Secondary battery

Abstract

To provide a secondary battery capable of improving discharge characteristics.SOLUTION: A secondary battery includes a positive electrode containing a croconic acid-based compound as a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolytic solution containing water as a solvent. The electrolyte solution contains lithium bis (trifluoromethanesulfonyl) imide (LiTFSI) and lithium bis (pentafluoroethanesulfonyl) imide (LiBETI), a ratio of an amount of substance of LiTFSI to a sum of amounts of substance of LiTFSI and LiBETI in the electrolyte solution is not lower than 0.6 and not higher than 0.8, and a ratio of an amount of substance of water to the sum of the amounts of substance of LiTFSI and LiBETI in the electrolyte solution is not lower than 1.8 and not higher than 4.0.SELECTED DRAWING: Figure 1

Description

[Technical Field]

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

[0002] Non-Patent Document 1 discloses a secondary battery that uses lithium croconic acid salt as a positive electrode active material for a non-aqueous battery. [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] Katsuyama Y, Kobayashi H, Iwase K, Gambe Y, Honma I. Adv Sci. 2022; 9:2200187. Summary of the Invention [Problem to be solved by the invention]

[0004] However, when an organic salt such as that disclosed in Non-Patent Document 1 is used as a positive electrode active material in a secondary battery using an aqueous electrolyte, the positive electrode active material may dissolve into the aqueous electrolyte, resulting in a deterioration in discharge characteristics.

[0005] The present invention has been made in view of the above problems, and an object of the present invention is to provide a secondary battery capable of improving discharge characteristics. [Means for solving the problem]

[0006] A secondary battery according to one embodiment of the present invention includes a positive electrode containing a croconic acid compound as a positive electrode active material, a negative electrode containing a negative electrode active material, and an electrolyte solution containing water as a solvent, the electrolyte solution containing lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), wherein in the electrolyte solution, the ratio of the amount of substance of the LiTFSI to the sum of the amounts of substance of the LiTFSI and the LiBETI is 0.6 or more and 0.8 or less, and the ratio of the amount of substance of the water to the sum of the amounts of substance of the LiTFSI and the LiBETI is 1.8 or more and 4.0 or less. [Effects of the Invention]

[0007] According to the present invention, a secondary battery capable of improving discharge characteristics can be provided. [Brief explanation of the drawings]

[0008] [Figure 1] FIG. 1 is a schematic cross-sectional view showing the configuration of a secondary battery according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic cross-sectional view showing the configuration of a secondary battery according to a first modified example. DETAILED DESCRIPTION OF THE INVENTION

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

[0010] <1. Embodiment (Secondary Battery)> First, a secondary battery according to an embodiment of the present invention will be described.

[0011] The secondary battery described in this disclosure is a secondary battery in which charge / discharge reactions proceed using the absorption and release of carrier ions, such as lithium ions, in a positive electrode. The secondary battery of this disclosure includes a positive electrode, a negative electrode, and an aqueous electrolyte. The aqueous electrolyte is an electrolyte containing an aqueous solvent. In this disclosure, the aqueous solvent refers to a solvent containing water molecules (HO).

[0012] <1-1.Configuration> FIG. 1 is a schematic cross-sectional view showing the configuration of a secondary battery according to an embodiment of the present invention.

[0013] As shown in Fig. 1, the secondary battery 1 includes an exterior body 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 a positive electrode 20, a negative electrode 30, an electrolyte solution 40, and the like, and has an internal space S.

[0015] Exterior body 10 includes at least one of a metal material, a glass material, a polymer compound, etc. 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, that is, the side where the positive electrode 20 faces the negative electrode 30. The positive electrode current collector 20A may also 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 hardly 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 contains 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 due to use of the 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, the positive electrode active material layer 20B is not provided on the connection terminal portion 20AT, which is a part of the positive electrode current collector 20A. The connection terminal portion 20AT is led out from the internal space S of the exterior body 10 to the outside.

[0019] (Cathode active material layer) The positive electrode active material layer 20B contains a positive electrode active material that absorbs and releases lithium ions, but may further contain at least one of a positive electrode binder and a positive electrode conductive agent.

[0020] The positive electrode active material includes a croconic acid-based compound. In this disclosure, the croconic acid-based compound refers to at least one of croconic acid and a croconic acid salt. Examples of croconic acid salts include dilithium croconic acid and disodium croconic acid salt. During charge / discharge reactions, the croconic acid salt releases cations such as lithium to form cyclopentane pentone, which then reacts with the cations to form croconic acid salt again. This reaction enables the croconic acid salt to absorb and release carrier ions such as lithium ions and to widen the potential window of the positive electrode active material. This allows for the realization of a metal-free positive electrode active material, reducing manufacturing costs, while also widening the potential window and improving discharge performance.

[0021] The positive electrode binder is not particularly limited and may include, for example, at least one of synthetic rubber and polymer compounds. Specific examples of synthetic rubber include styrene-butadiene rubber. Specific examples of polymer compounds include polyvinylidene fluoride and polyimide.

[0022] The positive electrode conductive agent is not particularly limited and may include, for example, at least one conductive material such as a carbon material, specific examples of which 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.

[0023] [Negative electrode] As shown in FIG. 1, the negative electrode 30 is disposed in the internal space S and absorbs and releases cations such as lithium ions. Here, the negative electrode 30 includes a negative electrode current collector 30A and a negative electrode active material layer 30B. 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.

[0024] (Negative electrode current collector) The negative electrode current collector 30A is a conductive support member that supports the negative electrode active material layer 30B, and has a pair of surfaces that support the negative electrode active material layer 30B. The negative electrode current collector 30A contains at least one conductive material selected from the group consisting of a metal material, a carbon material, and a conductive ceramic material.

[0025] Specific examples of metal materials include stainless steel (SUS), titanium, zinc, tin, lead, and alloys thereof. The stainless steel may be a highly corrosion-resistant stainless steel to which at least one of niobium, molybdenum, and other additive elements has been added. Specifically, the stainless steel may be SUS444 to which molybdenum has been added as an additive element. Details regarding the conductive ceramic material are as described above.

[0026] The material forming the negative electrode current collector 30A is preferably insoluble or hardly soluble in the electrolyte 40 and corrosion-resistant, and also has low reactivity with the negative electrode active material described below. Therefore, the negative electrode current collector 30A preferably contains a metal material. This suppresses deterioration of the negative electrode current collector 30A during charge and discharge.

[0027] The negative electrode current collector 30A may be a conductor whose surface is plated with the above-mentioned conductive material. The material for forming the conductor is not particularly limited and can be selected arbitrarily.

[0028] Here, the negative electrode current collector 30A includes a connection terminal portion 30AT on which the negative electrode active material layer 30B is not provided, and the connection terminal portion 30AT is led out from the inside (internal space S) of the exterior body 10 to the outside.

[0029] (Negative electrode active material layer) The negative electrode active material layer 30B is a layer that absorbs and releases cations such as lithium ions. Here, the negative electrode active material layer 30B is provided on both sides of the negative electrode current collector 30A. However, the negative electrode active material layer 30B may be provided on only one side of the negative electrode current collector 30A, on the side where the negative electrode 30 faces the positive electrode 20.

[0030] The negative electrode active material layer 30B may contain at least one negative electrode active material that absorbs and releases cations such as lithium ions, etc. However, the negative electrode active material layer 30B may further contain at least one of a negative electrode binder and a negative electrode conductive agent.

[0031] Specifically, the negative electrode active material that absorbs and releases lithium ions is lithium titanate, titanium oxide, carbon material, metal material, etc. A specific example of lithium titanate is Li4Ti5O 12 Specific examples of titanium oxide include anatase-type titanium oxide, rutile-type titanium oxide, and brookite-type titanium oxide. Specific examples of carbon materials include graphite. Metal-based materials are materials that contain, as a constituent element, at least one of metal elements and semimetal elements that can form an alloy with lithium.

[0032] Furthermore, the negative electrode active material layer 30B may contain at least one other negative electrode active material that absorbs and releases cations other than lithium ions, instead of the negative electrode active material that absorbs and releases lithium ions. The type of cations other than lithium ions may be one type or two or more types.

[0033] Specifically, other negative electrode active materials that absorb and release cations other than lithium ions include titanium-containing compounds, niobium-containing compounds, vanadium-containing compounds, iron-containing compounds, molybdenum-containing compounds, and zinc.

[0034] Titanium-containing compounds include alkali metal titanium composite oxides and titanium phosphates. Alkali metal titanium composite oxides include potassium titanium composite oxides, and specific examples of potassium titanium composite oxides include K2Ti3O7 and K4Ti5O 12 Specific examples of titanium phosphate oxides include TiP2O7 and NaTi2(PO4)3.

[0035] Niobium-containing compounds include hydrogen niobium compounds and titanium-niobium composite oxides. Specific examples of hydrogen niobium compounds include H4Nb6O 17Specific examples of titanium-niobium composite oxides include TiNb2O7 and Ti2Nb 10 O 29 And so on.

[0036] The vanadium-containing compound is vanadium oxide, etc. A specific example of the vanadium oxide is vanadium dioxide (VO2).

[0037] The iron-containing compound is an iron hydroxide, etc. A specific example of the iron hydroxide is iron oxyhydroxide (FeOOH).

[0038] Molybdenum-containing compounds include molybdenum oxide and cobalt-molybdenum composite oxide. A specific example of molybdenum oxide is molybdenum dioxide (MoO2). A specific example of cobalt-molybdenum composite oxide is CoMoO4.

[0039] Negative electrode active material layer 30B may contain both at least one type of negative electrode active material that absorbs and releases lithium ions and at least one type of other negative electrode active material that absorbs and releases cations other than lithium ions.

[0040] [Electrolyte] The electrolyte 40 is contained in the internal space S, and as described above, is an aqueous electrolyte. 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.

[0041] The secondary battery of this embodiment is a so-called single-liquid type lithium ion secondary battery that includes one type of aqueous electrolyte (electrolyte 40).

[0042] The electrolyte solution 40 includes an aqueous solvent and a solute. The solute includes an ionic substance that can be ionized in the aqueous solvent. More specifically, the electrolyte solution 40 used in the secondary battery includes lithium ions. As a result, the lithium ions are absorbed and released in the positive electrode 20 and the negative electrode 30, respectively. The aqueous solvent is pure water. The ionic substances are lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(fluorosulfonyl)imide (LiBETI).

[0043] The electrolyte salt may 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.

[0044] In the electrolyte, the ratio of the amount of LiTFSI to the total amount of LiTFSI and LiBETI is 0.6 to 0.8, inclusive. By setting it in this range, good discharge characteristics can be obtained.

[0045] In the electrolyte, the ratio of the amount of water to the sum of the amounts of LiTFSI and LiBETI is between 1.8 and 4.0. By setting the ratio within this range, the electrolyte becomes a liquid hydrate, a so-called hydrate melt, and both the croconate salt and the cyclopentane pentone are insoluble. This prevents the positive electrode active material from leaching into the electrolyte, improving discharge characteristics.

[0046] The components of the electrolyte solution 40 and the molar ratio of each component can be determined by a composition analysis method such as inductively coupled plasma (ICP) emission spectroscopy.

[0047] <1-2. Operation> The secondary battery 1 operates as described below.

[0048] When the 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.

[0049] When the 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.

[0050] <1-3. Manufacturing method> When producing the secondary battery 1, the positive electrode 20 and the negative electrode 30 are produced and the electrolyte solution 40 is prepared according to the procedure described below as an example, and then the secondary battery 1 is assembled.

[0051] [Preparation 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 positive electrode mixture. However, the composition of the positive electrode mixture can be changed as desired. Finally, the positive electrode mixture is compression-molded with a roller to form a positive electrode active material layer 20B. 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. Finally, the positive electrode 20 is produced by stacking the positive electrode active material layer 20B on the positive electrode current collector 20A.

[0052] [Preparation of negative electrode] First, a paste-like anode mixture slurry is prepared by adding a negative electrode mixture, which is a mixture of a negative electrode active material, a negative electrode binder, and a negative electrode conductive agent, to a solvent. Next, the negative electrode mixture slurry is applied to both sides of the negative electrode current collector 30A, excluding the connection terminal portion 30AT, to form the anode active material layer 30B, which is then compression-molded. This completes the anode 30.

[0053] [Preparation of electrolyte] The electrolytic solution 40 is prepared by adding an ionic substance to an aqueous solvent. The electrolytic solution 40 may be prepared by stirring while heating.

[0054] [Secondary battery assembly] 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 drawn out from the inside of the exterior body 10 (internal space S) to the outside so that they are exposed.

[0055] Subsequently, electrolyte solution 40 is supplied into internal space S through an injection hole provided in package 10 (not shown), and then the injection hole is sealed.

[0056] 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 secondary battery using one type of aqueous electrolyte solution (electrolyte solution 40) is completed.

[0057] <1-4. Actions and Effects> As described above, the secondary battery 1 of this embodiment includes a positive electrode 20 containing a croconic acid-based compound as a positive electrode active material, a negative electrode 30 containing a negative electrode active material, and an electrolyte solution 40 containing water as a solvent. The electrolyte solution 40 contains lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI). In the electrolyte solution 40, the ratio of the amount of LiTFSI to the total amount of LiTFSI and LiBETI is 0.6 or more and 0.8 or less. In the electrolyte solution 40, the ratio of the amount of water to the total amount of LiTFSI and LiBETI is 1.8 or more and 4.0 or less. This allows the electrolyte solution 40 to assume a so-called hydrate melt state, rendering both the croconic acid salt and cyclopentane pentone insoluble in the aqueous solvent. This prevents the positive electrode active material from leaching into the electrolyte. Therefore, the metal-free positive electrode active material can reduce manufacturing costs while widening the potential window and improving discharge characteristics.

[0058] In a preferred embodiment, the croconic acid compound is lithium croconic acid, which introduces lithium, a carrier ion, into the positive electrode active material, thereby improving the charge-discharge reaction in the positive electrode 20 and the discharge characteristics.

[0059] <2. Modifications> The configuration of the 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.

[0060] [First Modification] Fig. 2 is a schematic cross-sectional view showing the configuration of a secondary battery of Modified Example 1. As shown in Fig. 2, a secondary battery 1A of Modified Example 1 further includes a separator 70 that separates the positive electrode 20 and the negative electrode 30. The configuration of the secondary battery shown in Fig. 2 is the same as the configuration of the secondary battery 1 of the embodiment shown in Fig. 1, except for the points described below.

[0061] 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.

[0062] The material of separator 70 is not particularly limited and may include, for example, at least one of polymer compounds such as polyolefin, polyethylene, and polypropylene, and may be composed of multiple layers. Separator 70 may also be a laminate in which polymer compound films and inorganic particle films in which inorganic particles are dispersed in a fibrous material are alternately stacked. Even in this case, the number of layers of each of the polymer compound films and inorganic particle films is not particularly limited and can be set as desired.

[0063] In the secondary battery 1A 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 secondary battery 1 of the embodiment shown in Figure 1 can be obtained.

[0064] <3. Uses of secondary batteries> The uses (application examples) of secondary batteries are not particularly limited. Secondary batteries used as power sources may be the main power source for electronic devices, electric vehicles, etc., or may be auxiliary power sources. 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.

[0065] Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals. Storage devices such as backup power supplies and memory cards. Power tools such as power drills and power saws. Battery packs installed in electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars, including hybrid cars. Power storage systems such as home or industrial battery systems that store power in preparation for emergencies. In these uses, one or more secondary batteries may be used.

[0066] The battery pack may use a single cell or a battery pack. The electric vehicle is a vehicle that operates (travels) using a secondary battery as a driving power source, and may be a hybrid vehicle that also has a driving source other than the secondary battery. In a home power storage system, the power stored in the secondary battery, which is a power storage source, can be used to power home electrical appliances, etc.

[0067] Of course, the secondary battery may be used for purposes other than the series of purposes exemplified here. [Example]

[0068] An embodiment of the present invention will now be described.

[0069] <Examples 1 to 4 and Comparative Examples 1 and 2> As will be described below, after the secondary batteries were manufactured, the battery characteristics of the secondary batteries were evaluated.

[0070] [Preparation of electrochemical measurement cell] To evaluate the battery characteristics, an electrochemical measurement cell having a configuration similar to that of the single-liquid secondary battery (FIG. 2) described in the first modified example was fabricated by the procedure described below.

[0071] (Preparation of positive electrode) The positive electrode active material, the positive electrode binder, and the positive electrode conductive agent were mixed together to prepare a positive electrode mixture.

[0072] Dilithium croconic acid was used as the positive electrode active material. Dilithium croconic acid was synthesized by the following method. First, croconic acid (Tokyo Chemical Industry Co., Ltd., CAS RN: 488-86-8, purity >98.0%) was dissolved in ultrapure water at a concentration of 50 mmol / L to prepare a croconic acid aqueous solution. Next, lithium hydroxide monohydrate (Kojundo Chemical Laboratory Co., Ltd., CAS RN: 1310-66-3, purity >99%) was dissolved in ultrapure water at a concentration of 100 mmol / L to prepare a lithium hydroxide aqueous solution. Next, equal amounts of lithium hydroxide aqueous solution and croconic acid aqueous solution were mixed in a volume ratio and stirred to obtain a neutralized aqueous solution. The pH of the neutralized aqueous solution was confirmed to be around 7 using a pH meter. The resulting neutralized solution was injected into acetone (FUJIFILM Wako Pure Chemical Industries, Ltd., CAS RN: 67-64-1, purity >99.5%) with a 2.5 mL syringe while stirring to precipitate dilithium croconic acid. The procedure for precipitation of dilithium croconic acid involved four injections of 2.5 mL of the neutralized solution into 200 mL of acetone, totaling 10 mL, until the desired amount of dilithium croconic acid was obtained. The precipitate was then collected by suction filtration using a 0.45 m pore size membrane filter (Merck, model number: JHWP04700). The collected precipitate was vacuum dried overnight at 110 °C, and the resulting crystals were pulverized to obtain the cathode active material.

[0073] Polytetrafluoroethylene (PTFE) powder (Mitsui-Chemours Fluoroproducts, product number: 6-J) was used as the positive electrode binder.

[0074] Carboxyl group-modified multi-walled carbon nanotubes (COOH-MWCNT, Sigma-Aldrich Japan, catalog number: 755125, chemical modification rate >8%) were used as the positive electrode conductive agent.

[0075] The positive electrode mixture was obtained using the following method. First, an aqueous solution of the positive electrode active material was prepared using 10 mg of positive electrode active material and 1 mL of ultrapure water. A positive electrode conductive agent was added to the prepared aqueous solution of the positive electrode active material so that the mass ratio of the positive electrode active material to the positive electrode conductive agent was 1:3. The resulting aqueous solution was then subjected to ultrasonic treatment to composite the positive electrode active material and the positive electrode conductive agent. This treatment transformed the aqueous solution, with the positive electrode conductive agent suspended in it, into a slurry. After ultrasonic treatment, the solution was heated overnight on a hot plate at 80°C to remove moisture. The resulting powder was crushed and mixed in an agate mortar, and then further dried in vacuum at 110°C overnight to obtain a composite powder of the positive electrode active material and the positive electrode conductive agent. A positive electrode binder was added to the resulting composite powder so that the positive electrode binder accounted for 5% by mass of the total powder, and the mixture was thoroughly dry-mixed in the agate mortar. As a result, a positive electrode mixture was obtained in which the mass ratio of the positive electrode active material, the positive electrode conductive agent, and the positive electrode binder was 23.7:71.3:5.0.

[0076] The prepared positive electrode mixture was pressed with a roller to make the thickness uniform, and then punched out to a predetermined size to prepare the positive electrode active material layer 20B.

[0077] The electrochemical measurement cell had the positive electrode active material layer prepared above, and the negative electrode was aluminum foil coated with lithium titanate (Li4Ti5O 12A mixture slurry containing 100% ammonium nitrate (Al2O3) as the active material was applied and dried, and then punched out to a predetermined size. A porous polyethylene separator was used as the separator. The electrolyte solution was prepared using LiTFSI as the first solute, LiBETI as the second solute, and water as the solvent. The electrolyte solution was prepared by mixing the first solute, the second solute, and the solvent in a molar ratio of 7:3:20, and stirring and mixing while heating at 50°C until the solute was completely dissolved in the solvent. Thereafter, the separator and the negative electrode were stacked on the positive electrode, and the electrolyte solution was poured into the electrochemical measurement cell. The electrochemical measurement cell of Example 1 was prepared using the above method.

[0078] In Examples 2 to 4 and Comparative Example 1, electrochemical measurement cells were prepared in the same manner as in Example 1, except that the amount of solvent relative to the sum of the amounts of the first solute and the second solute was set to the amounts shown in Table 1 below.

[0079] In Comparative Example 2, the electrochemical measurement cell was prepared in the same manner as in Example 1, except that the electrolyte solution used lithium hexafluorophosphate (LiPF) as the solute and the solvent used was an organic solvent (EC+DMC) in which ethylene carbonate and dimethyl carbonate were mixed at a volume ratio of 1:1, and the solute and solvent were mixed at a molar ratio of 1.0:13.4.

[0080] [Evaluation of cathode characteristics] When the discharge characteristics of the positive electrode 20 were evaluated as an operating characteristic, the results shown in Table 1 were obtained.

[0081] [Table 1]

[0082] In Examples 1 to 4 and Comparative Examples 1 and 2, the discharge capacity at 0.5 C (0.5 C discharge capacity) was measured by the following method. The electrochemical measurement cell was charged and discharged in a room temperature (25°C) environment. During charging, the cell was charged at a constant current at a current density of 0.5 C until the cell voltage reached the cutoff voltage (2.5 V). During discharging, the cell was discharged at a constant current at a current density of 0.5 C until the cell voltage reached the cutoff voltage (0.0 V). This allowed the discharge capacity per unit mass of the positive electrode active material (mAh / g), which is an index for evaluating the charge / discharge characteristics, to be measured.

[0083] In Example 2 and Comparative Example 2, the discharge capacity at 10 C (10 C discharge capacity) was also measured. The 10 C discharge capacity was measured in the same manner as the 0.5 C discharge capacity, except that the current density during charge and discharge was changed from 0.5 C to 10 C.

[0084] As shown in Table 1, Example 2, in which the electrolyte solution was an aqueous solvent, was capable of high-rate (10 C) discharge, whereas Comparative Example 2, in which the electrolyte solution was an organic solvent, was unable to discharge at high rate (10 C). Furthermore, in Example 2, no elution of lithium chloronate, the positive electrode active material, occurred, whereas elution of lithium chloronate, the positive electrode active material, occurred in Comparative Example 2. Therefore, it is believed that using an aqueous solvent for the electrolyte solution suppresses elution of lithium chloronate and improves high-rate discharge characteristics.

[0085] As shown in Table 1, in Examples 1 to 4, in which the ratio of water to the sum of the amounts of substance of the first solute and the second solute was 1.0 or more and 4.0 or less, discharge at 0.5 C was possible, whereas in Comparative Example 1, in which the ratio of water to the sum of the amounts of substance of the first solute and the second solute was greater than 4.0, discharge at 0.5 C was not possible. Furthermore, in Examples 1 to 4, no elution of lithium chlorate, the positive electrode active material, occurred, whereas in Comparative Example 1, elution of lithium chlorate, the positive electrode active material, occurred. Therefore, by setting the ratio of water to the sum of the amounts of substance of the first solute and the second solute to 1.0 or more and 4.0 or less, the electrolyte becomes a hydrate melt (Li(TFSI) 0.7 (BETI) 0.3(H2O)2), which is thought to suppress the elution of lithium chlorate and enable discharge.

[0086] The configuration of the secondary battery of the present invention has been described above with reference to an embodiment and examples. However, the configuration of the secondary battery of the present invention is not limited to the configuration described in the embodiment and examples, and various modifications are possible.

[0087] 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. [Explanation of symbols]

[0088] 1 Secondary battery 10. Exterior body 20 positive electrode 30 negative electrode 40 Electrolyte 70 Separator

Claims

1. a positive electrode containing a croconic acid-based compound as a positive electrode active material; a negative electrode including a negative electrode active material; An electrolyte containing water as a solvent Equipped with The electrolyte solution contains lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) and lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), In the electrolyte solution, a ratio of the amount of substance of the LiTFSI to the sum of the amount of substance of the LiTFSI and the LiBETI is 0.6 or more and 0.8 or less; In the electrolyte solution, the ratio of the amount of water to the sum of the amounts of LiTFSI and LiBETI is 1.8 or more and 4.0 or less. Secondary battery.

2. The secondary battery according to claim 1 , wherein the croconic acid compound is lithium croconic acid.

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