Lithium ion secondary battery

Abstract

The present invention provides a lithium ion secondary battery which is capable of improving charge / discharge characteristics. This lithium ion secondary battery is provided with: a positive electrode that occludes and releases lithium ions; a negative electrode that occludes and releases the lithium ions; and an electrolyte solution that contains an aqueous solvent. The negative electrode is provided with a negative electrode active material layer that contains a negative electrode active material. The negative electrode active material contains at least one of titanium oxide and lithium titanium composite oxide. The pore volume of the negative electrode active material layer with respect to pores having a pore diameter of 0.005 μm to 50 µm inclusive as measured by a mercury intrusion method is 0.1 mL / g or more. The ratio of the pore volume of the negative electrode active material layer with respect to pores having a pore diameter of 0.01 μm to 0.1 µm inclusive as measured by the mercury intrusion method to the pore volume of the negative electrode active material layer with respect to pores having a pore diameter of 0.005 μm to 50 µm inclusive as measured by the mercury intrusion method is 0.5 or more.

Description

Lithium-ion secondary battery

[0001] The present invention relates to a lithium ion secondary battery.

[0002] Patent Document 1 discloses a two-liquid type aqueous lithium ion secondary battery that uses particles containing anatase type titanium oxide and having an average particle size of 100 nm or less as a negative electrode active material.

[0003] International Publication No. 2023 / 013585

[0004] However, since the electrolyte of the battery disclosed in Patent Document 1 is an aqueous electrolyte, it has a higher surface tension than non-aqueous electrolytes and may be difficult for the electrolyte to penetrate into the negative electrode, which may result in a decrease in charge / discharge characteristics such as charge capacity.

[0005] The present invention has been made in view of the above problems, and has an object to provide a lithium ion secondary battery that can improve charge / discharge characteristics.

[0006] A lithium-ion secondary battery according to one aspect of the present invention includes a positive electrode that absorbs and releases lithium ions, a negative electrode that absorbs and releases the lithium ions, and an electrolyte solution containing an aqueous solvent. The negative electrode includes a negative electrode active material layer that contains a negative electrode active material. The negative electrode active material includes at least one of titanium oxide and a lithium-titanium composite oxide. The pore volume of the negative electrode active material layer measured by mercury porosimetry is 0.1 mL / g or more for pores having a pore diameter of 0.005 μm or more and 50 μm or less. The pore volume ratio of the pore volume of the negative electrode active material layer measured by mercury porosimetry is 0.5 or more for pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to pores having a pore diameter of 0.005 μm or more and 50 μm or less.

[0007] According to the present invention, it is possible to provide a lithium ion secondary battery capable of improving charge / discharge characteristics.

[0008] FIG. 1 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a first embodiment of the present invention. FIG. 2 is a schematic cross-sectional view showing an enlarged view of the configuration of the negative electrode shown in FIG. 1. FIG. 3 is a schematic view showing an electron microscope photograph of a cross section of the negative electrode active material layer shown in FIG. 2. FIG. 4 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a second embodiment of the present invention. FIG. 5 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a first modified example. FIG. 6 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a third modified example. FIG. 7 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a fourth modified example.

[0009] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0010] 1. First Embodiment (Lithium-ion Secondary Battery) First, a lithium-ion secondary battery according to a first embodiment of the present invention will be described.

[0011] The lithium ion secondary battery described in the present disclosure is a secondary battery in which charge / discharge reactions proceed by utilizing the absorption and release of lithium ions in the positive electrode. The secondary battery of the present disclosure includes a positive electrode, a negative electrode, and an aqueous electrolyte solution. The aqueous electrolyte solution is an electrolyte solution containing an aqueous solvent. In the present disclosure, the aqueous solvent is an electrolyte solution containing water molecules (H ２ O).

[0012] <1-1. Configuration> Fig. 1 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a first embodiment of the present invention. Fig. 2 is a schematic cross-sectional view showing an enlarged view of the configuration of the negative electrode shown in Fig. 1. Fig. 3 is a schematic view showing an electron microscope photograph of a cross section of the negative electrode active material layer shown in Fig. 2.

[0013] 1 and 2, the lithium ion secondary battery 1 includes an outer casing 10, a positive electrode 20, a negative electrode 30, and an electrolyte solution 40. In Fig. 1, the electrolyte solution 40 is lightly shaded.

[0014] [Exterior Body] As shown in FIG. 1, the exterior body 10 is a substantially box-shaped exterior member that houses the positive electrode 20, the negative electrode 30, the electrolyte solution 40, and the like, and has an internal space S.

[0015] The exterior body 10 includes at least one of a metal material, a glass material, a polymer compound, etc. The exterior body 10 may be a rigid metal can, a glass case, a plastic case, etc., or a flexible metal foil, a polymer film, etc.

[0016] [Positive Electrode] As shown in FIG. 1 , the positive electrode 20 is disposed in the internal space S and absorbs and releases lithium ions. Here, the positive electrode 20 includes a positive electrode current collector 20A having a pair of surfaces and a positive electrode active material layer 20B provided on both surfaces of the positive electrode current collector 20A. However, the positive electrode active material layer 20B may be provided on only one surface of the positive electrode current collector 20A, on the side where the positive electrode 20 faces the negative electrode 30. Furthermore, the positive electrode current collector 20A may be omitted. That is, the positive electrode 20 may not include the positive electrode current collector 20A and may include only the positive electrode active material layer 20B.

[0017] (Positive Electrode Current Collector) The positive electrode current collector 20A is a conductive support member that supports the positive electrode active material layer 20B. The material of the positive electrode current collector 20A is preferably insoluble or poorly soluble in the electrolyte 40 and corrosion-resistant, and preferably has low reactivity with the positive electrode active material described below. From this perspective, the positive electrode current collector 20A preferably includes at least one conductive material, such as a metal material, a carbon material, or a conductive ceramic material. Specific examples of metal materials used for the positive electrode current collector 20A include titanium, aluminum, and their alloys. Specific examples of conductive ceramic materials include indium tin oxide (ITO). Using such materials for the positive electrode current collector 20A can suppress deterioration of the positive electrode current collector 20A during use of the lithium-ion secondary battery 1. The positive electrode current collector 20A may be a conductor whose surface is plated with the conductive material described above. The conductor material is not particularly limited and can be selected arbitrarily.

[0018] 1, a positive electrode active material layer 20B is not provided on a connection terminal 20AT, which is a part of a positive electrode current collector 20A. The connection terminal 20AT is led out from the internal space S of the exterior body 10 to the outside.

[0019] (Positive Electrode Active Material Layer) The positive electrode active material layer 20B contains a positive electrode active material that absorbs and releases lithium ions. However, the positive electrode active material layer 20B may further contain at least one of a positive electrode binder, a positive electrode conductive agent, and the like.

[0020] The positive electrode active material includes a lithium-containing compound. The lithium-containing compound is a compound containing lithium as a constituent element. The lithium-containing compound included in the positive electrode active material is not particularly limited, and 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. 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, and specific examples include nickel, cobalt, manganese, and iron.

[0021] The lithium-containing compound contained in the positive electrode active material has, for example, a layered rock salt type, a spinel type, an olivine type, or other crystal structure. A specific example of a lithium composite oxide having a layered rock salt type crystal structure is LiNiO 2、 LiCoO 2、 LiCo 0.98 Al 0.01 Mg 0.01 O 2、 LiNi 0.5 Co 0.2 Mn 0.3 O 2、 LiNi 0.8 Co 0.15 Al 0.05 O 2、 LiNi 0.33 Co 0.33 Mn 0.33 O 2、 Li 1.2 Mn 0.52 Co 0.175 Ni 0.1 O2 and Li 1.15 (Mn 0.65 Ni 0.22 Co 0.13 )O2, etc. A specific example of a lithium composite oxide having a spinel-type crystal structure is LiMn2O4, etc. A specific example of a lithium phosphate compound having an olivine-type crystal structure is LiFePO4、 LiMnPO 4、 LiMn 0.5 Fe 0.5 P.O. 4、 LiMn 0.7 Fe 0.3 PO4 and LiMn 0.75 Fe 0.25 PO4, etc.

[0022] The positive electrode binder includes at least one of synthetic rubber and polymer compounds. A specific example of the synthetic rubber is styrene-butadiene rubber. A specific example of the polymer compound is polyvinylidene fluoride and polyimide.

[0023] The positive electrode conductive agent contains at least one conductive material such as a carbon material, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black. However, the conductive material may also be a metal material, a conductive ceramic material, a conductive polymer, or the like.

[0024] [Negative Electrode] As shown in FIG. 1 , the negative electrode 30 is disposed in the internal space S and absorbs and releases lithium ions. Here, the negative electrode 30 includes a negative electrode current collector 30A having a pair of surfaces and a negative electrode active material layer 30B provided on both surfaces of the negative electrode current collector 30A. However, the negative electrode active material layer 30B may be provided on only one surface of the negative electrode current collector 30A, on the side where the negative electrode 30 faces the positive electrode 20. Furthermore, the negative electrode current collector 30A may be omitted. That is, the negative electrode 30 may include only the negative electrode active material layer 30B without including the negative electrode current collector 30A.

[0025] (Negative Electrode Current Collector) The negative electrode current collector 30A is a conductive support member that supports the negative electrode active material layer 30B. The material of the negative electrode current collector 30A is preferably insoluble or poorly soluble in the electrolyte 40 and corrosion-resistant, and preferably has low reactivity with the negative electrode active material described below. From this perspective, the negative electrode current collector 30A preferably includes at least one conductive material, such as a metal material, a carbon material, or a conductive ceramic material. Specific examples of the metal material include stainless steel (SUS), titanium, tin, lead, and their alloys. Here, the stainless steel may be highly corrosion-resistant stainless steel to which at least one additive element, such as niobium or molybdenum, has been added. Specifically, the stainless steel may be SUS444 to which molybdenum has been added as an additive element. Specific examples of the conductive ceramic material include indium tin oxide (ITO). Using the above materials for the negative electrode current collector 30A can suppress deterioration of the negative electrode current collector 30A during use of a lithium-ion secondary battery. The negative electrode current collector 30A may be a conductor whose surface is plated with the above-mentioned conductive material. The material of the conductor is not particularly limited and can be selected arbitrarily.

[0026] 1, the negative electrode active material layer 30B is not provided on the connection terminal 30AT, which is a part of the negative electrode current collector 30A. The connection terminal 30AT is led out from the internal space S of the exterior body 10 to the outside.

[0027] (Negative Electrode Active Material Layer) The negative electrode active material layer 30B contains a negative electrode active material that absorbs and releases lithium ions, but may further contain a negative electrode conductive agent or the like.

[0028] Specifically, as shown in Fig. 2, the negative electrode active material layer 30B includes a plurality of negative electrode active material particles 31. The plurality of negative electrode active material particles 31 are particles made of a negative electrode active material. Each of the plurality of negative electrode active material particles 31 is a so-called primary particle.

[0029] The negative electrode active material layer 30B preferably has a porous structure. In the present disclosure, a porous structure refers to a structure in which a plurality of voids (pores 32) are formed. In the example of FIG. 2 , a plurality of voids (pores 32) are formed between a plurality of negative electrode active material particles 31. The negative electrode active material layer 30B is preferably a sintered body of a plurality of negative electrode active material particles (negative electrode active material particles 31). In the present disclosure, a sintered body of a plurality of negative electrode active material particles 31 refers to a structure in which at least a portion of the plurality of negative electrode active material particles 31 are directly bonded to one another. Whether the negative electrode active material layer 30B has a porous structure or is a sintered body of a plurality of negative electrode active material particles 31 can be determined by the following method. First, the lithium-ion secondary battery 1 is disassembled to remove the negative electrode 30. Next, the negative electrode active material layer 30B is cut along the thickness direction. Then, a cross section of the negative electrode active material layer 30B along the thickness direction is observed using an electron microscope. The type of electron microscope is not particularly limited, and specifically, it is at least one of a scanning electron microscope (SEM) and a transmission electron microscope (TEM).

[0030] Pore ​​volume V of the negative electrode active material layer 30B for pores having a pore diameter of 0.005 μm or more and 50 μm or less Ｔ is 0.1 mL / g or more. Ｔ is measured by mercury intrusion porosimetry. More specifically, the negative electrode active material layer 30B is measured using a mercury intrusion porosimeter to obtain a differential pore size distribution. Then, the pore volume V is calculated by summing up the differential pore volumes in the pore diameter range of 0.005 μm to 50 μm. Ｔ This allows the surfaces inside the pores of the negative electrode active material layer 30B to come into contact with the aqueous electrolyte solution, thereby increasing the energy density of the negative electrode active material layer 30B and improving the electronic conductivity between the plurality of negative electrode active material particles 31. Therefore, the electrical resistance can be reduced without reducing the energy density of the negative electrode 30, and the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1 can be improved.

[0031] The ratio R of the pore volume of the negative electrode active material layer 30B for pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to pores having a pore diameter of 0.005 μm or more and 50 μm or less Ｈ is 0.5 or more. Ｈ is preferably 0.7 or more. Having many pores of 0.01 μm or more promotes the penetration of the electrolyte solution, thereby further improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1. Furthermore, having few pores larger than 0.1 μm increases the density of the negative electrode active material layer 30B, thereby further improving the energy density of the lithium-ion secondary battery 1.

[0032] Pore ​​volume ratio R Ｈ The pore volumes of the negative electrode active material layer 30B for pores having a pore diameter of 0.005 μm or more and 50 μm or less or 0.01 μm or more and 0.1 μm or less, which are used to calculate the ratio R, are all measured by mercury intrusion porosimetry. Ｈ is the ratio of the pore volume of the negative electrode active material layer 30B measured by mercury intrusion porosimetry. More specifically, the negative electrode active material layer 30B is measured using a mercury intrusion porosimeter to obtain a differential pore size distribution. Then, the differential pore volumes in the pore diameter ranges of 0.005 μm to 50 μm and 0.01 μm to 0.1 μm can be summed to calculate the respective pore volumes. The pore volume in the pore diameter range of 0.01 μm to 0.1 μm is then divided by the pore volume in the pore diameter range of 0.005 μm to 50 μm to obtain the pore volume ratio R Ｈ In the following description, the pore volume ratio R Ｈ is the first pore volume ratio R Ｈ It may be explained as:

[0033] The ratio R of the pore volume of the negative electrode active material layer 30B for pores having a pore diameter of 20 nm or more and 50 nm or less to pores having a pore diameter of 10 nm or more and 100 nm or less Ｇis preferably 0.5 or more. Having many pores of 20 nm or more further promotes penetration of the aqueous electrolyte solution, thereby further improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1. Furthermore, having many pores of 50 nm or less can improve the electronic conductivity between the plurality of negative electrode active material particles 31 in the negative electrode active material layer 30B, thereby further improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1.

[0034] Pore ​​volume ratio R Ｇ The pore volumes of the negative electrode active material layer 30B with pore diameters of 10 nm to 100 nm or 20 nm to 50 nm used in the calculation of the pore volume ratio R are all measured by a gas adsorption method. Ｇ is the ratio of the pore volume of the negative electrode active material layer 30B measured by the gas adsorption method. More specifically, the negative electrode active material layer 30B is measured using a gas adsorption measurement device to obtain a differential pore size distribution. Here, in measuring the pore volume by the gas adsorption method, as a pretreatment, a measurement cell containing a measurement sample is vacuum degassed at 200°C for 30 minutes, and measurement is performed using nitrogen gas at liquid nitrogen temperature (77K). Then, the pore volumes can be calculated by summing the differential pore volumes in the pore diameter range of 10 nm to 100 nm and the pore diameter range of 20 nm to 50 nm, respectively. Then, the pore volume ratio R is calculated by dividing the pore volume in the pore diameter range of 20 nm to 50 nm by the pore volume in the pore diameter range of 10 nm to 100 nm. Ｇ In the following description, the pore volume ratio R Ｇ The second pore volume ratio R Ｇ It may be explained as:

[0035] The negative electrode active material contains at least one of titanium oxide and lithium titanium composite oxide. Specific examples of lithium titanium composite oxide include Li ０．５ TiO ２ This allows the discharge capacity of the lithium ion secondary battery 1 to be increased.

[0036] At least one of titanium oxide and lithium titanium composite oxide used as the negative electrode active material preferably has at least one of a tetragonal and an orthorhombic crystal structure. A specific example of titanium oxide having a tetragonal crystal structure is titanium oxide having an anatase crystal structure. A specific example of titanium oxide having an orthorhombic crystal structure is titanium oxide having a brookite crystal structure. A specific example of titanium oxide having an orthorhombic crystal structure is Li ０．５ TiO ２ These crystal structures allow the charge / discharge reaction in the strongly alkaline electrolyte 40 described below to proceed stably, thereby stably obtaining a high discharge capacity. The crystal structure of the negative electrode active material can be measured using X-ray diffraction (XRD). If a peak attributable to at least one of a tetragonal and an orthorhombic crystal structure is observed in the XRD, it can be confirmed that the negative electrode active material has at least one of a tetragonal and an orthorhombic crystal structure.

[0037] Furthermore, the average particle size AS of the plurality of negative electrode active material particles 31, calculated based on the results of observation of a cross section of the negative electrode active material layer 30B using an electron microscope, is preferably 100 nm or less. This allows lithium ions to easily move inside each negative electrode active material particle 31. This also improves the energy density per weight of the negative electrode active material layer 30B, and makes it easier to form a plurality of pores 32 that serve as paths for lithium ions to move inside the negative electrode active material layer 30B. This therefore increases the discharge capacity of the lithium-ion secondary battery 1.

[0038] The average particle size AS is more preferably 30 nm or less, which allows lithium ions to move more easily inside the negative electrode active material particles 31. This also improves the energy density per weight of the negative electrode active material layer 30B, and makes it easier for a plurality of pores 32 to be formed inside the negative electrode active material layer 30B.

[0039] The average particle size AS is preferably 7 nm or more, which makes it easier to stably form a plurality of negative electrode active material particles 31.

[0040] The procedure for calculating the average particle size AS is as follows: To calculate the average particle size AS of the negative electrode active material layer 30B, an electron microscope photograph 100 shown in FIG.

[0041] Specifically, first, the lithium-ion secondary battery 1 is disassembled to remove the negative electrode 30. Next, the main surface of the negative electrode active material layer 30B is observed using an electron microscope to obtain an electron microscope photograph 100. The type of electron microscope is not particularly limited, and specifically, at least one of a scanning electron microscope (SEM) and a transmission electron microscope (TEM) is used. The observation conditions are an acceleration voltage of 5.0 kV and a magnification of 150,000 times.

[0042] In calculating the average particle size AS, the negative electrode 30 may be cut using an ion milling device or the like to expose a cross section of the negative electrode active material layer 30B, and the cross section of the negative electrode active material layer 30B may then be observed to obtain the electron microscope photograph 100. As the ion milling device, an ArBlade (registered trademark) 5000 ion milling device manufactured by Hitachi High-Tech Corporation or the like may be used.

[0043] As shown in Fig. 3, in the electron microscope photograph 100, a plurality of negative electrode active material particles 31 are directly bonded to one another, and a porous structure having a plurality of pores 32 is observed. In order to simplify the illustration, Fig. 3 shows a case where each of the plurality of negative electrode active material particles 31 has a rectangular planar shape.

[0044] Next, 50 negative electrode active material particles 31 are selected from the plurality of negative electrode active material particles 31 visible in the electron microscope photograph 100, and then the particle size (maximum outer diameter) of each negative electrode active material particle 31 is measured. This allows the particle sizes of the 50 particles to be obtained.

[0045] When selecting 50 negative electrode active material particles 31, the negative electrode active material particle 31 located nearest to the front among the multiple negative electrode active material particles 31 that overlap each other is selected. That is, negative electrode active material particles 31Y that overlap with one or more other negative electrode active material particles 31 and whose entire outer edge is not visible are not selected. In contrast, negative electrode active material particles 31X that do not overlap with one or more other negative electrode active material particles 31 and whose entire outer edge is visible are selected. In FIG. 3, some negative electrode active material particles 31X to be selected are shaded.

[0046] Finally, the average particle size AS can be calculated by calculating the arithmetic mean of the particle sizes of the 50 negative electrode active material particles 31 .

[0047] The negative electrode active material layer 30B may further contain at least one other negative electrode active material that absorbs and releases lithium ions. The type of the other negative electrode active material is not particularly limited, and specific examples thereof include rutile-type titanium oxide, spinel-type lithium titanate (Li4Ti5O 12 Other types of negative electrode active materials may be metal-based materials containing, as constituent elements, at least one of metal elements and semimetal elements that form an alloy with lithium.

[0048] When the negative electrode active material layer 30B contains other negative electrode active materials, the following procedure may be taken to calculate the average particle size AS.

[0049] The negative electrode active material layer 30B may contain other negative electrode active materials such as rutile-type titanium oxide and spinel-type lithium titanate (Li4Ti5O 12 In order to check whether the negative electrode active material layer 30B contains a tetragonal negative electrode active material such as tetragonal tetragonal oxide (LTO), the negative electrode active material layer 30B is analyzed using X-ray diffraction (XRD). This makes it possible to confirm the presence or absence of a tetragonal negative electrode active material based on the difference in crystal structure.

[0050] When the anode active material layer 30B contains a carbon material or a metal-based material as another anode active material, the anode active material layer 30B is analyzed using energy dispersive X-ray spectroscopy (EDX). In this case, the presence or location of the carbon material or the metal-based material can be confirmed using element mapping.

[0051] The negative electrode conductive agent contains at least one conductive material such as a carbon material, and specific examples of the carbon material include graphite, carbon black, acetylene black, and ketjen black. However, the conductive material may also be a metal material, a conductive ceramic material, a conductive polymer, or the like.

[0052] [Electrolyte] The electrolyte 40 is contained in the internal space S and is an aqueous electrolyte, as described above. That is, the electrolyte 40 is a solution in which an ionic substance that can be ionized in an aqueous solvent is dissolved or dispersed.

[0053] The lithium ion secondary battery of the first embodiment is a so-called single liquid type lithium ion secondary battery that includes one type of aqueous electrolyte (electrolyte 40).

[0054] The electrolyte solution 40 contains an aqueous solvent and a solute. The solute contains at least one type of ionic substance that can be ionized in an aqueous solvent. More specifically, the electrolyte solution 40 used in the lithium-ion secondary battery contains lithium ions. As a result, the lithium ions are absorbed and released in the positive electrode 20 and the negative electrode 30, respectively.

[0055] The type of aqueous solvent is not particularly limited, and specifically includes pure water.

[0056] The type of ionic substance is not particularly limited, and specifically includes at least one of acids, bases, and electrolyte salts. Specific examples of acids include carbonic acid, oxalic acid, nitric acid, sulfuric acid, hydrochloric acid, acetic acid, and citric acid. The electrolyte salt is a salt containing a cation and an anion, and more specifically includes at least one of lithium 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. Specific examples of imide salts include lithium bis(fluorosulfonyl)imide and lithium bis(trifluoromethanesulfonyl)imide.

[0057] The electrolyte salt may further contain at least one other metal salt in addition to the lithium salt. The type of other metal salt is not particularly limited, and specific examples include alkali metal salts other than lithium salts, alkaline earth metal salts, and transition metal salts. Specific examples of alkali metal salts include sodium salts and potassium salts. Specific examples of alkaline earth metal salts include calcium salts and magnesium salts.

[0058] The electrolytic solution 40 is preferably strongly alkaline, and preferably has a pH of 11 or higher. This allows lithium ions to move more easily in the electrolytic solution 40, facilitating the progress of charge / discharge reactions.

[0059] The electrolyte salt is preferably a basic salt such as lithium hydroxide, which allows the pH of the electrolyte solution 40 to be increased easily and stably. In this case, the concentration of the ionic substance in the electrolyte solution 40 is preferably 0.2 mol / kg or more and 4 mol / kg or less, which allows the pH of the electrolyte solution 40 to be increased more easily and stably.

[0060] Furthermore, the electrolytic solution 40 is more preferably a saturated solution of an electrolyte salt, which facilitates stable absorption and desorption of lithium ions during charging and discharging, thereby facilitating stable progress of the charge and discharge reaction.

[0061] To confirm whether the electrolyte solution 40 is a saturated solution of electrolyte salt, the lithium-ion secondary battery is disassembled and the internal space S is visually observed to determine whether electrolyte salt is precipitated. Specifically, the internal space S is observed by observing the liquid electrolyte solution 40, the surface of the positive electrode 20, and the inner wall surface of the exterior body 10. If the electrolyte solution 40 (liquid) and a precipitate of electrolyte salt (solid) coexist in the observed area, the electrolyte solution 40 is considered to be a saturated solution of electrolyte salt. To determine the composition of the precipitate, a surface analysis method such as X-ray photoelectron spectroscopy (XPS) or a composition analysis method such as inductively coupled plasma (ICP) emission spectroscopy may be used.

[0062] <1-2. Operation> The lithium ion secondary battery 1 operates as described below.

[0063] When the lithium ion secondary battery 1 is charged, lithium ions are released from the positive electrode 20 , and the lithium ions move to the negative electrode 30 via the electrolyte 40 , where they are absorbed.

[0064] When the lithium ion secondary battery 1 is discharged, lithium ions are released from the negative electrode 30 , and the lithium ions move to the positive electrode 20 via the electrolyte 40 , where they are absorbed.

[0065] <1-3. Manufacturing Method> When manufacturing the lithium ion secondary battery 1, the positive electrode 20 and the negative electrode 30 are each manufactured and the electrolyte solution 40 is prepared according to the procedure described below as an example, and then the lithium ion secondary battery 1 is assembled.

[0066] [Fabrication of Positive Electrode] First, a positive electrode active material, a positive electrode binder, and a positive electrode conductive agent are mixed together to obtain a mixture. However, the composition of the mixture can be changed as desired. Next, a paste-like positive electrode mixture slurry is prepared by adding the positive electrode mixture to a solvent. The solvent may be an aqueous solvent or an organic solvent. Finally, the positive electrode mixture slurry is applied to both surfaces of the positive electrode current collector 20A, excluding the connection terminal portions 20AT, to form the positive electrode active material layer 20B. Thereafter, the positive electrode active material layer 20B may be compression-molded using a roll press or the like. Here, the positive electrode active material layer 20B may be heated, or the compression molding of the positive electrode active material layer 20B may be repeated multiple times. In this manner, the positive electrode 20 is fabricated.

[0067] [Fabrication of Negative Electrode] First, a paste containing a plurality of negative electrode active material particles 31 and a negative electrode binder is obtained by mixing a negative electrode active material and a negative electrode binder. It is preferable to use a plurality of negative electrode active material particles 31 having an average particle size AS of 100 nm or less as the negative electrode active material to be mixed. The type of negative electrode binder is not particularly limited as long as it is a polymer compound. Specific examples of polymer compounds include polyethylene glycol, polyvinyl alcohol, and polyvinyl butyral. This can improve the strength of the molded body described below. It is preferable that the negative electrode binder be a polymer compound that is decomposed and degreased at or below the firing temperature described below.

[0068] The paste composition can be arbitrarily changed, and one or more additives may be added. The type of additive is not particularly limited, and specific examples include surfactants and sintering aids. Specific examples of surfactants include stearic acid. Specific examples of sintering aids include boron oxide and silicon oxide.

[0069] Next, the paste is applied to the negative electrode current collector 30A, dried, and then press-molded. Conditions such as the pressing pressure can be set as desired. As a result, the paste containing the plurality of negative electrode active material particles 31 is fixed to both sides of the negative electrode current collector 30A, resulting in a molded body. As a result, the plurality of negative electrode active material particles 31 are fixed to the surface of the negative electrode current collector 30A, forming a negative electrode active material layer 30B having a porous structure, and a plurality of pores 32 are formed between the plurality of negative electrode active material particles 31. At this time, the pore diameter and pore volume of the plurality of pores 32 can be adjusted by conditions such as the pressing pressure.

[0070] Furthermore, it is preferable that the molded body be fired in the atmosphere. The firing conditions, such as the firing temperature and firing time, are adjusted depending on the composition of the molded body, etc. In particular, it is preferable that the firing conditions are adjusted so that the plurality of negative electrode active material particles 31 are directly bonded to each other while maintaining the state of primary particles. Furthermore, the firing conditions may be adjusted so that the pore diameter of the pores formed between the secondary particles formed from the plurality of primary particles is reduced. For example, the maximum temperature during firing is 500°C or higher and 1200°C or lower. Note that firing may be performed in an oxygen atmosphere.

[0071] In the firing process, the negative electrode binder is degreased, so that the negative electrode active material particles 31 are directly bonded to one another, and the bonded body (sintered body) of the negative electrode active material particles 31 is fixed to the surface of the negative electrode current collector 30A. This allows the formation of a stronger negative electrode active material layer 30B. Furthermore, at this time, the pore diameter and pore volume of the multiple pores 32 formed between the multiple negative electrode active material particles 31 are adjusted by the firing conditions. The negative electrode 30 is produced by the above steps.

[0072] When producing the negative electrode 30, the bonding state of the plurality of negative electrode active material particles 31 (plurality of primary particles) and the pore volume and pore volume ratio R of the negative electrode active material layer 30B can be appropriately adjusted by adjusting the conditions such as the pressing pressure, the baking temperature, and the baking time. Ｈ , R Ｇ can be adjusted.

[0073] The method for producing the negative electrode 30 is not limited to the above method, and the procedure for producing the negative electrode 30 can be changed as appropriate as long as the negative electrode active material layer 30B is formed by fixing the plurality of negative electrode active material particles 31 to the surface of the negative electrode current collector 30A. Specifically, the negative electrode 30 may be produced by firing a powder compact obtained by press-molding a powder containing the plurality of negative electrode active material particles 31, without using a paste.

[0074] [Preparation of Electrolyte Solution] The electrolyte solution 40 is prepared by adding an ionic substance to an aqueous solvent. In preparing the electrolyte solution 40, the pH of the electrolyte solution 40 can be adjusted by adjusting conditions such as the type and concentration (mol / kg) of the ionic substance.

[0075] [Assembly of Lithium-Ion Secondary Battery] First, the positive electrode 20 and the negative electrode 30 are housed in the internal space S of the exterior body 10. Here, the connection terminal portions 20AT, 30AT are pulled out from the inside (internal space S) of the exterior body 10 so that they are exposed to the outside.

[0076] Subsequently, the electrolyte solution 40 is supplied into the internal space S through an injection hole provided in the exterior body 10 (not shown), and then the injection hole is sealed.

[0077] By the above procedure, the electrolyte solution 40 is accommodated in the internal space S in which the positive electrode 20 and the negative electrode 30 are respectively arranged, and thus a single-liquid type lithium ion secondary battery using one type of aqueous electrolyte solution (electrolyte solution 40) is completed.

[0078] <1-4. Actions and Effects> As described above, the lithium ion secondary battery of the first embodiment includes a positive electrode 20 that absorbs and releases lithium ions, a negative electrode 30 that absorbs and releases lithium ions, and an electrolyte solution 40 that contains an aqueous solvent. The negative electrode 30 includes a negative electrode active material layer 30B that contains a negative electrode active material. The negative electrode active material includes at least one of titanium oxide and a lithium-titanium composite oxide. The pore volume of the negative electrode active material layer 30B measured by mercury porosimetry, where the pore volume of pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.1 mL / g or more. The pore volume ratio of the negative electrode active material layer 30B measured by mercury porosimetry, where the pore volume of pores having a pore diameter of 0.01 μm or more and 0.1 μm or less is the ratio of the pore volume of pores having a pore diameter of 0.005 μm or more and 50 μm or less (first pore volume ratio R Ｈ ) is 0.5 or more.

[0079] This increases the density of the negative electrode active material layer 30B and makes it easier for the surfaces inside the pores of the negative electrode active material layer 30B to come into contact with the aqueous electrolyte solution, thereby increasing the energy density of the negative electrode active material layer 30B and improving the electronic conductivity between the plurality of negative electrode active material particles 31. Therefore, the electrical resistance can be reduced without reducing the energy density of the negative electrode 30, and the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1 can be improved.

[0080] In a desirable embodiment, the ratio of the pore volume of the negative electrode active material layer 30B measured by mercury porosimetry is the ratio of the pore volume of pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to the pores having a pore diameter of 0.005 μm or more and 50 μm or less (first pore volume ratio R Ｈ ) is 0.7 or more. This can increase the density of the negative electrode active material layer 30B, and the size of the pores 32 in the negative electrode active material layer 30B can be said to be suitable for the penetration of the aqueous electrolyte solution, thereby further improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1.

[0081] In a preferred embodiment, the ratio of the pore volume of the negative electrode active material layer 30B measured by a gas adsorption method is the ratio of the pore volume of pores having a pore diameter of 20 nm or more and 50 nm or less to the pores having a pore diameter of 10 nm or more and 100 nm or less (second pore volume ratio R Ｇ ) is 0.5 or more. This can increase the electronic conductivity of the anode active material layer 30B, and the size of the pores 32 in the anode active material layer 30B can be said to be more suitable for the penetration of the aqueous electrolyte solution, thereby further improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1.

[0082] In a preferred embodiment, at least one of titanium oxide and lithium-titanium composite oxide contained in the negative electrode active material has at least one of a tetragonal and an orthorhombic crystal structure, which allows the charge-discharge reaction in the strongly alkaline electrolyte 40 to proceed stably, thereby enabling a high discharge capacity to be obtained stably.

[0083] In a preferred embodiment, the negative electrode active material layer 30B has a porous structure. This allows the surfaces of the pores in the negative electrode active material layer 30B to easily come into contact with the aqueous electrolyte solution, thereby increasing the energy density of the negative electrode active material layer 30B and improving the electronic conductivity between the plurality of negative electrode active material particles 31. This reduces the electrical resistance without reducing the energy density of the negative electrode 30, thereby improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1.

[0084] In a preferred embodiment, the negative electrode active material layer 30B is a sintered body of a plurality of particles of the negative electrode active material (negative electrode active material particles 31). This results in a structure in which the negative electrode active material layer 30B has an interior in which the plurality of negative electrode active material particles 31 are interconnected, increasing the energy density of the negative electrode active material layer 30B and improving electronic conductivity between the plurality of negative electrode active material particles 31. Therefore, the electrical resistance can be reduced without reducing the energy density of the negative electrode 30, thereby improving the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1.

[0085] 2. Second Embodiment (Lithium-ion Secondary Battery) Next, a lithium-ion secondary battery according to a second embodiment of the present invention will be described.

[0086] Unlike the lithium ion secondary battery 1 of the first embodiment, which is a single-liquid type lithium ion secondary battery using one type of aqueous electrolyte (electrolyte solution 40), the lithium ion secondary battery 1A of the second embodiment is a dual-liquid type lithium ion secondary battery using two types of aqueous electrolyte (cathode electrolyte solution 61 and anode electrolyte solution 62).

[0087] 4 is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery according to a second embodiment of the present invention. The lithium-ion secondary battery 1A according to the second embodiment described here has the same configuration as the lithium-ion secondary battery 1 according to the first embodiment shown in FIG. 1, except for the points described below.

[0088] As shown in Fig. 4, the lithium ion secondary battery 1A further includes a partition wall 50. The lithium ion secondary battery 1A also includes a positive electrode electrolyte 61 and a negative electrode electrolyte 62 instead of the electrolyte 40 of the lithium ion secondary battery 1. In Fig. 4, the positive electrode electrolyte 61 is lightly hatched, and the negative electrode electrolyte 62 is darkly hatched.

[0089] The exterior body 10 has two spaces (a positive electrode chamber S1 and a negative electrode chamber S2) separated by a partition wall 50.

[0090] The partition wall 50 is disposed between the positive electrode 20 and the negative electrode 30, and separates the internal space S shown in Fig. 1 into a positive electrode chamber S1 and a negative electrode chamber S2. As a result, the positive electrode 20 and the negative electrode 30 are separated from each other via the partition wall 50 and also face each other via the partition wall 50.

[0091] The partition wall 50 does not allow anions to pass through, but selectively allows cations such as lithium ions that are absorbed and released in the positive electrode 20 and the negative electrode 30 to pass through. This makes it possible to prevent the positive electrode electrolyte 61 and the negative electrode electrolyte 62 from mixing with each other. In other words, the partition wall 50 allows cations such as lithium ions to pass between the positive electrode chamber S1 and the negative electrode chamber S2, but prevents anions from passing between the positive electrode chamber S1 and the negative electrode chamber S2.

[0092] The partition wall 50 includes at least one of an ion exchange membrane and a solid electrolyte membrane. The ion exchange membrane may be a porous membrane that is a cation exchange membrane that is permeable to lithium ions. The solid electrolyte membrane may be a membrane of a solid electrolyte that is conductive to lithium ions. This can improve the permeability of lithium ions through the partition wall 50.

[0093] Here, the solid electrolyte membrane may be a so-called inorganic particle membrane. The inorganic particle membrane is a composite membrane containing inorganic particles having lithium ion conductivity, a binder, and a fibrous material. The inorganic particles contain at least one inorganic material such as oxides, sulfides, hydroxides, carbonates, and sulfates. The binder contains at least one polymer compound such as polyvinyl formal, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polymethyl methacrylate, and polytetrafluoroethylene. The fibrous material contains at least one fibrous material such as cellulose fiber, polysaccharides, polyvinyl alcohol, polyacrylic acid, anionic derivatives of polystyrene, and cationic derivatives of polystyrene.

[0094] The partition wall 50 preferably has an ion exchange membrane, which allows the aqueous solvents in the positive electrode electrolyte 61 and the negative electrode electrolyte 62 to easily permeate into the interior of the partition wall 50, thereby improving the lithium ion conductivity inside the partition wall 50.

[0095] The positive electrode 20 is disposed inside the positive electrode chamber S1. The negative electrode 30 is disposed inside the negative electrode chamber S2. The positive electrode electrolyte 61 is accommodated inside the positive electrode chamber S1. The negative electrode electrolyte 62 is accommodated inside the negative electrode chamber S2. As a result, the positive electrode electrolyte 61 and the negative electrode electrolyte 62 are separated from each other by the partition wall 50 so as not to be mixed with each other. That is, the positive electrode electrolyte 61 accommodated inside the positive electrode chamber S1 is not in contact with the negative electrode 30, but is in contact only with the positive electrode 20. On the other hand, the negative electrode electrolyte 62 accommodated inside the negative electrode chamber S2 is not in contact with the positive electrode 20, but is in contact only with the negative electrode 30.

[0096] The positive electrode electrolyte 61 and the negative electrode electrolyte 62 are each an aqueous electrolyte.

[0097] The pH of the negative electrode electrolyte 62 is higher than the pH of the positive electrode electrolyte 61. As a result, the decomposition potential of the aqueous solvent shifts due to the difference in pH between the positive electrode electrolyte 61 and the negative electrode electrolyte 62, so that the decomposition reaction of the aqueous solvent can be thermodynamically suppressed and the potential window of the aqueous solvent is expanded. Therefore, a high voltage can be obtained, and the charge / discharge reaction utilizing the absorption / release of lithium ions can be facilitated to proceed sufficiently and stably.

[0098] The pH of the positive electrode electrolyte 61 is preferably less than 11, more preferably 3 or more and 8 or less, even more preferably 4 or more and 8 or less, and most preferably 4 or more and 6 or less. By setting the pH of the positive electrode electrolyte 61 to be in the range not exceeding the above upper limit, the pH of the positive electrode electrolyte 61 can be made sufficiently lower than the pH of the negative electrode electrolyte 62. Furthermore, it is possible to prevent the pH of the positive electrode electrolyte 61 from fluctuating due to charge / discharge reactions and becoming higher than the pH of the negative electrode electrolyte 62. Furthermore, by setting the pH of the positive electrode electrolyte 61 to be in the range not less than the above lower limit, the exterior body 10, the positive electrode current collector 20A, the negative electrode current collector 30A, etc. are less likely to corrode, thereby improving the durability and safety of the lithium-ion secondary battery.

[0099] The pH of the anode electrolyte 62 is preferably equal to or higher than 11, more preferably equal to or higher than 12, and even more preferably equal to or higher than 13. This allows the pH of the anode electrolyte 62 to be made sufficiently higher than the pH of the cathode electrolyte 61. Furthermore, it is possible to prevent the pH of the anode electrolyte 62 from fluctuating due to charge / discharge reactions and becoming lower than the pH of the cathode electrolyte 61.

[0100] At least one of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 is preferably a saturated solution of an electrolyte salt, similar to the electrolyte solution 40 of the first embodiment. This makes it easier for the charge / discharge reaction to proceed stably during charge / discharge. The method for checking whether each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 is a saturated solution of a lithium salt is the same as the method for checking whether the electrolyte solution 40 is a saturated solution of a lithium salt.

[0101] The type of electrolyte salt in the positive electrode electrolyte 61 and the type of electrolyte salt in the negative electrode electrolyte 62 are preferably different from each other. This allows the pH of the negative electrode electrolyte 62 to be higher than the pH of the positive electrode electrolyte 61.

[0102] Positive electrode electrolyte 61 preferably contains, as anions, at least one of sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, dihydrogen phosphate ions, and carboxylate ions. Specific examples of carboxylate ions include formate ions, acetate ions, propionate ions, tartrate ions, and citrate ions. This sufficiently suppresses pH fluctuations in positive electrode electrolyte 61 that accompany charge and discharge reactions, making it easier to maintain a low pH in positive electrode electrolyte 61.

[0103] The anode electrolyte 62 contains, as an anion, at least one of sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, and dihydrogen phosphate ions. The anode electrolyte 62 preferably contains hydroxide ions as an anion. This sufficiently suppresses fluctuations in pH of the anode electrolyte 62 due to charge / discharge reactions, making it easier to maintain a high pH of the anode electrolyte 62.

[0104] The positive electrode electrolyte 61 and the negative electrode electrolyte 62 are preferably isotonic solutions that are isotonic with each other. This makes it possible to equalize the osmotic pressures of the positive electrode electrolyte 61 and the negative electrode electrolyte 62, thereby maintaining a state in which the pH of the negative electrode electrolyte 62 is higher than the pH of the positive electrode electrolyte 61.

[0105] The types of aqueous solvents and the types and concentrations of ionic substances in the positive electrode electrolyte 61 and the negative electrode electrolyte 62 can be set arbitrarily.

[0106] For example, each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 may be a pH buffer solution. The pH buffer solution may be an aqueous solution in which a weak acid and its conjugate base are mixed, or an aqueous solution in which a weak base and its conjugate acid are mixed. This sufficiently suppresses fluctuations in the pH of each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62 due to charge / discharge reactions, making it easier to maintain the pH.

[0107] Furthermore, each of the cathode electrolyte 61 and the anode electrolyte 62 may contain at least one of trishydroxymethylaminomethane, ethylenediaminetetraacetic acid, and the like as a buffer.

[0108] <2-2. Operation> The lithium ion secondary battery 1A operates as described below.

[0109] During charging of the lithium-ion secondary battery 1A, lithium ions are released from the positive electrode 20, move to the negative electrode 30 through the positive electrode electrolyte 61, the partition wall 50, and the negative electrode electrolyte 62, and are absorbed in the negative electrode 30.

[0110] When lithium-ion secondary battery 1A is discharged, lithium ions are released from negative electrode 30, move to positive electrode 20 through negative electrode electrolyte 62, partition wall 50, and positive electrode electrolyte 61, and are absorbed in positive electrode 20.

[0111] 2-3. Manufacturing Method The manufacturing procedure for the lithium ion secondary battery 1A is the same as the manufacturing procedure for the lithium ion secondary battery 1 in the first embodiment described above, except for the points described below.

[0112] In preparing each of the positive electrode electrolyte 61 and the negative electrode electrolyte 62, the ionic substance is added to the aqueous solvent by adjusting the conditions such as the type and concentration (mol / kg) of the ionic substance so that the pH of the negative electrode electrolyte 62 is higher than the pH of the positive electrode electrolyte 61.

[0113] When assembling a lithium-ion secondary battery, first, the exterior housing 10 (cathode chamber S1 and anode chamber S2) to which the partition wall 50 has been attached is prepared. Next, the cathode 20 is housed inside the cathode chamber S1, and the connection terminal 20AT is extended from inside the cathode chamber S1 to the outside. Furthermore, the anode 30 is housed inside the anode chamber S2, and the connection terminal 30AT is extended from inside the anode chamber S2 to the outside. Finally, the cathode electrolyte 61 is supplied into the cathode chamber S1 through a cathode injection hole (not shown) provided in the exterior housing 10. Furthermore, the anode electrolyte 62 is supplied into the anode chamber S2 through a cathode injection hole (not shown) provided in the exterior housing 10. Then, the cathode injection hole and the anode injection hole are each sealed.

[0114] As a result, the positive electrode electrolyte 61 is accommodated inside the positive electrode chamber S1 in which the positive electrode 20 is disposed, and the negative electrode electrolyte 62 is accommodated inside the negative electrode chamber S2 in which the negative electrode 30 is disposed. In this manner, a dual liquid type lithium ion secondary battery using two types of aqueous electrolytes (positive electrode electrolyte 61 and negative electrode electrolyte 62) is completed.

[0115] <2-4. Actions and Effects> As described above, the lithium-ion secondary battery 1A of the second embodiment further includes a positive electrode chamber S1 that houses the positive electrode 20, an anode chamber S2 that houses the anode 30, and a partition wall 50 that is disposed between the positive electrode chamber and the anode chamber and that allows lithium ions to pass through. The electrolyte includes a positive electrode electrolyte 61 housed in the positive electrode chamber S1 and an anode electrolyte 62 housed in the anode chamber S2. The pH of the anode electrolyte 62 is higher than the pH of the positive electrode electrolyte 61.

[0116] Even in this case, the surfaces inside the pores of the negative electrode active material layer 30B are more likely to come into contact with the aqueous electrolyte solution, which increases the energy density of the negative electrode active material layer 30B and improves the electronic conductivity between the plurality of negative electrode active material particles 31. Therefore, the electrical resistance can be reduced without reducing the energy density of the negative electrode 30, and the charge / discharge efficiency and discharge capacity of the lithium-ion secondary battery 1 can be improved.

[0117] 3. Modifications The configuration of the lithium ion secondary battery can be modified as appropriate, as described below. Regarding the series of modifications described below, any two or more of the modifications may be combined with each other.

[0118] [First Modification] Fig. 5 is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery of the first modification. As shown in Fig. 5, the lithium-ion secondary battery 1B of the first modification further includes a separator 70 that separates the positive electrode 20 and the negative electrode 30. The configuration of the lithium-ion secondary battery shown in Fig. 5 is the same as the configuration of the lithium-ion secondary battery 1 of the first embodiment shown in Fig. 1, except for the points described below.

[0119] The separator 70 is disposed between the positive electrode 20 and the negative electrode 30 and is adjacent to each of the positive electrode 20 and the negative electrode 30. The separator 70 is an insulating porous membrane that separates the positive electrode 20 and the negative electrode 30 from each other and allows lithium ions to pass through. The separator 70 is impregnated with the electrolyte solution 40.

[0120] The separator 70 contains at least one of polymer compounds such as polyolefin, polyethylene, and polypropylene.

[0121] The inorganic particle film used in the separator 70 contains inorganic particles, a binder, and a fibrous material.

[0122] The inorganic particles are a plurality of particulate inorganic materials. The inorganic material is a compound containing at least one of metal elements (cations) such as Mg, Al, Si, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, In, Ba, Hf, Ta, W, Re, Ir, Pt, and Au as a constituent element. The inorganic material includes at least one of oxides, sulfides, hydroxides, carbonates, sulfates, etc.

[0123] The inorganic material is preferably an inorganic solid electrolyte having excellent alkali metal ion conductivity and high water resistance. Specifically, the inorganic solid electrolyte having excellent alkali metal ion conductivity has a NASICON structure, and more specifically, a lithium phosphate solid electrolyte represented by the general formula LiM2(PO4)3. Here, M is at least one metal element selected from the group consisting of Ti, Ge, Sr, Zr, Sn, and Al. It is particularly preferred that M contains at least one metal element selected from the group consisting of Ge, Zr, and Ti, and Al. This can suppress hydrolysis reactions within the lithium ion secondary battery.

[0124] A specific example of a lithium phosphate solid electrolyte having a NASICON structure is LATP (Li 1+x Al x Ti 2-x (PO4)3), Li 1+x Al x Ge 2-x (PO4)3 and Li 1+x Al x Zr 2-x (PO4)3, etc., where x satisfies 0<x≦5, and preferably 0.1≦x≦0.5. The lithium phosphate solid electrolyte is preferably LATP. This improves the water resistance of the separator 70 and suppresses hydrolysis reactions inside the lithium ion secondary battery.

[0125] The inorganic material may be an oxide-based solid electrolyte. Specifically, the oxide-based solid electrolyte may be amorphous LIPON (Li 2.9 P.O. 3.3 N 0.46 ) and LLZ (LiLaZrO) with a garnet structure 12 ) etc.

[0126] The inorganic material may be oxide ceramics, carbonates, sulfates, nitride ceramics, etc. Specific examples of oxide ceramics include alumina, silica, zirconia, yttria, magnesium oxide, calcium oxide, barium oxide, strontium oxide, and vanadium oxide. Specific examples of carbonates include sodium carbonate, potassium carbonate, magnesium carbonate, calcium carbonate, barium carbonate, lanthanum carbonate, and cerium carbonate. Specific examples of sulfates include calcium sulfate, magnesium sulfate, aluminum sulfate, gypsum, and barium sulfate. Specific examples of phosphates include hydroxyapatite, zirconium phosphate, and titanium phosphate. Specific examples of nitride ceramics include silicon nitride, titanium nitride, and boron nitride. Among these, alumina, silica, and calcium oxide are preferably in the form of glass ceramics.

[0127] The shape of the inorganic particles, the average particle size of the inorganic particles, and the content of the inorganic particles in the inorganic particle film are not particularly limited and can be set as desired. It is preferable that the content of the inorganic particles in the inorganic particle film is sufficiently large. This increases the density of the separator 70 and improves the hydrophobicity of the separator 70.

[0128] The binder contains at least one polymer compound. The polymer compound is a compound obtained by polymerizing a hydrocarbon monomer having a predetermined functional group, and the functional group contains at least one element selected from O, S, N, and F as a constituent element. Specific examples of the polymer compound include polyvinyl formal, polyvinyl alcohol, polyvinyl acetal, polyvinyl butyral, polymethyl methacrylate, and polytetrafluoroethylene. The molecular weight of the binder and the content of the binder in the inorganic particle film are not particularly limited and can be set as desired.

[0129] The fibrous substance is one or more types of fibrous material. The fibrous material preferably contains at least one type of hydrophilic functional group. Specific examples of hydrophilic functional groups include hydroxyl groups, sulfonic groups, and carboxyl groups. Specific examples of fibrous materials 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. An example of a cationic derivative of polystyrene is polystyrene trialkylbenzylammonium. The fibrous material is preferably cellulose fibers. The fibrous material may be a derivative of the above-mentioned fibrous material or a copolymer consisting of two or more types of monomers that constitute the above-mentioned fibrous material. This facilitates the incorporation of the electrolyte solution 40 between two or more fibrous materials containing hydrophilic functional groups. Therefore, the separator 70 is more likely to swell when impregnated with the electrolyte solution 40. The average fiber diameter of the fibrous material and the content of the fibrous material in the inorganic particle film are not particularly limited and can be set as desired.

[0130] Here, the separator 70 may be a laminate in which polymer compound films and inorganic particle films are alternately stacked. In this case, the number of stacked polymer compound films and inorganic particle films is not particularly limited and can be set as desired.

[0131] When producing an inorganic particle film, first, an inorganic particle film, a binder, and a fibrous material are introduced into a solvent such as an organic solvent to prepare a slurry. Then, the slurry is poured into a mold. Finally, the slurry is dried to volatilize the solvent, and the mold is removed. This allows the production of an inorganic particle film containing inorganic particles, a binder, and a fibrous material.

[0132] In the lithium ion secondary battery 1B of the first modified example, lithium ions can also move through the separator 70 between the positive electrode 20 and the negative electrode 30, so that the same effect as that of the lithium ion secondary battery 1 of the first embodiment shown in Figure 1 can be obtained.

[0133] [Second Modification] In the second embodiment, the partition walls 50 may be the inorganic particle film described in the first modification. Details regarding the inorganic particle film are as described above.

[0134] In the second modified example, lithium ions can also move between the positive electrode 20 and the negative electrode 30 via the partition wall 50, and therefore the same effects as those of the lithium ion secondary battery 1A of the second embodiment shown in FIG. 4 can be obtained.

[0135] [Third Modification] Fig. 6 is a schematic cross-sectional view showing the configuration of a lithium-ion secondary battery according to a third modification. As shown in Fig. 6, the lithium-ion secondary battery 1C according to the third modification further includes a separator 70 and electrolyte layers 81 and 82, which are gel electrolytes. The configuration of the lithium-ion secondary battery 1C shown in Fig. 6 is the same as the configuration of the lithium-ion secondary battery 1 according to the first embodiment shown in Fig. 1, except as described below.

[0136] As described above, the separator 70 is disposed between the positive electrode 20 and the negative electrode 30. The electrolyte layer 81 is disposed between the positive electrode 20 and the separator 70. The electrolyte layer 82 is disposed between the negative electrode 30 and the separator 70. As a result, the electrolyte layer 81 is adjacent to both the positive electrode 20 and the separator 70. The electrolyte layer 82 is adjacent to both the negative electrode 30 and the separator 70.

[0137] Each of the electrolyte layers 81 and 82 contains a polymer compound together with the electrolytic solution 40, and the electrolytic solution 40 is held by the polymer compound. The type of polymer compound is not particularly limited, and specifically, it is at least one of polyvinylidene fluoride, polyethylene oxide, etc. In Figure 6, each of the electrolyte layers 81 and 82 is lightly shaded.

[0138] The separator 70 can be similar in detail to the separator 70 of the first modified example, except that it is an insulating porous membrane that separates the electrolyte layers 81 and 82 while allowing lithium ions to pass through. The separator 70 has at least one of a polymer compound membrane and an inorganic particle membrane. The polymer compound membrane contains at least one type of polymer compound such as polyolefin. Specific examples of polyolefin include polyethylene and polypropylene. The inorganic particle membrane can be the same as that described in the first modified example.

[0139] When forming the electrolyte layer 81, the electrolytic solution 40, the polymer compound, and a dilution solvent are mixed to prepare a sol-like precursor solution, and then the precursor solution is applied to the surface of the positive electrode 20. The procedure for forming the electrolyte layer 82 is the same as the procedure for forming the electrolyte layer 81, except that the precursor solution is applied to the surface of the negative electrode 30.

[0140] In the lithium ion secondary battery 1C of the second modified example, lithium ions are also able to move through the electrolyte layers 81, 82 between the positive electrode 20 and the negative electrode 30, and therefore the same effects as those of the lithium ion secondary battery 1 of the first embodiment shown in Fig. 1 can be obtained. In particular, the lithium ion secondary battery 1C of the second modified example can prevent leakage of the electrolyte.

[0141] [Fourth Modification] Fig. 7 is a schematic cross-sectional view showing the configuration of a lithium ion secondary battery according to a fourth modification. In the second embodiment, as shown in Fig. 7, a lithium ion secondary battery 1D according to the fourth modification differs from the lithium ion secondary battery 1A according to the second embodiment in that it includes electrolyte layers 91, 92, which are gel electrolytes, instead of the positive electrode electrolyte 61 and the negative electrode electrolyte 62. The configuration of the lithium ion secondary battery 1D shown in Fig. 7 is the same as the configuration of the lithium ion secondary battery 1A shown in Fig. 4, except as described below.

[0142] The electrolyte layer 91 is disposed between the positive electrode 20 and the partition wall 50. The electrolyte layer 92 is disposed between the negative electrode 30 and the partition wall 50. As a result, the electrolyte layer 91 is adjacent to both the positive electrode 20 and the partition wall 50. The electrolyte layer 92 is adjacent to both the negative electrode 30 and the partition wall 50.

[0143] The electrolyte layer 91 includes a cathode electrolyte 61 and a polymer compound. The cathode electrolyte 61 is held by the polymer compound. The electrolyte layer 92 includes an anode electrolyte 62 and a polymer compound. The anode electrolyte 62 is held by the polymer compound. The type of polymer compound may be the same as the polymer compound described in the second modified example. In FIG. 7 , the electrolyte layer 91 including the cathode electrolyte 61 is lightly shaded, and the electrolyte layer 92 including the anode electrolyte 62 is darkly shaded.

[0144] When forming the electrolyte layer 91, a sol-like precursor solution is prepared by mixing the positive electrode electrolyte 61, a polymer compound, and a dilution solvent, and the precursor solution is then applied to the surface of the positive electrode 20. When forming the electrolyte layer 92, a sol-like precursor solution is prepared by mixing the negative electrode electrolyte 62, a polymer compound, and a dilution solvent, and the precursor solution is then applied to the surface of the negative electrode 30.

[0145] The details of the configuration of the partition walls 50 are as described in the second embodiment. However, the partition walls 50 may be the inorganic particle film described in the first modified example. The inorganic particle film may be the same as that described in the second embodiment and the first modified example.

[0146] In the lithium ion secondary battery 1D of the fourth modification, lithium ions are also able to move between the positive electrode 20 and the negative electrode 30 via the electrolyte layers 91, 92, and therefore the same effects as those of the lithium ion secondary battery 1A of the second embodiment shown in Fig. 4 can be obtained. The lithium ion secondary battery 1D of the fourth modification can particularly prevent leakage of the electrolyte.

[0147] 4. Uses of Lithium-Ion Secondary Batteries There are no particular limitations on the uses (application examples) of lithium-ion secondary batteries. Lithium-ion secondary batteries used as power sources may be the main power source for electronic devices, electric vehicles, and the like, or may be an auxiliary power source. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source is a power source that is used in place of the main power source, or a power source that can be switched from the main power source.

[0148] Specific examples of uses for lithium ion secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals; Storage devices such as backup power supplies and memory cards; Power tools such as power drills and power saws; Battery packs installed in electronic devices; Medical electronic devices such as pacemakers and hearing aids; Electric vehicles such as electric cars including hybrid cars; Power storage systems such as home or industrial battery systems that store power in preparation for emergencies, etc. In these uses, one lithium ion secondary battery may be used, or multiple lithium ion secondary batteries may be used.

[0149] The battery pack may use a single cell or a battery pack. The electric vehicle is a vehicle that operates (travels) using a lithium-ion secondary battery as its driving power source, and may be a hybrid vehicle that also has a driving source other than the lithium-ion secondary battery. In a home power storage system, power stored in a lithium-ion secondary battery, which is a power storage source, can be used to power household electrical appliances, etc.

[0150] Of course, the lithium ion secondary battery may be used for purposes other than the series of applications exemplified here.

[0151] An embodiment of the present invention will now be described.

[0152] Examples 1 to 5 and Comparative Examples 1 to 3 After an electrochemical measurement cell was fabricated using the negative electrode 30, the charge / discharge characteristics of the negative electrode 30 were evaluated.

[0153] [Fabrication of Electrochemical Measurement Cells] In Examples 1 to 4, electrochemical measurement cells having substantially the same configuration as the single-liquid type lithium ion secondary battery (FIG. 1) described in the first embodiment were fabricated by the procedure described below.

[0154] (Fabrication of Negative Electrode) First, 100 parts by mass of a plurality of negative electrode active material particles 31 containing anatase-type titanium oxide (TiO2) as the negative electrode active material and 5 parts by mass of methyl cellulose as the negative electrode binder were dispersed in water and mixed to obtain a paste. The average particle diameters AS (nm) of the plurality of negative electrode active material particles 31 in Examples 1 to 4 are as shown in Table 1.

[0155] Next, using a coating device, the paste was applied to a mesh-like titanium foil (200 μm thick) as the negative electrode current collector 30A, dried, and then pressed to obtain a molded body. Furthermore, the molded body was fired in air. As a result, the plurality of negative electrode active material particles 31 were directly bonded to each other, and negative electrode active material layers 30B, which were sintered bodies of the plurality of negative electrode active material particles 31, were formed on both sides of the negative electrode current collector 30A. Here, by appropriately adjusting the pressing pressure and the firing temperature within the range of 500°C or higher and 650°C or lower, negative electrodes 30 according to Examples 1 to 4 with different pore distributions were obtained.

[0156] In Example 5 and Comparative Example 3, instead of a plurality of negative electrode active material particles 31 containing anatase-type titanium oxide, lithium titanium composite oxide (Li4Ti5O 12 A negative electrode 30 was fabricated in the same manner as in Examples 1 to 4, except that a plurality of negative electrode active material particles 31 containing LTO (Lithium Toluene Oxide) was used.

[0157] In Comparative Example 1, the negative electrode 30 was fabricated by applying a negative electrode mixture slurry instead of firing the negative electrode 30. The detailed manufacturing method will be described below. First, lithium titanium composite oxide (Li4Ti5O 12A negative electrode mixture was prepared by mixing 91 parts by mass of polyvinylidene fluoride (LTO), 3 parts by mass of polyvinylidene fluoride as a negative electrode binder, and 6 parts by mass of graphite as a negative electrode conductive agent. Subsequently, the negative electrode mixture was added to an organic solvent, N-methyl-2-pyrrolidone, and the solvent was stirred to prepare a paste-like negative electrode mixture slurry. Finally, using a coating device, similar to the negative electrodes according to Examples 1 to 4, the negative electrode mixture slurry was applied to both sides of a mesh-like titanium foil (thickness 200 μm) as a negative electrode current collector 30A, excluding the connection terminal portion 30AT, and then dried to form a negative electrode active material layer 30B. Thus, a negative electrode 30 was produced. The average particle size AS (nm) of the plurality of negative electrode active material particles 31 is as shown in Table 1.

[0158] (Measurement of Pore Volume of Negative Electrode) The pore volume of the negative electrode active material layer 30B of the negative electrode 30 fabricated by the above method was measured by both mercury porosimetry and gas adsorption. More specifically, the negative electrode 30 for measurement was fabricated by the same manufacturing method as in each of Examples 1 to 5 and Comparative Examples 1 to 3. Then, the negative electrode active material layer 30B was taken out from the fabricated negative electrode 30 for measurement to serve as a measurement sample.

[0159] In the measurement of pore volume by mercury intrusion porosimetry, a mercury intrusion porosimeter (AutoPore IV manufactured by Micromeritics) was used as the measuring device, and the pore size distribution of the differential pore volume was measured in the pore size range of 0.005 μm to 50 μm. The measurement sample was dried under reduced pressure at a temperature of 200°C for 30 minutes, then appropriately cut and introduced into a measurement cell, and the filling pressure was 0.22 lb / in. ２ Mercury intrusion measurement was performed from a pressure of 1517 Pa (1517 Pa), and the pore distribution was analyzed using the attached analysis software. The pore volume V was calculated by summing up the differential pore volumes in the pore diameter range of 0.005 μm to 50 μm. Ｔ The pore volume obtained by summing up the differential pore volumes in the pore diameter range of 0.01 μm to 0.1 μm was referred to as the pore volume V Ｔ By dividing by , the first pore volume ratio R Ｈ was calculated.

[0160] In the measurement of pore volume by gas adsorption method, a gas adsorption measuring device (Microtrack Bell, Belsorp II) was used as the measuring device, and the pore size distribution of the differential pore volume was measured in the pore diameter range of 10 nm to 100 nm. In the measurement of pore volume by gas adsorption method, as a pretreatment, the measurement cell containing the measurement sample was vacuum degassed at a temperature of 200 ° C. for 30 minutes, and measured using nitrogen gas at liquid nitrogen temperature (77 K). Then, using commercially available software (Microtrack Bell, BELMaster), the pore volumes were calculated by summing the differential pore volumes in the pore diameter range of 10 nm to 100 nm and the pore diameter range of 20 nm to 50 nm based on the BJH analysis method. Then, the pore volume for the pore diameter range of 20 nm to 50 nm was divided by the pore volume for the pore diameter range of 10 nm to 100 nm, to obtain the second pore volume ratio R Ｇ was calculated.

[0161] (Preparation of Electrolyte Solution) In Examples 1 to 5 and Comparative Example 1, an electrolyte salt was added as an ionic substance to water, which was an aqueous solvent, and the solvent was then stirred to prepare an aqueous electrolyte solution 40. Lithium hydroxide (LiOH) was used as the electrolyte salt. The concentration of the electrolyte salt in the electrolyte solution 40 was adjusted to 4.0 mol / kg.

[0162] In Comparative Examples 2 and 3, a nonaqueous electrolyte solution was prepared by adding an electrolyte salt to a nonaqueous solvent (EC-DMC) prepared by mixing ethylene carbonate (EC) and dimethyl carbonate (DMC) in a mass ratio of 1:1, and then stirring the solvent. Lithium hexafluorophosphate (LiPF) was used as the electrolyte salt. The concentration of the electrolyte salt in the nonaqueous electrolyte solution was adjusted to 1.0 mol / kg.

[0163] (Assembly of electrochemical measurement cell) First, the positive electrode 20 and the negative electrode 30 were housed in the internal space S of a glass beaker as a glass exterior body 10. Nickel metal foil was used for the positive electrode 20. Furthermore, the connection terminals 20AT and 30AT were each drawn from the inside of the exterior body 10 to the outside. Next, a silver / silver chloride (Ag / AgCl) electrode was placed in the internal space S as a reference electrode. Finally, the electrolyte solution 40 or a nonaqueous electrolyte solution was supplied to the internal space S. As a result, the electrolyte solution 40 was housed in the internal space S, and the electrochemical measurement cell was completed.

[0164] [Evaluation of Negative Electrode Characteristics] When the charge / discharge characteristics were evaluated as the operating characteristics of the negative electrode 30, the results shown in Table 1 were obtained. In the "Discharge Possible" column in Table 1, "Y" indicates that discharge was possible, and "N" indicates that discharge was not possible.

[0165]

[0166] In Examples 1 to 4 and Comparative Example 1, the discharge capacity and charge / discharge efficiency were measured by the following method. The electrochemical measurement cell was charged and discharged in a room temperature (23°C) environment. During charging, the cell was charged at a constant current of 1 C until the voltage relative to the reference electrode reached a cutoff potential (-1.45 V), and then charged at a constant voltage until the current reached 0.5 C at the cutoff potential. During discharging, the cell was discharged at a constant current of 1 C until the voltage relative to the reference electrode reached a cutoff potential (-1.00 V). This allowed the discharge capacity (mAh / g) per unit mass of the negative electrode active material (plurality of negative electrode active material particles 31), which is an index for evaluating charge / discharge characteristics, to be measured.

[0167] Then, the charge / discharge efficiency, which is an index for evaluating the charge / discharge characteristics, was calculated based on the formula: charge / discharge efficiency (%)=(discharge capacity / charge capacity)×100.

[0168] In Example 5, the discharge capacity and charge / discharge efficiency were measured in the same manner as in Examples 1 to 4 and Comparative Example 1, except that the cutoff potentials during charging and discharging were changed to −1.8 V and −1.0 V, respectively.

[0169] In Comparative Examples 2 and 3, the discharge capacity could not be obtained because the cutoff potential was reached quickly during discharge, and therefore the discharge capacity and charge / discharge efficiency could not be measured.

[0170] As shown in Table 1, Examples 1 to 5, in which the electrolyte solution was an aqueous solvent, were able to discharge, whereas Comparative Examples 2 and 3, in which the electrolyte solution was an organic solvent, were unable to discharge. This shows that the negative electrode of the present disclosure functions in aqueous lithium ion secondary batteries but does not function in nonaqueous lithium ion secondary batteries.

[0171] As shown in Table 1, the first pore volume ratio R Ｈ In Examples 1 to 5, the first pore volume ratio R Ｈ The discharge capacity and charge / discharge efficiency were improved compared to Comparative Example 1 in which the first pore volume ratio R Ｈ It can be seen that the charge-discharge characteristics can be improved by making the ratio 0.5 or more.

[0172] As shown in Table 1, the first pore volume ratio R Ｈ In Examples 1 to 4, the first pore volume ratio R Ｈ The charge-discharge efficiency was improved compared to Example 5 in which the first pore volume ratio R Ｈ It can be seen that the charge-discharge characteristics can be further improved by making the ratio 0.7 or more.

[0173] As shown in Table 1, in Examples 1 to 4 in which the crystalline structure of the negative electrode active material was tetragonal, the charge-discharge efficiency was improved compared to Example 5 in which the crystalline structure of the negative electrode active material was cubic. Ｈ It can be seen that the charge-discharge characteristics can be further improved by making the ratio 0.7 or more.

[0174] As shown in Table 1, the second pore volume ratio R Ｇ In Examples 1 to 3, the second pore volume ratio R Ｇ The charge-discharge efficiency was improved compared to Example 4 in which the second pore volume ratio R Ｇ It can be seen that the charge-discharge characteristics can be further improved by making the ratio 0.5 or more.

[0175] Examples 6 and 7 Furthermore, after a lithium ion secondary battery was fabricated using the negative electrode 30, the charge / discharge characteristics of the lithium ion secondary battery were evaluated.

[0176] [Fabrication of Lithium Ion Secondary Battery] The two-liquid type lithium ion secondary battery described in the second embodiment shown in FIG. 4 was fabricated according to the procedure described below.

[0177] (Preparation of Positive Electrode) First, 91 parts by mass of LiFePO4, a lithium phosphate compound, was used as the positive electrode active material, 3 parts by mass of polyvinylidene fluoride as a positive electrode binder, and 6 parts by mass of graphite as a positive electrode conductive agent were mixed to form a positive electrode mixture. Subsequently, the positive electrode mixture was added to N-methyl-2-pyrrolidone, an organic solvent, and the solvent was stirred to prepare a paste-like positive electrode mixture slurry. Finally, using a coating device, the positive electrode mixture slurry was applied to both sides of the positive electrode current collector 20A, excluding the connection terminal portion 20AT of a 10 μm-thick titanium foil, and then dried to form a positive electrode active material layer 20B. Thus, the positive electrode 20 was prepared.

[0178] (Fabrication of Negative Electrode) In Example 6, a negative electrode was fabricated using anatase-type titanium oxide as the material for forming the plurality of negative electrode active material particles 31, as in Example 2.

[0179] In Example 7, the same negative electrode as in Example 5 was produced using lithium titanium composite oxide as the material for forming the plurality of negative electrode active material particles 31 .

[0180] (Preparation of Positive Electrolyte Solution) Lithium sulfate (LiSO4), an electrolyte salt, was added as an ionic substance to pure water, an aqueous solvent, and the mixture was stirred to prepare a water-based electrolyte solution, positive electrode electrolyte solution 61. Positive electrode electrolyte solution 61 was prepared so that the concentration of the electrolyte salt was 2 mol / kg and the pH was 5.

[0181] (Preparation of Negative Electrolyte) The same electrolyte 40 as in Examples 1 to 4 was used as the negative electrode electrolyte 62. The negative electrode electrolyte 62 was prepared so that the concentration of the electrolyte salt was 4 mol / kg and the pH was 12.

[0182] (Assembly of Lithium-Ion Secondary Battery) First, a glass container was prepared as the exterior body 10, with a cation exchange membrane Nafion 115 (registered trademark) (manufactured by Sigma-Aldrich Japan LLC) attached thereto as the partition wall 50. Inside the exterior body 10, the positive electrode chamber S1 and the negative electrode chamber S2 were separated from each other via the partition wall 50. Next, the positive electrode 20 was housed inside the positive electrode chamber S1, and then the negative electrode 30 was housed inside the negative electrode chamber S2. Then, the connection terminals 20AT and 30AT were each extended from inside the exterior body 10 to the outside. Next, a silver / silver chloride (Ag / AgCl) electrode was installed as a reference electrode in the negative electrode chamber S2. Next, a positive electrode electrolyte 61 was supplied into the positive electrode chamber S1, and a negative electrode electrolyte 62 was supplied into the negative electrode chamber S2. As a result, the positive electrode electrolyte 61 was accommodated inside the positive electrode chamber S1, and the negative electrode electrolyte 62 was accommodated inside the negative electrode chamber S2, completing a dual liquid type lithium ion secondary battery.

[0183] [Evaluation of Lithium Ion Secondary Battery Characteristics] In Example 6, the charge / discharge characteristics of the lithium ion secondary battery were evaluated according to the procedure described below, and the results shown in Table 2 were obtained.

[0184]

[0185] (Charge / Discharge Characteristics) First, the charge capacity was measured by charging the lithium ion secondary battery in a room temperature (23° C.) environment. In Example 6, the battery was charged at a constant current of 2 C until the voltage relative to the reference electrode reached −1.45 V. In Example 7, the battery was charged at a constant current of 2 C until the voltage relative to the reference electrode (charge end voltage (negative electrode)) reached −1.45 V.

[0186] Subsequently, the lithium ion secondary battery was discharged under the same environment as that during charging to measure the discharge capacity. During discharge, regardless of the type of material forming the plurality of negative electrode active material particles 31, constant current discharge was performed at a current of 2 C until the voltage relative to the reference electrode (discharge end voltage (negative electrode)) reached −1 V.

[0187] Finally, the charge / discharge efficiency, which is an index for evaluating the charge / discharge characteristics, was calculated based on the formula: charge / discharge efficiency (%)=(discharge capacity / charge capacity)×100.

[0188] In Example 7, as shown in Table 2, the charge-discharge efficiency was measured in the same manner as in Example 6, except that constant current charging was performed until the voltage relative to the reference electrode (charge end voltage (negative electrode)) reached −1.8 V.

[0189] As shown in Table 2, in Example 6, in which tetragonal titanium oxide was used as the negative electrode active material, the charge-discharge efficiency was improved compared to Example 7, in which cubic LTO was used as the negative electrode active material. This shows that the charge-discharge characteristics can be improved by using tetragonal titanium oxide as the negative electrode active material.

[0190] The configuration of the lithium ion secondary battery of the present invention has been described above with reference to an embodiment and examples. However, the configuration of the lithium ion secondary battery of the present invention is not limited to the configuration described in the embodiment and examples, and various modifications are possible.

[0191] The effects described in this specification are merely examples, and the effects of the present invention are not limited to the effects described in this specification. Therefore, other effects may be obtained with respect to the present invention.

[0192] The present invention may take the following forms: (1) A lithium ion secondary battery comprising: a positive electrode that absorbs and releases lithium ions; a negative electrode that absorbs and releases lithium ions; and an electrolyte solution containing an aqueous solvent, wherein the negative electrode comprises a negative electrode active material layer containing a negative electrode active material, the negative electrode active material including at least one of titanium oxide and a lithium-titanium composite oxide, wherein the pore volume of the negative electrode active material layer measured by mercury porosimetry, the pore volume for pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.1 mL / g or more, and wherein the ratio of the pore volume of the negative electrode active material layer measured by mercury porosimetry, the pore volume of pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to the pore volume of pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.5 or more. (2) The lithium ion secondary battery according to (1), wherein the ratio of the pore volume of the negative electrode active material layer measured by mercury porosimetry, where the ratio of the pore volume of pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to the pore volume of pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.7 or more. (3) The lithium ion secondary battery according to (1) or (2), wherein the ratio of the pore volume of the negative electrode active material layer measured by gas adsorption, where the ratio of the pore volume of pores having a pore diameter of 20 nm or more and 50 nm or less to the pore volume of pores having a pore diameter of 10 nm or more and 100 nm or less, is 0.5 or more. (4) The lithium ion secondary battery according to any one of (1) to (3), wherein at least one of the titanium oxide and the lithium titanium composite oxide has at least one of a tetragonal crystal structure and an orthorhombic crystal structure. (5) The lithium ion secondary battery according to any one of (1) to (4), further comprising: a positive electrode chamber accommodating the positive electrode, an anode chamber accommodating the anode, and a partition wall disposed between the positive electrode chamber and the anode chamber and allowing the lithium ions to pass through, wherein the electrolyte includes a positive electrode electrolyte accommodated in the positive electrode chamber and an anode electrolyte accommodated in the anode chamber, and a pH of the anode electrolyte is higher than a pH of the positive electrode electrolyte. (6) The lithium ion secondary battery according to any one of (1) to (5), wherein the anode active material layer has a porous structure.(7) The lithium ion secondary battery according to any one of (1) to (6), wherein the negative electrode active material layer is a sintered body of a plurality of particles of the negative electrode active material.

[0193] REFERENCE SIGNS LIST 1 Lithium ion secondary battery 10 Exterior body 20 Positive electrode 30 Negative electrode 40 Electrolyte 50 Partition wall 62 Negative electrode electrolyte 61 Positive electrode electrolyte 70 Separator 81, 82, 91, 92 Electrolyte layer S1 Positive electrode chamber S2 Negative electrode chamber

Claims

1. A lithium ion secondary battery comprising: a positive electrode that absorbs and releases lithium ions; a negative electrode that absorbs and releases the lithium ions; and an electrolyte solution containing an aqueous solvent, wherein the negative electrode comprises a negative electrode active material layer containing a negative electrode active material, the negative electrode active material including at least one of titanium oxide and a lithium titanium composite oxide, wherein the pore volume of the negative electrode active material layer measured by mercury porosimetry, the pore volume for pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.1 mL / g or more, and wherein the ratio of the pore volume of the negative electrode active material layer measured by mercury porosimetry, the pore volume of pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to the pore volume of pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.5 or more.

2. The lithium-ion secondary battery according to claim 1, wherein the ratio of the pore volume of the negative electrode active material layer measured by mercury porosimetry, in which the ratio of the pore volume of pores having a pore diameter of 0.01 μm or more and 0.1 μm or less to the pores having a pore diameter of 0.005 μm or more and 50 μm or less, is 0.7 or more.

3. The lithium ion secondary battery according to claim 1 or 2, wherein the ratio of the pore volume of the negative electrode active material layer measured by a gas adsorption method, in which the ratio of the pore volume of pores having a pore diameter of 20 nm or more and 50 nm or less to the pore volume of pores having a pore diameter of 10 nm or more and 100 nm or less, is 0.5 or more.

4. The lithium ion secondary battery according to any one of claims 1 to 3, wherein at least one of the titanium oxide and the lithium titanium composite oxide has at least one of a tetragonal and an orthorhombic crystal structure.

5. The lithium ion secondary battery according to any one of claims 1 to 4, further comprising: a positive electrode chamber that houses the positive electrode; an anode chamber that houses the anode; and a partition wall that is disposed between the positive electrode chamber and the anode chamber and that allows the lithium ions to pass through, wherein the electrolyte includes a positive electrode electrolyte housed inside the positive electrode chamber and a negative electrode electrolyte housed inside the anode chamber, and the pH of the negative electrode electrolyte is higher than the pH of the positive electrode electrolyte.

6. The lithium ion secondary battery according to any one of claims 1 to 5, wherein the negative electrode active material layer has a porous structure.

7. The lithium ion secondary battery according to any one of claims 1 to 6, wherein the negative electrode active material layer is a sintered body of a plurality of particles of the negative electrode active material.

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