiment. However, according to the results shown in Table 1, the same tendency can be obtained when examining the dual-liquid secondary battery described in the second embodiment.

[0225] [Summary] From the results shown in Table 1, in a secondary battery equipped with an aqueous electrolyte solution (electrolyte solution 40), when the positive electrode active material layer 20B contains a positive electrode active material (lithium-containing compound) and a positive electrode binder (a water-insoluble polar polymer compound), the water solubility of the water-insoluble polar polymer compound is 1 g or less, and the density ratio of the positive electrode active material layer 20B is 30% to 70%, a high discharge capacity and a high capacity retention rate were obtained. Therefore, both the discharge characteristics and the cycle characteristics were improved, and thus excellent battery characteristics were obtained in the secondary battery.

[0226] 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.

[0227] 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.

[0228] The present technology may also be configured as follows: <1> A secondary battery comprising: a positive electrode including a positive electrode active material layer, a negative electrode, and an electrolyte solution including an aqueous solvent, wherein the positive electrode active material layer includes: a positive electrode active material including a lithium-containing compound that absorbs and releases lithium ions; and a positive electrode binder including a polymer compound, wherein the polymer compound includes at least one of a repeating unit represented by formula (1) and a repeating unit represented by formula (2), wherein the solubility of the polymer compound in 100 g of water is 1 g or less, and a ratio of the volume density of the positive electrode active material layer to the true density of the positive electrode active material is 30% or more and 70% or less. (R1 is either a hydrocarbon group or an oxygen-containing hydrocarbon group, and R2 is either a hydrogen group or a hydrocarbon group. R3 is either a hydrocarbon group or an oxygen-containing hydrocarbon group. n1 and n2 each are an integer of 2 or more. An asterisk (*) represents a dangling bond.) <2> The secondary battery according to <1>, wherein the hydrocarbon group for each of R1 and R3 is an alkyl group having 1 to 10 carbon atoms, and the hydrocarbon group for R2 is an alkyl group having 1 or 2 carbon atoms. <3> The secondary battery according to <1> or <2>, wherein the positive electrode further includes a positive electrode current collector supporting the positive electrode active material layer, and the positive electrode current collector includes titanium as a constituent element. <4> The secondary battery according to any one of <1> to <3>, wherein the lithium-containing compound includes a lithium composite oxide having a spinel-type crystal structure and including manganese as a constituent element, or a lithium phosphate compound having an olivine-type crystal structure and including iron as a constituent element. <5> The secondary battery according to any one of <1> to <4>, wherein the positive electrode active material layer further contains a positive electrode conductive agent, and the positive electrode conductive agent contains a carbon material. <6> The secondary battery according to any one of <1> to <5>, wherein a pH of the electrolyte is less than 11. <7> The secondary battery according to any one of <1> to <5>, further comprising: a positive electrode chamber accommodating the positive electrode; an anode chamber accommodating the negative electrode; and a partition wall disposed between the positive electrode chamber and the anode chamber and allowing the lithium ions to pass through, wherein the electrolyte includes a positive electrode electrolyte contained in the positive electrode chamber, and an anode electrolyte contained in the anode chamber and having a pH higher than a pH of the positive electrode electrolyte.

[0229] 20... Positive electrode, 20A... Positive electrode current collector, 20B... Positive electrode active material layer, 30... Negative electrode, 40... Electrolyte, 50... Partition wall, 61... Positive electrode electrolyte, 62... Negative electrode electrolyte, S1... Positive electrode chamber, S2... Negative electrode chamber

Claims

1. A secondary battery comprising: a positive electrode including a positive electrode active material layer; a negative electrode; and an electrolyte including an aqueous solvent, wherein the positive electrode active material layer includes a positive electrode active material including a lithium-containing compound that absorbs and releases lithium ions; and a positive electrode binder including a polymer compound, wherein the polymer compound includes at least one of a repeating unit represented by formula (1) and a repeating unit represented by formula (2), wherein the solubility of the polymer compound in 100 g of water is 1 g or less, and a ratio of the volume density of the positive electrode active material layer to the true density of the positive electrode active material is 30% or more and 70% or less. (R1 is either a hydrocarbon group or an oxygen-containing hydrocarbon group, and R2 is either a hydrogen group or a hydrocarbon group. R3 is either a hydrocarbon group or an oxygen-containing hydrocarbon group. Each of n1 and n2 is an integer of 2 or more. An asterisk (*) represents a dangling bond.) 2. The secondary battery according to claim 1, wherein the hydrocarbon group for each of R1 and R3 is an alkyl group having 1 or more and 10 or less carbon atoms, and the hydrocarbon group for R2 is an alkyl group having 1 or 2 carbon atoms.

3. The secondary battery according to claim 1 or 2, wherein the positive electrode further includes a positive electrode current collector supporting the positive electrode active material layer, and the positive electrode current collector includes titanium as a constituent element.

4. The secondary battery according to any one of claims 1 to 3, wherein the lithium-containing compound comprises a lithium composite oxide having a spinel-type crystal structure and containing manganese as a constituent element, or a lithium phosphate compound having an olivine-type crystal structure and containing iron as a constituent element.

5. The secondary battery according to any one of claims 1 to 4, wherein the positive electrode active material layer further contains a positive electrode conductive agent, and the positive electrode conductive agent contains a carbon material.

6. The secondary battery according to any one of claims 1 to 5, wherein the pH of the electrolyte is less than 11.

7. The secondary battery according to any one of claims 1 to 5, further comprising: a positive electrode chamber for accommodating the positive electrode therein; a negative electrode chamber for accommodating the negative electrode therein; and a partition wall disposed between the positive electrode chamber and the negative electrode chamber and allowing the lithium ions to pass through, wherein the electrolyte comprises a positive electrode electrolyte contained in the positive electrode chamber; and a negative electrode electrolyte contained in the negative electrode chamber and having a pH higher than a pH of the positive electrode electrolyte.

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