Secondary battery

Abstract

Provided is a secondary battery capable of achieving excellent battery characteristics. This secondary battery comprises: a negative electrode electrolyte containing an aqueous solvent; a positive electrode electrolyte containing an aqueous solvent; a partition wall disposed between the negative electrode electrolyte and the positive electrode electrolyte and permeable to alkali metal ions; a negative electrode that is immersed in the negative electrode electrolyte and adsorbs and releases alkali metal ions; and a positive electrode that is immersed in the positive electrode electrolyte and adsorbs and releases alkali metal ions. The negative electrode electrolyte has a pH greater than the pH of the positive electrode electrolyte.

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. Among these, secondary batteries using electrolytes containing aqueous solvents have been developed, and various studies have been conducted on the structure of these secondary batteries.

[0003] Specifically, the negative electrode electrolyte and the positive electrode electrolyte are separated from each other by a cation exchange membrane, and the negative electrode electrolyte has a higher pH than the positive electrode electrolyte (see, for example, Patent Document 1). A polymer compound with ion exchange properties is used as the separator, and the water content of the polymer compound is specified (see, for example, Patent Documents 2 and 3). The water content of the redox flow electrolyte membrane is specified (see, for example, Patent Document 4).

[0004] International Publication No. 2020 / 218456 Japanese Patent Laid-Open No. 05-217571 Japanese Patent Laid-Open No. 2016-031832 Japanese Patent Laid-Open No. 2013-168365

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

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

[0007] According to one embodiment of the present technology, a secondary battery includes: an anode electrolyte containing an aqueous solvent; a cathode electrolyte containing an aqueous solvent; a partition wall disposed between the anode electrolyte and the cathode electrolyte and allowing alkali metal ions to pass therethrough; an anode immersed in the anode electrolyte and occluding and releasing alkali metal ions; and a cathode immersed in the cathode electrolyte and occluding and releasing alkali metal ions. The anode electrolyte has a pH greater than that of the cathode electrolyte. The partition wall has a liquid absorption rate calculated by Equation (1), and the liquid absorption rate satisfies a condition expressed by Equation (2).

[0008] W=(W1−W0) / W0 (1) (W is the liquid absorption rate. W0 is the weight of the partition wall in a state where the negative electrode electrolyte and the positive electrode electrolyte have been removed. W1 is the weight of the partition wall in a state where the negative electrode electrolyte and the positive electrode electrolyte have been impregnated.) −0.1<W<0.1 (2)

[0009] Details of a procedure for calculating the liquid absorption rate, including a procedure for measuring a weight W0 of the partition wall in a state where the anode electrolyte and the cathode electrolyte have each been removed, and a procedure for measuring a weight W1 of the partition wall in a state where the anode electrolyte and the cathode electrolyte have each been impregnated, will be described later.

[0010] According to a secondary battery according to one embodiment of the present technology, a partition wall that allows alkali metal ions to pass through is disposed between an anode electrolyte containing an aqueous solvent and a cathode electrolyte containing an aqueous solvent. Furthermore, an anode that occludes and releases alkali metal ions is immersed in the anode electrolyte, and a cathode that occludes and releases alkali metal ions is immersed in the cathode electrolyte, and the anode electrolyte has a pH higher than that of the cathode electrolyte. Furthermore, the liquid absorption rate of the partition wall calculated using formula (1) satisfies the condition expressed by formula (2). Therefore, excellent battery characteristics can be obtained.

[0011] Note that the effects of the present technology are not necessarily limited to the effects described here, but may be any of a series of effects related to the present technology described below.

[0012] 1 is a cross-sectional view illustrating a configuration of a secondary battery according to an embodiment of the present technology, and FIG. 2 is a cross-sectional view illustrating a configuration of a secondary battery according to a second modification.

[0013] Hereinafter, an embodiment of the present technology will be described in detail with reference to the drawings. The description will be made in the following order: 1. Secondary battery 1-1. Configuration 1-2. Physical properties 1-3. Operation 1-4. Manufacturing method 1-5. Actions and effects 2. Modifications 3. Uses of secondary battery

[0014] 1. Secondary Battery First, a secondary battery according to an embodiment of the present technology will be described.

[0015] The secondary battery described here utilizes the absorption and desorption of alkali metal ions and includes a positive electrode, a negative electrode, and an electrolyte solution (a so-called aqueous electrolyte solution) that is a liquid electrolyte containing an aqueous solvent. In this secondary battery, the absorption and desorption of alkali metal ions is utilized to promote charge and discharge reactions, thereby obtaining battery capacity.

[0016] The type of alkali metal ion is not particularly limited, but specific examples include lithium ions, sodium ions, and potassium ions, because this allows a high voltage to be obtained while the charge / discharge reaction proceeds stably.

[0017] <1-1. Structure> FIG. 1 shows a cross-sectional structure of a secondary battery.

[0018] As shown in Fig. 1, this secondary battery includes an exterior member 11, a partition wall 12, an anode 13, a cathode 14, an anode electrolyte 15, and a cathode electrolyte 16. In Fig. 1, the anode electrolyte 15 is shaded darkly, and the cathode electrolyte 16 is shaded lightly.

[0019] Each of the anode electrolyte 15 and the cathode electrolyte 16 is the above-described water-based electrolyte, and therefore contains an aqueous solvent. This water-based electrolyte contains an ionic substance that can be ionized in the aqueous solvent, and the ionic substance is dissolved or dispersed in the aqueous solvent. Details of the anode electrolyte 15 and the cathode electrolyte 16 will be described later.

[0020] [Exterior Member] The exterior member 11 has an internal space for accommodating the partition wall 12, the negative electrode 13, the positive electrode 14, the negative electrode electrolyte 15, the positive electrode electrolyte 16, and the like.

[0021] This exterior member 11 contains one or more of materials such as a metal material, a glass material, a polymer compound, etc. Specifically, the exterior member 11 may be any of a metal can, a glass case, a plastic case, etc., which have rigidity, or any of a metal foil, a polymer film, etc., which have flexibility or pliability.

[0022] [Partition Wall] Partition wall 12 is disposed between anode electrolyte 15 and cathode electrolyte 16, and separates the internal space of exterior member 11 into two spaces. These two spaces are an anode chamber S1 that accommodates anode electrolyte 15 and a cathode chamber S2 that accommodates cathode electrolyte 16.

[0023] That is, because partition wall 12 is located between the anode chamber S1 and the cathode chamber S2, it separates anode electrolyte 15 contained in anode chamber S1 from cathode electrolyte 16 contained in cathode chamber S2. As a result, anode 13 immersed in anode electrolyte 15 and cathode 14 immersed in cathode electrolyte 16 face each other with partition wall 12 interposed therebetween.

[0024] This partition wall 12 does not allow anions to pass between the anode electrolyte 15 and the cathode electrolyte 16, but allows substances (excluding anions) such as alkali metal ions (cations) that are occluded and released in the anode 13 and the cathode 14 to pass between them. This is because the partition wall 12 allows alkali metal ions to move between the anode 13 and the cathode 14 while preventing the anode electrolyte 15 and the cathode electrolyte 16 from mixing with each other. As a result, the partition wall 12 allows alkali metal ions to pass from the anode electrolyte 15 to the cathode electrolyte 16, and also allows alkali metal ions to pass from the cathode electrolyte 16 to the anode electrolyte 15.

[0025] The partition wall 12 includes one or more types of functional membranes that allow substances such as alkali metal ions to pass through. Specific examples of the functional membranes include a cation exchange membrane and a solid electrolyte membrane.

[0026] In this secondary battery, the physical property (liquid absorption rate W, which will be described later) of the partition walls 12 is optimized in order to improve the battery characteristics. The details of this physical property will be described later.

[0027] (Cation Exchange Membrane) A cation exchange membrane is a porous membrane that is permeable to substances such as alkali metal ions. As will be described below, this cation exchange membrane is a polymer membrane having so-called anionic groups.

[0028] The cation exchange membrane contains multiple anion groups (-X -　), and its multiple anionic groups are hydrogen ions (H +　 ) and metal ions (M n+ ), etc., where n is an integer of 1 or more. Specific examples of metal ions include lithium ions, sodium ions, and potassium ions. As a result, the cation exchange membrane has multiple ion exchange groups (for example, -X -　 H +　 and -X -　 M +　 etc.)

[0029] The type of anionic group is not particularly limited, but specifically, a sulfonic acid ion group (-S(=O) 2　 -O -　 ) and a carboxylate ion group (—C(═O)—O -　 ) and both thereof. The type of ion exchange group is not particularly limited, but specifically, a sulfonic acid group (-S(=O) 2　 -O -　 H +　 ), a carboxylic acid group (—C(═O)—O -　 H +　 ), sulfonic acid metal complex (-S(=O) 2　 -O -　 H +　 ) and carboxylic acid metal complexes (—C(═O)—O -　 M +　 The sulfonic acid metal complex is a so-called metal sulfonate base, and the carboxylic acid metal complex is a so-called metal carboxylate base.

[0030] The type of skeleton to which multiple ion-exchange groups are bonded is not particularly limited, as long as it is a polymeric compound to which multiple ion-exchange groups can be bonded. Specific examples of polymeric compounds that form this skeleton include perfluorohydrocarbons, polymeric compounds polymerized from monomers having ethylenically unsaturated double bonds, polysulfones, polyphenylene sulfides, polyether ketones, polyether ether ketones, polyetherimides, polyphenylene oxides, polyether sulfones, polybenzimidazoles, polystyrene-poly(ethylene-butylene)-polystyrene triblock copolymers, polystyrene-poly(ethylene-propylene)-polystyrene triblock copolymers, and styrene-based elastomers. Polymeric compounds polymerized from monomers having ethylenically unsaturated double bonds include vinyl-based, styrene-based, and acrylic-based polymeric compounds. Styrene-based elastomers include polystyrene-polyisoprene block copolymers and hydrogenated products of polystyrene-polyisoprene block copolymers.

[0031] When the partition wall 12 includes a cation exchange membrane, an appropriate amount of the aqueous solvent contained in the anode electrolyte 15 easily permeates into the partition wall 12, and an appropriate amount of the aqueous solvent contained in the cathode electrolyte 16 easily permeates into the partition wall 12. This improves ionic conductivity inside the partition wall 12.

[0032] (Solid electrolyte membrane) The solid electrolyte membrane has alkali metal ion conductivity and therefore has the property of allowing alkali metal ions to pass through. The type of solid electrolyte membrane having alkali metal ion conductivity is not particularly limited, but it is preferable that the membrane contains an inorganic solid electrolyte that has excellent alkali metal ion conductivity and high water resistance. This is because hydrolysis is less likely to occur inside the secondary battery.

[0033] Specifically, the inorganic solid electrolyte having alkali metal ion conductivity has a NASICON structure, more specifically, LiM 2　 (P.O. 4　 ) 3　The lithium phosphate solid electrolyte preferably contains one or more of the lithium phosphate solid electrolytes represented by the general formula: M = Ti, Ge, Sr, Zr, Sn, Al, or the like. In particular, M preferably contains one or more of the metal elements Ge, Zr, and Ti, and Al.

[0034] A specific example of a lithium phosphate solid electrolyte having a NASICON structure is LATP (Li 1+x　 Al x　 Ti 2-x　 (P.O. 4　 ) 3　 ), Li 1+x　 Al x　 Ge 2-x　 (P.O. 4　 ) 3　 and Li 1+x　 Al x　 Zr 2-x　 (P.O. 4　 ) 3　 etc. However, x satisfies 0<x≦5, and preferably 0.1≦x≦0.5. Among these, it is preferable that the lithium phosphate solid electrolyte contains LATP, because this provides excellent water resistance and makes it difficult for hydrolysis to occur inside the secondary battery.

[0035] Alternatively, the inorganic solid electrolyte having alkali metal ion conductivity includes one or more oxide-based solid electrolytes.

[0036] A specific example of an oxide-based solid electrolyte is amorphous LIPON (Li 2.9　 P.O. 3.3　 N 0.46 ) and LLZ (Li 7　 La 3　 Zr 2　 O 12 ) etc.

[0037] Alternatively, the inorganic solid electrolyte having alkali metal ion conductivity includes one or more sodium-containing solid electrolytes having excellent sodium ion conductivity, and the sodium-containing solid electrolyte is preferably in a glass-ceramic state.

[0038] Specific examples of sodium-containing solid electrolytes include sodium phosphorus sulfide and sodium phosphorus oxide.

[0039] The solid electrolyte membrane may be a solid electrolyte formed into a membrane, or may be a solid electrolyte particle membrane containing solid electrolyte particles together with a binder and a fibrous material.

[0040] The binder contains one or more polymeric compounds. These polymeric compounds are compounds formed by polymerizing hydrocarbons (monomers) having specific functional groups, and the functional groups contain one or more of elements such as O, S, N, and F as constituent elements. Specific examples of polymeric compounds include polyvinyl formal, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polymethyl methacrylate, and polytetrafluoroethylene.

[0041] The weight-average molecular weight of the binder and the content of the binder in the solid electrolyte particle membrane are not particularly limited, but the liquid absorption rate W of the partition walls 12, which will be described later, can be adjusted by adjusting the weight-average molecular weight and the content.

[0042] The fibrous substance is a plurality of fibrous materials and includes one or more of the following fibrous materials. The fibrous material preferably includes one or more of the following hydrophilic functional groups, such as hydroxyl groups, sulfonic groups, and carboxyl groups. Specific examples of the fibrous material include cellulose fibers, polysaccharides, polyvinyl alcohol, polyacrylic acid, anionic derivatives of polystyrene, and cationic derivatives of polystyrene. An example of an anionic derivative of polystyrene is polystyrene sulfonate, and an example of a cationic derivative of polystyrene is polystyrene trialkylbenzylammonium.

[0043] Among these, the fibrous material preferably contains cellulose fibers. However, specific examples of the fibrous material may be derivatives of the above-mentioned series of specific examples, or may be copolymers of two or more of the above-mentioned series of specific examples.

[0044] As described above, the fibrous material contains a hydrophilic functional group, and therefore each of the anode electrolyte 15 and the cathode electrolyte 16 is easily trapped between two or more strands of the fibrous material. The average fiber diameter of the fibrous material and the content of the fibrous material in the solid electrolyte particle membrane are not particularly limited. However, by adjusting the average fiber diameter, the content, and the like, the liquid absorption rate W of the partition wall 12, which will be described later, can be adjusted.

[0045] To form a solid electrolyte particle membrane, first, solid electrolyte particles, a binder, and a fibrous material are introduced into a solvent such as an organic solvent to prepare a slurry. The slurry is then poured into a mold. Finally, the slurry is dried to volatilize the solvent, and the mold is then removed. This results in a solid electrolyte particle membrane containing solid electrolyte particles, a binder, and a fibrous material.

[0046] [Negative Electrode] The negative electrode 13 absorbs and releases alkali metal ions that pass through the partition wall 12. The negative electrode 13 is disposed inside the negative electrode chamber S1 and is therefore immersed in the negative electrode electrolyte 15.

[0047] Specifically, the negative electrode 13 includes a negative electrode current collector 13A and a negative electrode active material layer 13B. However, the negative electrode current collector 13A may be omitted.

[0048] The negative electrode current collector 13A is a conductive support member that supports the negative electrode active material layer 13B and has a pair of surfaces on which the negative electrode active material layer 13B is provided. The negative electrode current collector 13A contains one or more conductive materials such as a metal material, a carbon material, and a conductive ceramic material. Here, the negative electrode current collector 13A includes a connection terminal portion 13AT, which is one end portion of the negative electrode current collector 13A, and the connection terminal portion 13AT is extended to the outside of the exterior member 11.

[0049] Specific examples of metal materials include stainless steel (SUS), titanium, zinc, tin, lead, etc., and alloys of one or more of these may also be used. The stainless steel may be highly corrosion-resistant stainless steel to which one or more of additive elements such as niobium and molybdenum have been added. Specifically, the stainless steel may be SUS444 to which molybdenum has been added. A specific example of a conductive ceramic material is indium tin oxide (ITO).

[0050] This negative electrode current collector 13A is preferably insoluble or hardly soluble in the negative electrode electrolyte 15 and corrosion-resistant, and also has low reactivity with the negative electrode active material described below. For this reason, the negative electrode current collector 13A preferably contains the above-mentioned metal material, because this makes the negative electrode current collector 13A less susceptible to deterioration during operation of the secondary battery.

[0051] The negative electrode current collector 13A may be a conductor whose surface is coated with one or more of the above-mentioned conductive materials. The material of this conductor is not particularly limited as long as it is conductive. When the conductive material contains a metal material, the surface of the conductor may be plated with the metal material.

[0052] Here, the negative electrode active material layer 13B is provided on both sides of the negative electrode current collector 13A, but the negative electrode active material layer 13B may be provided on only one side of the negative electrode current collector 13A.

[0053] The negative electrode active material layer 13B contains one or more types of negative electrode active materials that absorb and release alkali metal ions, but may also contain other materials such as a negative electrode binder and a negative electrode conductive agent.

[0054] The negative electrode active material is a titanium-containing compound, a niobium-containing compound, a vanadium-containing compound, an iron-containing compound, a molybdenum-containing compound, etc. This is because the charge / discharge reaction tends to proceed smoothly and stably even when two types of aqueous electrolytes (negative electrode electrolyte 15 and positive electrode electrolyte 16) are used.

[0055] The titanium-containing compound includes titanium oxide, alkali metal titanium composite oxide, titanium phosphate, alkali metal titanium phosphate compound, and hydrogen titanium compound.

[0056] The titanium oxide is a compound represented by formula (3), that is, bronze-type titanium oxide, etc.

[0057] TiO w　 ... (3) (W satisfies 1.85≦W≦2.15.)

[0058] Specific examples of titanium oxide include anatase, rutile, or brookite titanium oxide (TiO 2　 However, the titanium oxide may be a composite oxide containing one or more of phosphorus, vanadium, tin, copper, nickel, iron, cobalt, etc. as constituent elements in addition to titanium. Specific examples of composite oxides include TiO 2　 -P 2　 O 5　 , TiO 2　 -V 2　 O 5　 , TiO 2　 -P 2　 O 5　 -SnO 2　 and TiO 2　 -P 2　 O 5　 -MeO, etc., where Me is one or more of copper, nickel, iron, cobalt, etc.

[0059] Among alkali metal titanium composite oxides, lithium titanium composite oxides include compounds represented by formulas (4) to (6), such as ramsdellite-type lithium titanate. M2 in formula (4) is a metal element that can become a divalent ion. M3 in formula (5) is a metal element that can become a trivalent ion. M4 in formula (6) is a metal element that can become a tetravalent ion.

[0060] Li [Li x　 M2 (1-3x) / 2 Ti (3+x) / 2　 ]O 4　 ... (4) (M2 is at least one of Mg, Ca, Cu, Zn, and Sr. x satisfies 0≦x≦1 / 3.)

[0061] Li [Li y　 M3 1-3y Ti 1+2y ]O 4　 ... (5) (M3 is at least one of Al, Sc, Cr, Mn, Fe, Ge, and Y, and y satisfies 0≦y≦1 / 3.)

[0062] Li [Li 1 / 3　 M4 z　 Ti (5 / 3)-z　 ]O 4　 ... (6) (M4 is at least one of V, Zr, and Nb. z satisfies 0≦z≦2 / 3.)

[0063] Specific examples of the lithium titanium composite oxide shown in formula (4) include Li 3.75 Ti 4.875　 Mg 0.375　 O 12 A specific example of the lithium titanium composite oxide shown in formula (5) is LiCrTiO 4　 Specific examples of the lithium titanium composite oxide shown in formula (6) include Li 4　 Ti 5　 O 12 and Li 4　 Ti 4.95 Nb 0.05 O 12 And so on.

[0064] Specific examples of potassium titanium composite oxides among alkali metal titanium composite oxides include K 2　 Ti 3　 O 7　 and K. 4　 Ti 5　 O 12 And so on.

[0065] A specific example of titanium phosphate is titanium phosphate (TiP 2　 O 7　 ) and the like. Specific examples of lithium titanium phosphate compounds among alkali metal titanium phosphate compounds include LiTi 2　 (P.O. 4　 ) 3　 Specific examples of sodium titanium phosphate compounds among alkali metal titanium phosphate compounds include NaTi 2　 (P.O. 4　 ) 3　 Specific examples of hydrogen titanium compounds include H 2　 Ti 3　 O 7　 (3TiO 2　 ・1H 2　 O), H 6　 Ti 12 O 27 (3TiO 2　 ・0.75H 2　 O), H 2　 Ti 6　 O 13 (3TiO 2　 ・0.5H 2　 O), H 2　 Ti 7　 O 15 (3TiO 2　 ・0.43H 2　 O) and H 2　 Ti 12 O 25 (3TiO 2　 ・0.25H 2　 O) etc.

[0066] Niobium-containing compounds include alkali metal niobium composite oxides, hydrogen niobium compounds, and titanium niobium composite oxides, although materials that fall under the category of niobium-containing compounds are excluded from titanium-containing compounds.

[0067] A specific example of the alkali metal niobium composite oxide is LiNbO 2　 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 However, the titanium-niobium composite oxide may be intercalated with an alkali metal.

[0068] The vanadium-containing compounds include vanadium oxides and alkali metal vanadium composite oxides, etc. However, materials that fall under the category of vanadium-containing compounds are excluded from both titanium-containing compounds and niobium-containing compounds.

[0069] A specific example of vanadium oxide is vanadium dioxide (VO 2　 ) and the like. Specific examples of alkali metal vanadium composite oxides include LiV 2　 O 4　 and LiV 3　 O 8　 And so on.

[0070] The iron-containing compound is iron hydroxide, etc. However, materials that fall under the category of iron-containing compounds are excluded from each of titanium-containing compounds, niobium-containing compounds, and vanadium-containing compounds.

[0071] A specific example of iron hydroxide is iron oxyhydroxide (FeOOH), etc. However, the iron oxyhydroxide may be α-iron oxyhydroxide, β-iron oxyhydroxide, γ-iron oxyhydroxide, δ-iron oxyhydroxide, or any two or more of them.

[0072] The molybdenum-containing compounds include molybdenum oxides and cobalt-molybdenum composite oxides, etc. However, materials that fall under the category of molybdenum-containing compounds are excluded from the titanium-containing compounds, niobium-containing compounds, vanadium-containing compounds, and iron-containing compounds.

[0073] Specific examples of molybdenum oxide include molybdenum dioxide (MoO2　 ) and the like. A specific example of the cobalt-molybdenum composite oxide is CoMoO 4　 And so on.

[0074] The negative electrode binder contains one or more of materials such as synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber. Specific examples of polymer compounds include polyvinylidene fluoride and polyimide.

[0075] The negative electrode conductive agent contains one or more conductive materials such as a carbon material, a metal material, a conductive ceramic material, and a conductive polymer, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black.

[0076] [Positive Electrode] The positive electrode 14 absorbs and releases alkali metal ions that pass through the partition wall 12. The positive electrode 14 is disposed inside the positive electrode chamber S2 and is therefore immersed in the positive electrode electrolyte 16.

[0077] Specifically, the positive electrode 14 includes a positive electrode current collector 14A and a positive electrode active material layer 14B. However, the positive electrode current collector 14A may be omitted.

[0078] The positive electrode current collector 14A is a conductive support member that supports the positive electrode active material layer 14B and has a pair of surfaces on which the positive electrode active material layer 14B is provided. The positive electrode current collector 14A includes one or more conductive materials such as a metal material, a carbon material, and a conductive ceramic material. Here, the positive electrode current collector 14A includes a connection terminal portion 14AT, which is one end portion of the positive electrode current collector 14A, and the connection terminal portion 14AT is extended to the outside of the exterior member 11. The extension direction of the connection terminal portion 14AT is the same as the extension direction of the connection terminal portion 13AT.

[0079] Specific examples of the metal material include titanium, aluminum, and alloys thereof. Details regarding the conductive ceramic material are as described above.

[0080] This positive electrode current collector 14A is preferably insoluble or hardly soluble in the positive electrode electrolyte 16 and corrosion-resistant, and also has low reactivity with the positive electrode active material described below. For this reason, the positive electrode current collector 14A preferably contains the above-mentioned metal material, because this makes the positive electrode current collector 14A less susceptible to deterioration during operation of the secondary battery.

[0081] The positive electrode current collector 14A may be a conductor whose surface is coated with one or more of the above-mentioned conductive materials. The material of this conductor is not particularly limited as long as it is conductive. When the conductive material contains a metal material, the surface of the conductor may be plated with the metal material.

[0082] Here, the positive electrode active material layer 14B is provided on both sides of the positive electrode current collector 14A, but the positive electrode active material layer 14B may be provided on only one side of the positive electrode current collector 14A.

[0083] The positive electrode active material layer 14B contains one or more positive electrode active materials that absorb and release alkali metal ions, but may also contain other materials such as a positive electrode binder and a positive electrode conductive agent.

[0084] The positive electrode active material that absorbs and releases lithium ions as alkali metal ions includes a lithium-containing compound. The type of lithium-containing compound is not particularly limited, but specific examples include lithium composite oxides and lithium phosphate compounds. The lithium composite oxide is an oxide containing lithium and one or more transition metal elements as constituent elements, and the lithium phosphate compound is a phosphate compound containing lithium and one or more transition metal elements as constituent elements. The type of transition metal element is not particularly limited, but specific examples include nickel, cobalt, manganese, and iron.

[0085] A specific example of the layered rock salt type lithium composite oxide is LiNiO 2　 , LiCoO 2　 , LiCo 0.98 Al 0.01 Mg 0.01 O2　 , 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 spinel-type lithium composite oxides include LiMn 2　 O 4　 Specific examples of olivine-type lithium phosphate compounds include LiFePO 4　 , LiMnPO 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.

[0086] The positive electrode active material that absorbs and releases sodium ions as alkali metal ions includes a sodium-containing compound, etc. The type of the sodium-containing compound is not particularly limited, but specifically includes a Prussian blue analogue represented by formula (7).

[0087] Na x　 K y　 M5 z　 Fe(CN) 6　 ・aH 2　 O (7) (M5 is at least one of Mn and Zn. x, y, and z satisfy 0.5<x≦2, 0≦y≦0.5, and 0≦z≦2. a is an arbitrary value. However, y may satisfy 0.05≦y≦0.2.)

[0088] A specific example of the Prussian blue analogue shown in formula (7) is Na 2　 MnFe(CN 6　 ), Na 1.42 K 0.09 Mn 1.13 Fe(CN) 6　 ・3H 2　 O and Na 0.83 K 0.12 Zn 1.49 Fe(CN) 6　 ・3.2H 2　 O, etc.

[0089] The positive electrode active material that absorbs and releases potassium ions as alkali metal ions includes potassium-containing compounds. Specific examples of potassium-containing compounds include K 0.7　 Fe 0.6　 Mn 0.6　 O 2　 , K. 0.6　 MnO 2　 , K. 0.3　 MnO 2　 , K. 0.31 CoO 2　 , KCrO 2　 , K. 0.6　 CoO 2　 , K. 2 / 3　 Mn 2 / 3　 Co 1 / 3　 Ni 1 / 3　 O 2　 , K. 2 / 3　 Ni 2 / 3　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 6　 Co 1 / 2　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 2　 Mn 1 / 6　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 2　 Cu 1 / 6　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 3　 Zn 1 / 3　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 6　 Mg1 / 2　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 2　 Co 1 / 6　 Te 1 / 3　 O 2　 , K. 2 / 3　 Ni 1 / 3　 Mg 1 / 3　 Te 1 / 3　 O 2　 and K. 2 / 3　 Ni 1 / 3　 Co 1 / 3　 Te 1 / 3　 O 2　 And so on.

[0090] The details regarding the positive electrode binder are the same as those regarding the negative electrode binder, and the details regarding the positive electrode conductive agent are the same as those regarding the negative electrode conductive agent.

[0091] [Negative Electrolyte] The negative electrode electrolyte 15 is accommodated in the negative electrode chamber S1. As a result, the negative electrode electrolyte 15 is separated from the positive electrode electrolyte 16 accommodated in the positive electrode chamber S2 via the partition wall 12.

[0092] Specifically, the negative electrode electrolyte 15 contains an aqueous solvent and an ionic substance. The ionic substance is a substance that ionizes in the aqueous solvent and contains alkali metal ions.

[0093] (Aqueous Solvent) The type of aqueous solvent is not particularly limited, but specifically, it is one or more of pure water and the like.

[0094] (Ionic Substance) The type of ionic substance is not particularly limited, but specifically, it is any one or two or more types of electrolyte salts. However, the ionic substance may further contain any one or two or more types of acids and bases. Specific examples of acids include carbonic acid, oxalic acid, nitric acid, sulfuric acid, hydrochloric acid, acetic acid, and citric acid.

[0095] The electrolyte salt is a salt containing a cation and an anion. More specifically, the electrolyte salt contains, as cations, alkali metal ions that are absorbed and released in the negative electrode 13 and the positive electrode 14, and therefore contains one or more types of alkali metal salts that have these alkali metal ions as cations. This is because a high voltage can be obtained.

[0096] Hereinafter, the alkali metal ions absorbed and released in the negative electrode 13 and the positive electrode 14 will also be simply referred to as "alkali metal ions."

[0097] The types of alkali metal ions absorbed and released in the negative electrode 13 and the positive electrode 14 are not particularly limited, but specific examples include lithium ions, sodium ions, and potassium ions.

[0098] Alkali metal salts having the alkali metal ions absorbed and released in the negative electrode 13 and the positive electrode 14 as cations include lithium salts, sodium salts, and potassium salts. Specific examples of lithium salts include lithium carbonate, lithium oxalate, lithium nitrate, lithium sulfate, lithium chloride, lithium acetate, lithium citrate, lithium hydroxide, and imide salts. Examples of imide salts include lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide. Specific examples of sodium salts include compounds in which the lithium ions of the above-mentioned specific lithium salts are substituted with sodium ions. Specific examples of potassium salts include compounds in which the lithium ions of the above-mentioned specific lithium salts are substituted with potassium ions.

[0099] In addition, since the electrolyte salt further contains one or more types of alkali metal ions that are not absorbed or released in the negative electrode 13 and the positive electrode 14, it may also contain one or more types of alkali metal salts that have these alkali metal ions as cations.

[0100] Specific examples of alkali metal ions that are not occluded or released in the negative electrode 13 and the positive electrode 14 are the same as the specific examples of alkali metal ions that are occluded or released in the negative electrode 13 and the positive electrode 14. However, the types of alkali metal ions that are not occluded or released in the negative electrode 13 and the positive electrode 14 are different from the types of alkali metal ions that are occluded or released in the negative electrode 13 and the positive electrode 14.

[0101] Specific examples of alkali metal salts having as their cations alkali metal ions that are not occluded or released in the negative electrode 13 and the positive electrode 14 are the same as the specific examples of alkali metal salts having as their cations alkali metal ions that are occluded or released in the negative electrode 13 and the positive electrode 14. However, the types of alkali metal salts having as their cations alkali metal ions that are not occluded or released in the negative electrode 13 and the positive electrode 14 are different from the types of alkali metal salts having as their cations alkali metal ions that are occluded or released in the negative electrode 13 and the positive electrode 14.

[0102] Furthermore, the electrolyte salt further contains one or more types of other metal ions different from the alkali metal ions that are absorbed and released in the negative electrode 13 and the positive electrode 14, respectively, and the alkali metal ions that are not absorbed and released in the negative electrode 13 and the positive electrode 14, respectively, and therefore may contain one or more types of other metal salts having the other metal ions as cations.

[0103] The other metal ions are alkaline earth metal ions, transition metal ions, and other metal ions. Specific examples of alkaline earth metal ions include magnesium ions and calcium ions. Specific examples of transition metal ions include titanium ions, vanadium ions, iron ions, manganese ions, nickel ions, cobalt ions, and copper ions. Specific examples of other metal ions include aluminum ions and zinc ions.

[0104] Specific examples of other metal salts having other metal ions as cations include compounds in which the lithium ions of the specific examples of lithium salts mentioned above are substituted with alkaline earth metal ions, transition metal ions, or other metal ions.

[0105] In particular, the electrolyte salt preferably contains alkali metal ions that are occluded and released in the negative electrode 13 and the positive electrode 14, respectively, as well as alkali metal ions that are not occluded and released in the negative electrode 13 and the positive electrode 14. This preferably contains one or more types of alkali metal salts having, as cations, alkali metal ions that are occluded and released in the negative electrode 13 and the positive electrode 14, respectively, as well as one or more types of alkali metal salts having, as cations, alkali metal ions that are not occluded and released in the negative electrode 13 and the positive electrode 14. This is because, as will be described later, the pH of the negative electrode electrolyte 15 is more easily controlled to be sufficiently higher than the pH of the positive electrode electrolyte 16. This makes it easier to maintain the relationship between the pH of the negative electrode electrolyte 15 and the pH of the positive electrode electrolyte 16.

[0106] In this case, the alkali metal ions absorbed and released in each of the negative electrode 13 and the positive electrode 14 preferably contain lithium ions, because this allows a sufficiently high voltage to be obtained and also allows a high energy density to be obtained.

[0107] Furthermore, when the alkali metal ions absorbed and released in each of anode 13 and cathode 14 contain lithium ions, the alkali metal ions not absorbed and released in each of anode 13 and cathode 14 preferably contain potassium ions. That is, the electrolyte salt preferably contains potassium ions as cations in addition to lithium ions. This is because the above-described relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 is more easily maintained.

[0108] The electrolyte salt may further contain one or more non-electrolytes.

[0109] The type of anion is not particularly limited. In particular, the anion preferably includes a hydroxide ion. This is because fluctuations in the pH of anode electrolyte 15 are sufficiently suppressed, and the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16, which will be described later, is more likely to be sufficiently maintained.

[0110] The composition of the negative electrode electrolyte 15 (type of aqueous solvent and type of electrolyte salt) and the composition of the positive electrode electrolyte 16 (type of aqueous solvent and type of electrolyte salt) may be the same as or different from each other.

[0111] The content of the ionic substance in the anode electrolyte 15, i.e., the concentration (mol / kg) of the anode electrolyte 15, is not particularly limited and can be set arbitrarily.

[0112] (pH) The negative electrode electrolyte 15 has a pH higher than the pH of the positive electrode electrolyte 16 .

[0113] The reason why anode electrolyte 15 has a higher pH than cathode electrolyte 16 is that the decomposition potential of the aqueous solvent shifts due to the difference in pH between the two, compared to when anode electrolyte 15 has a pH equal to or lower than the pH of cathode electrolyte 16. As a result, during charge and discharge, the decomposition reaction of the aqueous solvent is thermodynamically suppressed and the potential window of the aqueous solvent is expanded. Therefore, a high voltage is obtained and the charge and discharge reaction utilizing the absorption and release of alkali metal ions proceeds sufficiently and stably.

[0114] In particular, it is preferable that the composition (type of electrolyte salt) of anode electrolyte 15 and the composition (type of electrolyte salt) of cathode electrolyte 16 are different from each other. This is because the pH of anode electrolyte 15 can be more easily controlled to be higher than the pH of cathode electrolyte 16.

[0115] The pH of anode electrolyte 15 is not particularly limited as long as anode electrolyte 15 has a higher pH than the pH of cathode electrolyte 16. In particular, the pH of anode electrolyte 15 is preferably 11 or higher, more preferably 12 or higher, and even more preferably 13 or higher. This is because the pH of anode electrolyte 15 becomes sufficiently high, and therefore the pH of anode electrolyte 15 is likely to become higher than the pH of cathode electrolyte 16. In addition, the difference between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 becomes sufficiently large, and therefore the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 is easily maintained.

[0116] The pH of the negative electrode electrolyte 15 is preferably set so as to make the negative electrode current collector 13A and the negative electrode active material layer 13B less likely to corrode, because this makes it easier for the charge / discharge reaction using the negative electrode 13 to proceed stably and continuously.

[0117] (Preferred Configuration) Note that, it is preferable that negative electrode electrolyte 15 contains a saturated solution of an electrolyte salt, that is, it contains a saturated solution of an alkali metal salt in which the alkali metal ions absorbed and released in each of negative electrode 13 and positive electrode 14 are used as cations. This is because the alkali metal ion absorption and release reaction tends to proceed stably during charge and discharge, and thus the charge and discharge reaction tends to proceed more easily.

[0118] To confirm whether the anode electrolyte 15 is a saturated solution of electrolyte salt, the secondary battery is disassembled, and then the inside of the anode chamber S1 is visually inspected for the presence of precipitated electrolyte salt. The inside of the anode chamber S1 specifically refers to the liquid anode electrolyte 15, the surface of the partition wall 12, the surface of the anode 13, and the inner wall surface of the exterior member 11. When the anode electrolyte 15 (liquid) and a precipitate (solid) of electrolyte salt coexist inside the anode chamber S1 due to the precipitation of electrolyte salt, the anode electrolyte 15 is considered to be a saturated solution of electrolyte salt. To examine 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.

[0119] Furthermore, anode electrolyte 15 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 an aqueous solution in which a weak base and its conjugate acid are mixed, or both. This is because pH fluctuations are sufficiently suppressed, making it easier to maintain the pH of anode electrolyte 15.

[0120] The negative electrode electrolyte 15 may further contain one or more buffers, such as trishydroxymethylaminomethane and ethylenediaminetetraacetic acid.

[0121] Furthermore, anode electrolyte 15 may be an isotonic solution that is isotonic with cathode electrolyte 16. This is because the osmotic pressure of anode electrolyte 15 is optimized, making it easier to maintain the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16.

[0122] [Positive Electrolyte] The positive electrode electrolyte 16 is accommodated in the positive electrode chamber S2. As a result, the positive electrode electrolyte 16 is separated from the negative electrode electrolyte 15 accommodated in the negative electrode chamber S1 via the partition wall 12.

[0123] The configuration of the positive electrode electrolyte 16 is similar to the configuration of the negative electrode electrolyte 15 described above, except as described below. That is, the details regarding the aqueous solvent and the ionic substance are as described above.

[0124] The type of anion is not particularly limited. Among them, the anion preferably includes one or more of nitrate ions, 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 16 are sufficiently suppressed, making it easier to sufficiently maintain the relationship between the pH of the negative electrode electrolyte 15 and the pH of the positive electrode electrolyte 16, which will be described later. Specific examples of carboxylate ions include formate ions, acetate ions, propionate ions, tartrate ions, and citrate ions.

[0125] In particular, it is more preferable that the anions include one or more of nitrate ions, sulfate ions, hydrogensulfate ions, carbonate ions, hydrogencarbonate ions, phosphate ions, monohydrogenphosphate ions, and dihydrogenphosphate ions, because this further suppresses fluctuations in the pH of positive electrode electrolyte 16, making it easier to maintain the relationship between the pH of negative electrode electrolyte 15 and the pH of positive electrode electrolyte 16, which will be described later.

[0126] The content of the ionic substance in the positive electrode electrolyte 16, that is, the concentration (mol / kg) of the positive electrode electrolyte 16, is not particularly limited and can be set arbitrarily.

[0127] As described above, the negative electrode electrolyte 15 has a higher pH than the positive electrode electrolyte 16 , and therefore the positive electrode electrolyte 16 has a lower pH than the negative electrode electrolyte 15 .

[0128] The composition (type of electrolyte salt) of cathode electrolyte 16 and the composition (type of electrolyte salt) of anode electrolyte 15 are preferably different from each other. This is because the pH of cathode electrolyte 16 can be more easily controlled to be lower than the pH of anode electrolyte 15.

[0129] The pH of positive electrode electrolyte 16 is not particularly limited as long as it has a lower pH than the pH of negative electrode electrolyte 15. In particular, the pH of positive electrode electrolyte 16 is preferably 3 to 8, more preferably 4 to 8, and even more preferably 4 to 6. This is because the pH of positive electrode electrolyte 16 becomes sufficiently low, so that the pH of positive electrode electrolyte 16 is likely to become lower than the pH of negative electrode electrolyte 15. Furthermore, the difference between the pH of positive electrode electrolyte 16 and the pH of negative electrode electrolyte 15 becomes sufficiently large, so that the relationship between the pH of positive electrode electrolyte 16 and the pH of negative electrode electrolyte 15 is easily maintained. Furthermore, corrosion of exterior member 11, negative electrode current collector 13A, positive electrode current collector 14A, and the like is suppressed.

[0130] The pH of the positive electrode electrolyte 16 is preferably set so as to make it difficult for the positive electrode current collector 14A and the positive electrode active material layer 14B to corrode, because this makes it easier for the charge / discharge reaction using the positive electrode 14 to proceed stably and continuously.

[0131] Positive electrode electrolyte 16 is preferably a saturated solution of an electrolyte salt, that is, a saturated solution of an alkali metal salt having, as cations, alkali metal ions absorbed and released in negative electrode 13 and positive electrode 14. This is because the alkali metal ion absorption and release reaction tends to proceed stably during charge and discharge, and therefore the charge and discharge reaction tends to proceed easily.

[0132] The method for checking whether positive electrode electrolyte 16 is a saturated solution of electrolyte salt is similar to the method for checking whether negative electrode electrolyte 15 is a saturated solution of electrolyte salt described above, except that positive electrode chamber S2 is checked instead of negative electrode chamber S1.

[0133] Furthermore, the positive electrode electrolyte 16 may be a pH buffer solution, because the pH fluctuation is sufficiently suppressed, and the pH of the positive electrode electrolyte 16 is easily maintained.

[0134] The positive electrode electrolyte 16 may further contain one or more buffers, the details of which are as described above.

[0135] Furthermore, positive electrode electrolyte 16 may be an isotonic solution that is isotonic with negative electrode electrolyte 15. This is because the osmotic pressure of positive electrode electrolyte 16 is optimized, making it easier to maintain the relationship between the pH of positive electrode electrolyte 16 and the pH of negative electrode electrolyte 15.

[0136] <1-2. Physical Properties> As described above, in this secondary battery, the physical properties of the partition walls 12 are optimized to improve the battery characteristics.

[0137] [Physical Property Conditions] The physical properties of the partition walls 12 satisfy the following predetermined conditions.

[0138] Specifically, the partition walls 12 have a liquid absorption rate W calculated by formula (1), and the liquid absorption rate W satisfies the condition (hereinafter referred to as the “physical property condition”) expressed by formula (2).

[0139] W=(W1−W0) / W0 (1) (W is the liquid absorption rate. W0 is the weight of partition wall 12 in a state where both anode electrolyte 15 and cathode electrolyte 16 have been removed. W1 is the weight of partition wall 12 in a state where both anode electrolyte 15 and cathode electrolyte 16 have been impregnated.)

[0140] -0.1<W<0.1...(2)

[0141] The liquid absorption rate W is an index representing the extent to which partition wall 12 is impregnated with each of anode electrolyte 15 and cathode electrolyte 16 when partition wall 12 is disposed between anode electrolyte 15 and cathode electrolyte 16 and thereby comes into contact with each of anode electrolyte 15 and cathode electrolyte 16.

[0142] Weight W0 is the weight (g) of partition wall 12 measured in a state in which partition wall 12 recovered from a secondary battery is dried under predetermined conditions to remove anode electrolyte 15 and cathode electrolyte 16 impregnated in partition wall 12, as will be described later.

[0143] As will be described later, weight W1 is the weight (g) of partition wall 12 that is measured in a state in which partition wall 12 is impregnated with anode electrolyte 15 and cathode electrolyte 16 by performing a simple treatment under predetermined conditions on partition wall 12 recovered from a secondary battery.

[0144] The details of the procedure for calculating the liquid absorption rate W, including the procedure for measuring the weights W0 and W1, will be described later.

[0145] As is clear from the formula (2), the value of the liquid absorption rate W may be a positive value, a negative value, or zero.

[0146] Specifically, when partition wall 12 is disposed between anode electrolyte 15 and cathode electrolyte 16 inside the secondary battery, partition wall 12 is impregnated with anode electrolyte 15 and cathode electrolyte 16, respectively.

[0147] In this case, when weight W0 is used as a reference when partition wall 12 is recovered from the secondary battery and then anode electrolyte 15 and cathode electrolyte 16 are each removed from partition wall 12, weight W1 is naturally greater than weight W0. As a result, when liquid absorption rate W is calculated based on formula (1), the value of liquid absorption rate W becomes a positive value.

[0148] In contrast, there is a case where partition wall 12 is pre-impregnated with water before partition wall 12 is disposed between anode electrolyte 15 and cathode electrolyte 16. This water pre-impregnated in partition wall 12 is moisture in the air or the like, and is impregnated in partition wall 12 in an environment of a temperature of 23°C and a humidity of 50%.

[0149] In this case, when partition wall 12 is disposed between anode electrolyte 15 and cathode electrolyte 16, partition wall 12 is impregnated with anode electrolyte 15 and cathode electrolyte 16, respectively, and water that has been impregnated in partition wall 12 in advance is released into anode electrolyte 15 and cathode electrolyte 16, respectively.

[0150] As a result, when the total amount of water released from partition wall 12 into anode electrolyte 15 and cathode electrolyte 16 becomes larger than the total amount of anode electrolyte 15 and cathode electrolyte 16 impregnated in partition wall 12, weight W1 becomes smaller than weight W0. As a result, when the liquid absorption rate W is calculated based on formula (1), the value of the liquid absorption rate W becomes a negative value.

[0151] Note that when the total amount of anode electrolyte 15 and cathode electrolyte 16 impregnated in partition wall 12 is equal to the total amount of water released from partition wall 12 into anode electrolyte 15 and cathode electrolyte 16, weight W1 and weight W0 become equal. As a result, when the liquid absorption rate W is calculated based on formula (1), the value of the liquid absorption rate W becomes 0.

[0152] The liquid absorption rate W changes depending on the configuration of partition wall 12 and the respective configurations of anode electrolyte 15 and cathode electrolyte 16 impregnated in partition wall 12. In this way, the liquid absorption rate W can be adjusted depending on the configuration of partition wall 12 and the respective configurations of anode electrolyte 15 and cathode electrolyte 16 so that physical property conditions are satisfied.

[0153] The configuration of partition wall 12 used to adjust the liquid absorption rate W is determined by the type and number of anionic groups, etc. The configuration of each of anode electrolyte 15 and cathode electrolyte 16 used to adjust the liquid absorption rate W is determined by the type and concentration of an ionic substance (electrolyte salt), etc., and the type of the ionic substance is determined by the type of alkali metal ion (cation) and the type of anion, etc.

[0154] The reason why the physical property conditions regarding the physical property (liquid absorption rate W) of the partition walls 12 are satisfied is that the charge / discharge reaction using alkali metal ions tends to proceed sufficiently and smoothly during charge / discharge.

[0155] Specifically, when the liquid absorption rate W is greater than 0.1, the absolute value of the liquid absorption rate W becomes too large. In this case, the aqueous solvent that accounts for a large portion of each of the negative electrode electrolyte 15 and the positive electrode electrolyte 16 is likely to excessively impregnate the partition wall 12, and the aqueous solvent is likely to inhibit the permeation of alkali metal ions through the partition wall 12 during charge and discharge. This reduces the permeation efficiency of the alkali metal ions that permeate the partition wall 12, making it difficult for the charge and discharge reaction using the alkali metal ions to proceed during charge and discharge.

[0156] On the other hand, when the liquid absorption rate W is smaller than −0.1, the absolute value of the liquid absorption rate W becomes too large. In this case, the amount of the aqueous solvent impregnated into the partition walls 12 is significantly reduced, and the function of the ion exchange groups in the partition walls 12 is substantially reduced. This reduces the permeation efficiency of alkali metal ions passing through the partition walls 12, making it difficult for the charge / discharge reaction using alkali metal ions to proceed during charge / discharge.

[0157] In contrast, when the liquid absorption rate W satisfies the physical property condition (−0.1<W<0.1), the absolute value of the liquid absorption rate W becomes sufficiently small. In this case, an appropriate amount of the aqueous solvent contained in the negative electrode electrolyte 15 permeates into the partition wall 12, and an appropriate amount of the aqueous solvent contained in the positive electrode electrolyte 16 permeates into the partition wall 12. This makes it less likely that the aqueous solvent will inhibit the permeation of alkali metal ions through the partition wall 12 during charge and discharge, and improves the ion exchange performance of the partition wall 12. Therefore, the permeation efficiency of alkali metal ions permeating through the partition wall 12 is improved, which facilitates the progress of charge and discharge reactions using alkali metal ions during charge and discharge, as described above.

[0158] In particular, in a secondary battery using two types of aqueous electrolyte solutions (negative electrode electrolyte solution 15 and positive electrode electrolyte solution 16), the liquid absorption rate W satisfies the physical property conditions, and therefore the permeation efficiency of alkali metal ions is sufficiently improved, making it easier for charge / discharge reactions to proceed smoothly and stably.

[0159] [Suitable configuration of partition walls] The partition walls 12 preferably include a cation exchange membrane rather than a solid electrolyte membrane. This is because an appropriate amount of aqueous solvent can easily permeate the partition walls 12, thereby improving ionic conductivity inside the partition walls 12 and making it easier for the physical property conditions regarding the physical property (liquid absorption rate W) of the partition walls 12 to be satisfied.

[0160] When the partition walls 12 include a cation exchange membrane, the ion exchange capacity of the cation exchange membrane is not particularly limited. In particular, the ion exchange capacity of the cation exchange membrane is preferably 0.5 meq / g or more, and more preferably 1.0 meq / g or more. This is because the permeation efficiency of alkali metal ions passing through the partition walls 12 is further improved, and thus the charge / discharge reaction using alkali metal ions is more likely to proceed during charge / discharge.

[0161] [Configuration of Suitable Ionic Substance (Electrolyte Salt)] As described above, it is preferable that one or both of the anode electrolyte 15 and the cathode electrolyte 16 contain lithium ions as alkali metal ions to be absorbed and released in the anode 13 and the cathode 14, respectively. This is because, as described above, a sufficiently high voltage and a high energy density can be obtained.

[0162] In this case, as described above, it is more preferable that one or both of anode electrolyte 15 and cathode electrolyte 16 further contain potassium ions as alkali metal ions that are not occluded or released in anode 13 and cathode 14. As described above, this is because the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 is more easily maintained, and the physical property condition regarding the physical property (liquid absorption rate W) of partition wall 12 is more easily satisfied.

[0163] [Calculation Procedure for Liquid Absorption Rate] The calculation procedure for the liquid absorption rate W, including the procedures for measuring the weights W0 and W1, is as follows. The series of procedures described below is performed in a normal temperature and normal humidity environment (temperature = 23°C, humidity = 50%). In this case, environmental testing equipment such as a constant temperature and humidity bath and a constant temperature and humidity room may be used to perform the series of procedures in the normal temperature and normal humidity environment.

[0164] First, to put the secondary battery into a discharged state, the secondary battery is discharged until the potential reaches a predetermined discharge cutoff potential. A specific example of the discharge cutoff potential is 1.5 V. The current during discharge is not particularly limited and can be set arbitrarily.

[0165] Next, the secondary battery is disassembled to recover partition wall 12. This partition wall 12 was in contact with both anode electrolyte 15 and cathode electrolyte 16 inside the secondary battery, and therefore is impregnated with both anode electrolyte 15 and cathode electrolyte 16.

[0166] (Simple Treatment) Subsequently, the partition wall 12 is subjected to a simple treatment to remove excess anode electrolyte 15 and excess cathode electrolyte 16 from the partition wall 12 .

[0167] In this simple treatment, a liquid-absorbing waste cloth is laid on a workbench. Kimwipes manufactured by Nippon Paper Crecia Co., Ltd., or the like can be used as this waste cloth. Then, the partition wall 12 is placed on the waste cloth and left as it is. As a result, droplets of anode electrolyte 15 and cathode electrolyte 16 that have not impregnated the interior of the partition wall 12 and are attached to the surface of the partition wall 12 are absorbed by the waste cloth. Thus, excess anode electrolyte 15 and excess cathode electrolyte 16 are removed from the partition wall 12.

[0168] Note that the time for which partition wall 12 is placed on the waste cloth (so-called leaving time) is preferably sufficiently short. If the leaving time is too long, not only the excess anode electrolyte 15 and cathode electrolyte 16 attached to the surface of partition wall 12 but also the anode electrolyte 15 and cathode electrolyte 16 impregnated in partition wall 12 are removed. A specific example of the leaving time is one minute.

[0169] Here, the above-described "anode electrolyte 15 and cathode electrolyte 16 impregnated in partition 12" refer to the anode electrolyte 15 and cathode electrolyte 16 impregnated in partition 12 inside the secondary battery, that is, the anode electrolyte 15 and cathode electrolyte 16 present inside partition 12, and therefore are the anode electrolyte 15 and cathode electrolyte 16 necessary for measuring weight W1.

[0170] In contrast, the above-described "anode electrolyte 15 and cathode electrolyte 16 attached to the surface of partition wall 12" are anode electrolyte 15 and cathode electrolyte 16 that are not impregnated into partition wall 12 inside the secondary battery, that is, anode electrolyte 15 and cathode electrolyte 16 that are present outside partition wall 12, and therefore are anode electrolyte 15 and cathode electrolyte 16 that are not necessary for measuring weight W1.

[0171] (Measurement of weight W1) After the simple treatment of partition wall 12, weight W1 of partition wall 12 is immediately measured. As described above, weight W1 is the weight of partition wall 12 measured in a state in which partition wall 12 is impregnated with anode electrolyte 15 and cathode electrolyte 16, respectively.

[0172] (Drying Treatment) Subsequently, the partition wall 12 is subjected to a drying treatment to remove the anode electrolyte 15 and the cathode electrolyte 16 from the partition wall 12 .

[0173] In this drying treatment, the partition wall 12 is stored in the above-described normal-temperature and normal-humidity environment without a rag placed thereon, thereby drying the partition wall 12. As a result, the anode electrolyte 15 and the cathode electrolyte 16 impregnated in the partition wall 12 are removed.

[0174] Note that the time for drying partition wall 12 (so-called drying time) is preferably sufficiently longer than the above-described standing time. If the drying time is too short, anode electrolyte 15 and cathode electrolyte 16 remain inside partition wall 12.

[0175] The sufficiently long drying time described here refers to the time required for the weight of the partition walls 12 to reach a state where it does not substantially fluctuate when the partition walls 12 are stored in the above-described normal temperature and humidity environment. More specifically, the sufficiently long drying time is the drying time required for the weight of the partition walls 12 to fluctuate to less than 0.1% 24 hours after the start of storage when the partition walls 12 are stored in a normal temperature and humidity environment while the weight of the partition walls 12 is measured. This fluctuation rate is calculated based on the following formula: Fluctuation rate (%) = [(weight of the partition walls 12 at the start of storage - weight of the partition walls 12 24 hours after the start of storage) / weight of the partition walls 12 at the start of storage] × 100.

[0176] The drying time can be set arbitrarily depending on the combination of the type of partition wall 12, the composition of negative electrode electrolyte 15, and the composition of positive electrode electrolyte 16. A specific example of the drying time is 72 hours.

[0177] (Measurement of Weight W0) After the partition wall 12 is dried, the weight W0 of the partition wall 12 is measured. As described above, the weight W0 is the weight of the partition wall 12 measured in a state where the anode electrolyte 15 and the cathode electrolyte 16 have both been removed.

[0178] (Calculation of Liquid Absorption Rate W) Finally, based on the measurement results of the weights W0 and W1, the liquid absorption rate W is calculated using the calculation formula shown in Equation (1). The value of this liquid absorption rate W is rounded to two decimal places.

[0179] [Procedure for Measuring Ion Exchange Capacity] The procedure for measuring the ion exchange capacity is as follows: The series of procedures described below are carried out in a normal temperature and humidity environment (temperature = 23°C, humidity = 50%).

[0180] First, the secondary battery is discharged in the same manner as in the procedure for calculating the liquid absorption rate W, and then the secondary battery is disassembled to recover the partition walls 12 .

[0181] Next, the partition wall 12 is immersed in a sufficient amount of hydrochloric acid (concentration = 10%) (immersion time = 24 hours). Next, the partition wall 12 is taken out from the hydrochloric acid, and then washed with pure water, thereby substituting the plurality of ion exchange groups so that the partition wall 12 becomes completely H-type.

[0182] Subsequently, the partition wall 12 is immersed in an aqueous sodium chloride solution (concentration: 5%) (immersion time: 24 hours) to replace the multiple ion exchange groups so that they become completely sodium type.

[0183] Next, the sodium chloride aqueous solution (so-called treatment liquid) in which the partition wall 12 was immersed was collected, and then the hydrogen ions (H +　 ) is measured.

[0184] Finally, the concentration is divided by the weight (g) of the partition walls 12 in the above-described normal temperature and normal humidity environment to calculate the ion exchange capacity (meq / g).

[0185] <1-3. Operation> The secondary battery operates as follows.

[0186] During charging, alkali metal ions are released from positive electrode 14 and move to negative electrode 13 through positive electrode electrolyte 16, partition wall 12, and negative electrode electrolyte 15, and are thereby occluded in negative electrode 13.

[0187] During discharge, alkali metal ions are released from negative electrode 13 and move to positive electrode 14 through negative electrode electrolyte 15, partition wall 12, and positive electrode electrolyte 16, and the alkali metal ions are thereby occluded in positive electrode 14.

[0188] <1-4. Manufacturing Method> When manufacturing this secondary battery, partition wall 12, negative electrode 13, and positive electrode 14 are each fabricated, and negative electrode electrolyte 15 and positive electrode electrolyte 16 are each prepared, according to the procedure of an example described below, and then the secondary battery is assembled.

[0189] The manufacturing procedure when a cation exchange membrane is used as the partition wall 12 will be described below.

[0190] [Fabrication of Negative Electrode] First, a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent are mixed together to form a negative electrode mixture. Next, the negative electrode mixture is poured into a solvent to prepare a paste-like negative electrode mixture slurry. The solvent may be an aqueous solvent or an organic solvent. Finally, the negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 13A, excluding the connection terminal portion 13AT, to form the negative electrode active material layer 13B. The negative electrode active material layer 13B may then be compression-molded using a compression device such as a roll press. In this case, the negative electrode active material layer 13B may be heated, or the compression molding may be repeated multiple times. As a result, the negative electrode active material layer 13B is formed on both surfaces of the negative electrode current collector 13A, thereby fabricating the negative electrode 13.

[0191] [Fabrication of Positive Electrode] Positive electrode active material layers 14B are formed on both surfaces of the positive electrode current collector 14A using a procedure similar to that for fabricating the negative electrode 13 described above. Specifically, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed together to form a positive electrode mixture, and the positive electrode mixture is then poured into a solvent to prepare a paste-like positive electrode mixture slurry. Next, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 14A, excluding the connection terminal portion 14AT, to form the positive electrode active material layers 14B. The positive electrode active material layers 14B may then be compression-molded. As a result, positive electrode active material layers 14B are formed on both surfaces of the positive electrode current collector 14A, thereby fabricating the positive electrode 14.

[0192] [Fabrication of Partition Wall] As described above, the partition wall 12 is made of one or more functional membranes such as a cation exchange membrane and a solid electrolyte membrane.

[0193] In this case, as described above, the liquid absorption rate W can be adjusted by changing the type and number of anionic groups.

[0194] [Preparation of Positive Electrolyte Solution] An ionic substance is added to an aqueous solvent, whereby the ionic substance is dispersed or dissolved in the aqueous solvent, and thus positive electrode electrolyte solution 16 is prepared.

[0195] When preparing this positive electrode electrolyte 16, as described above, the liquid absorption rate W can be adjusted by changing the type and concentration of the ionic substance (electrolyte salt) and by changing the type of alkali metal ion (cation) and the type of anion.

[0196] [Preparation of Negative Electrolyte] Negative electrolyte 15 is prepared using the same procedure as that for preparing positive electrode electrolyte 16 described above, so as to have a pH higher than that of positive electrode electrolyte 16.

[0197] When preparing this negative electrode electrolyte 15, as described above, the liquid absorption rate W can be adjusted by changing the type and concentration of the ionic substance (electrolyte salt) and by changing the type of alkali metal ion (cation) and the type of anion.

[0198] [Assembly of Secondary Battery] First, the partition wall 12 is disposed inside the exterior member 11, and then the partition wall 12 is attached to the exterior member 11. As a result, the negative electrode chamber S1 and the positive electrode chamber S2 separated from each other by the partition wall 12 are formed.

[0199] Next, the negative electrode 13 and the positive electrode 14 are respectively placed inside the exterior member 11, and thereby attached to the exterior member 11. In this case, the negative electrode 13 is placed inside the negative electrode chamber S1, and the connection terminal portion 13AT is led out to the outside of the exterior member 11. Furthermore, the positive electrode 14 is placed inside the positive electrode chamber S2, and the connection terminal portion 14AT is led out to the outside of the exterior member 11.

[0200] Next, anode electrolyte 15 is supplied to anode chamber S1 from a supply port (not shown) provided in exterior member 11, and cathode electrolyte 16 is supplied to cathode chamber S2 from a supply port (not shown) provided in exterior member 11. As a result, anode electrolyte 15 is accommodated in anode chamber S1, and anode 13 is immersed in anode electrolyte 15. Furthermore, cathode electrolyte 16 is accommodated in cathode chamber S2, and anode 14 is immersed in cathode electrolyte 16.

[0201] Finally, the supply port used to supply the negative electrode electrolyte 15 is sealed, and the supply port used to supply the positive electrode electrolyte 16 is also sealed.

[0202] In this way, a secondary battery using two types of aqueous electrolytes (negative electrode electrolyte 15 and positive electrode electrolyte 16) is completed.

[0203] <1-5. Actions and Effects> In this secondary battery, partition wall 12 that is permeable to alkali metal ions is disposed between anode electrolyte 15 containing an aqueous solvent and cathode electrolyte 16 containing an aqueous solvent. Furthermore, anode 13 that occludes and releases alkali metal ions is immersed in anode electrolyte 15, and cathode 14 that occludes and releases alkali metal ions is immersed in cathode electrolyte 16, and anode electrolyte 15 has a higher pH than cathode electrolyte 16. Furthermore, the liquid absorption rate W of partition wall 12 satisfies the physical property condition (-0.1<W<0.1).

[0204] In this case, because the negative electrode electrolyte 15 has a higher pH than the positive electrode electrolyte 16, the decomposition potential of the aqueous solvent shifts as described above. As a result, the decomposition reaction of the aqueous solvent is thermodynamically suppressed during charge and discharge, and the potential window of the aqueous solvent is expanded. Therefore, a high voltage is obtained, and the charge and discharge reaction utilizing the absorption and release of alkali metal ions proceeds sufficiently and stably.

[0205] Moreover, because the liquid absorption rate W satisfies the physical property condition, an appropriate amount of aqueous solvent is impregnated into the partition wall 12 from each of the anode electrolyte 15 and the cathode electrolyte 16, as described above. This makes it less likely that the aqueous solvent will inhibit the permeation of alkali metal ions through the partition wall 12 during charge and discharge, and improves the ion exchange performance of the partition wall 12. Therefore, the permeation efficiency of the alkali metal ions permeating through the partition wall 12 is improved, which makes it easier for a charge and discharge reaction using the alkali metal ions to proceed during charge and discharge.

[0206] In this case, even when two types of aqueous electrolyte solutions (negative electrode electrolyte solution 15 and positive electrode electrolyte solution 16) are used, the permeation efficiency of alkali metal ions is sufficiently improved, making it easier for charge / discharge reactions to proceed smoothly and stably.

[0207] These factors enable a high voltage to be obtained. In addition, a high discharge capacity can be obtained even when a large current is drawn, resulting in high output characteristics. Therefore, excellent battery characteristics can be obtained.

[0208] In particular, when the partition walls 12 include a cation exchange membrane, an appropriate amount of aqueous solvent can easily penetrate into the partition walls 12. As a result, the ionic conductivity inside the partition walls 12 is improved, and the physical property conditions regarding the physical property (liquid absorption rate W) of the partition walls 12 are easily satisfied, thereby obtaining a higher effect.

[0209] Furthermore, if the cation exchange membrane contains one or both of sulfonic acid ion groups and carboxylate ion groups, the permeation efficiency of alkali metal ions passing through the partition wall 12 is sufficiently improved, and thus the charge / discharge reaction using alkali metal ions can be more easily and satisfactorily carried out during charge / discharge, thereby achieving even greater effects.

[0210] Furthermore, if anode electrolyte 15 contains hydroxide ions as anions and cathode electrolyte 16 contains one or more of nitrate ions, sulfate ions, hydrogensulfate ions, carbonate ions, hydrogencarbonate ions, phosphate ions, monohydrogenphosphate ions, dihydrogenphosphate ions, and carboxylate ions as anions, the pH of each of anode electrolyte 15 and cathode electrolyte 16 is less likely to fluctuate. Therefore, the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 is more easily maintained, and a greater effect can be obtained.

[0211] Furthermore, if each of the negative electrode electrolyte 15 and the positive electrode electrolyte 16 contains lithium ions as cations, a high voltage can be obtained, and a high energy density can also be obtained, resulting in a greater effect.

[0212] In this case, if each of anode electrolyte 15 and cathode electrolyte 16 further contains potassium ions as cations, the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 is more easily maintained, and therefore an even greater effect can be obtained.

[0213] Furthermore, if the pH of anode electrolyte 15 is 11 or higher and the pH of cathode electrolyte 16 is 3 to 8, the relationship between the pH of anode electrolyte 15 and the pH of cathode electrolyte 16 is more easily maintained, and therefore a greater effect can be obtained.

[0214] 2. Modifications The configuration of the secondary battery described above can be modified as appropriate, as described below. However, the series of modifications described below may be combined with each other.

[0215] [Variation 1] When the partition wall 12 includes a cation exchange membrane, the cation exchange membrane may be previously ion-exchanged with alkali metal ions occluded and released in each of the negative electrode 13 and the positive electrode 14. In this case, too, the alkali metal ions permeate the partition wall 12, and therefore the same effect can be obtained.

[0216] In this case, alkali metal ions absorbed and released in the negative electrode 13 and the positive electrode 14 can easily pass through the partition wall 12, thereby achieving a higher effect.

[0217] [Variation 2] Examples of the negative electrode active material include a titanium-containing compound, a niobium-containing compound, a vanadium-containing compound, an iron-containing compound, and a molybdenum-containing compound. However, the negative electrode active material may be other compounds as long as the charge / discharge reaction proceeds smoothly and stably even when two types of aqueous electrolytes (negative electrode electrolyte 15 and positive electrode electrolyte 16) are used. In this case, the same effect can be obtained.

[0218] 1, the electrolyte solutions (anode electrolyte 15 and cathode electrolyte 16) are liquid electrolytes. However, as shown in FIG. 2, which corresponds to FIG. 1, electrolyte layers 17 and 18, which are gel electrolytes, may be used instead of the electrolyte solutions.

[0219] 2 , when the electrolyte layers 17 and 18 are used, the electrolyte layer 17 is interposed between the partition wall 12 and the negative electrode 13, and the electrolyte layer 18 is interposed between the partition wall 12 and the positive electrode 14. As a result, the electrolyte layer 17 is adjacent to the partition wall 12 and the negative electrode 13, and the electrolyte layer 18 is adjacent to the partition wall 12 and the positive electrode 14, respectively.

[0220] Specifically, electrolyte layer 17 contains a polymer compound together with anode electrolyte 15, and anode electrolyte 15 is held by the polymer compound. This is because leakage of anode electrolyte 15 is prevented. In Fig. 2, electrolyte layer 17 containing anode electrolyte 15 is shaded darkly.

[0221] The electrolyte layer 18 contains a polymer compound together with the positive electrode electrolyte 16, and the positive electrode electrolyte 16 is held by the polymer compound. This is because leakage of the positive electrode electrolyte 16 is prevented. In Fig. 2, the electrolyte layer 18 containing the positive electrode electrolyte 16 is lightly shaded.

[0222] The type of polymer compound is not particularly limited, but specifically, it is one or more of polyvinylidene fluoride, polyethylene oxide, and the like.

[0223] When forming electrolyte layer 17, a precursor solution containing anode electrolyte 15, a polymer compound, an organic solvent, etc. is prepared, and then the precursor solution is applied to anode 13. When forming electrolyte layer 18, a precursor solution containing cathode electrolyte 16, a polymer compound, an organic solvent, etc. is prepared, and then the precursor solution is applied to cathode 14.

[0224] In this case, alkali metal ions can also move between the negative electrode 13 and the positive electrode 14 via the electrolyte layers 17 and 18, so that the same effect can be obtained.

[0225] 3. Uses of Secondary Batteries The uses of secondary batteries are not particularly limited as long as they can be used for machines, devices, instruments, equipment, and systems (assemblies of multiple devices, etc.) that can use the secondary battery as a driving power source and a power storage source for storing power. A secondary battery used as a power source may be a main power source or an auxiliary power source. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source may be a power source used in place of the main power source, or may be a power source that can be switched from the main power source as needed. When a secondary battery is used as an auxiliary power source, the type of main power source is not limited to a secondary battery.

[0226] Specifically, specific examples of uses of secondary batteries are as follows: Electronic devices (including portable electronic devices) such as video cameras, digital still cameras, mobile phones, notebook computers, cordless phones, headphone stereos, portable radios, portable televisions, and portable information terminals; Portable household appliances such as electric shavers; Storage devices such as backup power supplies and memory cards; Power tools such as power drills and power saws; Battery packs installed in notebook computers and the like as removable power sources; Medical electronic devices such as pacemakers and hearing aids; Electric vehicles such as electric cars (including hybrid cars); Power storage systems such as home battery systems that store power in preparation for emergencies. Of course, secondary batteries may be used for uses other than the series of uses exemplified here.

[0227] An embodiment of the present technology will be described.

[0228] Examples 1 to 10 and Comparative Examples 1 to 3 As will be described below, secondary batteries were fabricated, and then the battery characteristics of the secondary batteries were evaluated.

[0229] [Fabrication of Secondary Battery] The secondary battery shown in FIG. 1 was fabricated according to the procedure described below.

[0230] (Preparation of Negative Electrode) First, 89 parts by mass of a negative electrode active material (anatase-type titanium oxide, which is a titanium-containing compound (titanium oxide)), 10 parts by mass of a negative electrode binder (polyvinylidene fluoride), and 1 part by mass of a negative electrode conductive agent (graphite) were mixed together to prepare a negative electrode mixture. Next, the negative electrode mixture was added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry was applied to both sides of the negative electrode current collector 13A (titanium foil having a thickness of 20 μm) excluding the connection terminal portion 13AT using a coating device, and the negative electrode mixture slurry was then dried to form a negative electrode active material layer 13B. In this way, the negative electrode 13 was prepared.

[0231] (Preparation of Positive Electrode) First, a positive electrode active material (a lithium composite oxide, LiMn 2　 O4　 A positive electrode mixture was prepared by mixing 91 parts by mass of a positive electrode binder (polyvinylidene fluoride), 3 parts by mass of a positive electrode binder, and 6 parts by mass of a positive electrode conductive agent (graphite). The positive electrode mixture was then added to an organic solvent (N-methyl-2-pyrrolidone), and the organic solvent was stirred to prepare a paste-like positive electrode mixture slurry. The positive electrode mixture slurry was then applied to both sides of the positive electrode current collector 14A (titanium foil having a thickness of 20 μm) excluding the connection terminal portion 14AT using a coating device, and the positive electrode mixture slurry was then dried to form the positive electrode active material layer 14B. This produced the positive electrode 14.

[0232] (Preparation of negative electrode electrolyte solution and positive electrode electrolyte solution) Ionic substances (one or two types of alkali metal salts as electrolyte salts) were added to an aqueous solvent (pure water), and then the aqueous solvent was stirred. In this way, negative electrode electrolyte solution 15 and positive electrode electrolyte solution 16 were prepared, respectively. The types, concentrations (mol / kg), and pH of the ionic substances are as shown in Table 1.

[0233] Here, lithium ions were used as the alkali metal ions absorbed and released in the negative electrode 13 and the positive electrode 14, respectively, and lithium salts were used as the alkali metal salts having the alkali metal ions as cations.

[0234] In addition, potassium ions were used as the alkali metal ions that were not absorbed or released in the negative electrode 13 and the positive electrode 14, and potassium salts were used as the alkali metal salts having the alkali metal ions as cations.

[0235] Specifically, lithium salts include lithium hydroxide (LiOH) and lithium nitrate (LiNO 3　 ) and lithium sulfate (Li 2　 SO 4　 As potassium salts, potassium hydroxide (KOH) and potassium nitrate (KNO 3　 ), potassium sulfate (K 2　 SO 4　 ) and monopotassium hydrogen phosphate (KH 2　 P.O. 4　 ) was used.

[0236] (Assembly of Secondary Battery) First, a glass container (glass case) was prepared as the exterior member 11, and then the partition wall 12 was placed inside the exterior member 11. As a result, an anode chamber S1 and a cathode chamber S2 were formed inside the exterior member 11.

[0237] As the partition walls 12, five types of cation exchange membranes (cation exchange membranes A to E) and one type of solid electrolyte membrane having lithium ion conductivity were used, as will be described below.

[0238] Cation exchange membrane A (fluorine-based sulfonic acid cation exchange membrane for electrolysis and electrodialysis, FORBLUE (registered trademark) S-series Sx-2301DH, manufactured by AGC Inc.)

[0239] Cation exchange membrane B (Nafion membrane, Nafion (registered trademark) NRE212, purchased from Sigma-Aldrich Japan, LLC)

[0240] Cation exchange membrane C (PFSA melt-extruded ionomer membrane Aquivion (registered trademark) E87-05S, manufactured by Solvay Japan Co., Ltd.)

[0241] Cation exchange membrane D (Selemion (registered trademark) CMVN ion exchange membrane manufactured by AGC Inc.)

[0242] Cation exchange membrane E (Nafion membrane, Nafion (registered trademark) 115, purchased from Sigma-Aldrich Japan, LLC)

[0243] Solid electrolyte membrane (Li-ion conductive Li manufactured by Ohara Co., Ltd.) 2　 O-Al 2　 O 3　 -SiO 2　 -P 2　 O 5　 -TiO 2　 Solid electrolyte LICGC (registered trademark) PW

[0244] Subsequently, the negative electrode 13 was placed inside the negative electrode chamber S1, and the positive electrode 14 was placed inside the positive electrode chamber S2. In this case, the connection terminals 13AT and 14AT were led out to the outside of the exterior member 11.

[0245] Finally, anode electrolyte 15 was supplied into anode chamber S1, and cathode electrolyte 16 was supplied into cathode chamber S2. As a result, anode 13 was immersed in anode electrolyte 15, and cathode 14 was immersed in cathode electrolyte 16, completing the secondary battery.

[0246] When producing this secondary battery, the configuration (type) of partition wall 12, the configuration (type and concentration of ionic substance) of anode electrolyte 15, and the configuration (type and concentration of ionic substance) of cathode electrolyte 16 were each changed to change the liquid absorption rate W as shown in Table 1.

[0247] [Evaluation of Battery Characteristics] When the charge / discharge characteristics of the secondary battery were evaluated as the battery characteristics, the results shown in Table 1 were obtained.

[0248] When evaluating the charge / discharge characteristics, the secondary battery was first charged and discharged in a room temperature environment (temperature = 23°C) to measure the discharge capacity at the first cycle. In this case, the battery was charged at a current of 0.2 C until the voltage reached 2.8 V, and then discharged at a current of 0.2 C until the voltage reached 1.5 V. Note that 0.2 C is the current value at which the battery capacity (theoretical capacity) is fully discharged in 5 hours.

[0249] The secondary battery was then charged and discharged in the same environment to measure the discharge capacity at the second cycle. The charge and discharge conditions were the same as those at the first cycle, except that the discharge current was changed from 0.2 C to 1 C. 1 C is the current value at which the battery capacity is fully discharged in 1 hour.

[0250] Finally, the capacity ratio, which is an index for evaluating the charge-discharge characteristics, was calculated based on the formula: Capacity ratio (%) = (Discharge capacity at the second cycle / Discharge capacity at the first cycle) × 100. This capacity ratio value was rounded to one decimal place.

[0251]

[0252] [Discussion] As shown in Table 1, in a secondary battery using two types of aqueous electrolytes (negative electrode electrolyte 15 and positive electrode electrolyte 16) and in which negative electrode electrolyte 15 had a higher pH than positive electrode electrolyte 16, the capacity ratio varied depending on the liquid absorption rate W.

[0253] Specifically, when the liquid absorption rate W did not satisfy the physical property condition (-0.1<W<0.1) (Comparative Examples 1 to 3), the capacity ratio decreased significantly. In contrast, when the liquid absorption rate W satisfied the physical property condition (Examples 1 to 10), the capacity ratio increased.

[0254] In particular, when the liquid absorption rate W satisfies the physical property conditions (Examples 1 to 10), a series of trends described below were obtained.

[0255] First, the capacity ratio increased more when the partition walls 12 contained a cation exchange membrane than when the partition walls 12 contained a solid electrolyte membrane.

[0256] Second, when each of the negative electrode electrolyte 15 and the positive electrode electrolyte 16 contains lithium ions as cations (alkali metal ions), a sufficient capacity ratio was obtained.

[0257] In this case, when each of the negative electrode electrolyte 15 and the positive electrode electrolyte 16 further contained potassium ions as cations, the capacity ratio was further increased.

[0258] Third, when anode electrolyte 15 contains hydroxide ions as anions and cathode electrolyte 16 contains any one of nitrate ions, sulfate ions, and dihydrogen phosphate ions as anions, a sufficient capacity ratio was obtained.

[0259] In this case, it is believed that a similar tendency can be obtained even if positive electrode electrolyte 16 contains any one of hydrogen sulfate ions, carbonate ions, bicarbonate ions, phosphate ions, monohydrogen phosphate ions, and carboxylate ions as anions.

[0260] Fourth, when the pH of the negative electrode electrolyte 15 was 11 or higher and the pH of the positive electrode electrolyte 16 was 3 to 8, a sufficient capacity ratio was obtained.

[0261] [Summary] From the results shown in Table 1, in a secondary battery including two types of aqueous electrolytes (anode electrolyte 15 and cathode electrolyte 16), when anode electrolyte 15 had a higher pH than cathode electrolyte 16 and the liquid absorption rate W satisfied the physical property condition (-0.1<W<0.1), the charge / discharge characteristics were improved. Therefore, excellent battery characteristics were obtained in the secondary battery using the two types of aqueous electrolytes.

[0262] 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.

[0263] 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.

[0264] The present technology may also be configured as follows: <1> A secondary battery comprising: an anode electrolyte containing an aqueous solvent, a cathode electrolyte containing an aqueous solvent, a partition wall disposed between the anode electrolyte and the cathode electrolyte and allowing alkali metal ions to pass therethrough, an anode immersed in the anode electrolyte and occluding and desorbing the alkali metal ions, and a cathode immersed in the cathode electrolyte and occluding and desorbing the alkali metal ions, wherein the anode electrolyte has a pH higher than a pH of the cathode electrolyte, the partition wall has a liquid absorption rate calculated by formula (1), and the liquid absorption rate satisfies a condition expressed by formula (2). W=(W1-W0) / W0 (1) (W is a liquid absorption rate. W0 is the weight of the partition wall when the anode electrolyte and the cathode electrolyte have been removed. W1 is the weight of the partition wall when the partition wall is impregnated with the anode electrolyte and the cathode electrolyte.) -0.1<W<0.1 (2) <2> The secondary battery according to <1>, wherein the partition wall includes a cation exchange membrane. <3> The cation exchange membrane contains a sulfonic acid ion group (-S(=O) 2　 -O -　 ) and a carboxylate ion group (—C(═O)—O -　<4> The secondary battery according to any one of <1> to <3>, wherein the anode electrolyte contains hydroxide ions as anions, and the cathode electrolyte contains at least one of nitrate ions, sulfate ions, hydrogensulfate ions, carbonate ions, hydrogencarbonate ions, phosphate ions, monohydrogenphosphate ions, dihydrogenphosphate ions, and carboxylate ions as anions. <5> The secondary battery according to any one of <1> to <4>, wherein the anode electrolyte and the cathode electrolyte each contain lithium ions as cations. <6> The secondary battery according to <5>, wherein the anode electrolyte and the cathode electrolyte each further contain potassium ions as cations. <7> The secondary battery according to any one of <1> to <6>, wherein the anode electrolyte has a pH of 11 or more, and the cathode electrolyte has a pH of 3 or more and 8 or less.

[0265] 11... exterior member, 12... partition wall, 13... negative electrode, 14... positive electrode, 15... negative electrode electrolyte, 16... positive electrode electrolyte, 17, 18... electrolyte layer, S1... negative electrode chamber, S2... positive electrode chamber

Claims

1. a negative electrode electrolyte including an aqueous solvent; a positive electrode electrolyte including an aqueous solvent; a partition wall disposed between the anode electrolyte and the cathode electrolyte and allowing alkali metal ions to pass therethrough; a negative electrode immersed in the negative electrode electrolyte and absorbing and releasing the alkali metal ions; a positive electrode immersed in the positive electrode electrolyte and absorbing and releasing the alkali metal ions; Equipped with the anode electrolyte has a pH greater than a pH of the cathode electrolyte; The partition walls have a liquid absorption rate calculated by formula (1), The liquid absorption rate satisfies the condition represented by formula (2): Secondary battery. W=(W1-W0) / W0...(1) (W is the liquid absorption rate. W0 is the weight of the partition wall in a state where the anode electrolyte and the cathode electrolyte have both been removed. W1 is the weight of the partition wall in a state where the anode electrolyte and the cathode electrolyte have both been impregnated.) -0.1<W<0.1...(2)

2. The partition includes a cation exchange membrane. The secondary battery according to claim 1 .

3. The cation exchange membrane contains a sulfonic acid ion group (-S(=O) 2 -O - ) and a carboxylate ion group (—C(═O)—O - ) The secondary battery according to claim 2 .

4. the negative electrode electrolyte contains hydroxide ions as anions, the positive electrode electrolyte contains, as an anion, at least one of nitrate ions, sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, dihydrogen phosphate ions, and carboxylate ions; The secondary battery according to claim 1 .

5. Each of the negative electrode electrolyte and the positive electrode electrolyte contains lithium ions as cations. The secondary battery according to claim 1 .

6. Each of the negative electrode electrolyte and the positive electrode electrolyte further contains potassium ions as the cations. The secondary battery according to claim 5 .

7. The pH of the negative electrode electrolyte is 11 or more, The pH of the positive electrode electrolyte is 3 or more and 8 or less. The secondary battery according to claim 1 .