Secondary battery

Abstract

Provided is a secondary battery in which charge / discharge characteristics can be improved. The secondary battery comprises: a positive electrode that includes positive electrode active material particles; a negative electrode; and an electrolyte that contains an aqueous solvent. The positive electrode active material particles include a lithium manganese composite oxide having a spinel crystal structure. The pH of the electrolyte is smaller than 9. The ratio I2 / I1 of the maximum value I1 of scattered light intensity in the range of 600 cm-1 to 700 cm-1 and the maximum value I2 of scattered light intensity in the range of 950 cm-1 to 1000 cm-1 as measured by Raman spectroscopy using excitation light of 532 nm on the surface of the positive electrode active material particles is 0.2 to 2, inclusive. The molar ratio of phosphorus to manganese as measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 to 5, inclusive.

Description

secondary battery

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

[0002] Patent Document 1 discloses a secondary battery including a positive electrode containing lithium manganate and an aqueous electrolyte solution containing phosphate.

[0003] International Publication No. 2020 / 218456

[0004] However, in the battery disclosed in Patent Document 1, the lithium manganese oxide in the positive electrode may dissolve into the electrolyte, which may result in a deterioration in charge-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 charge / discharge characteristics.

[0006] A secondary battery according to one aspect of the present invention includes a positive electrode including positive electrode active material particles, a negative electrode, and an electrolyte including an aqueous solvent, wherein the positive electrode active material particles include a lithium manganese composite oxide having a spinel crystal structure, and the pH of the electrolyte is less than 9, and the pH of the electrolyte is less than 600 cm as measured by Raman spectroscopy using 532 nm excitation light on the surface of the positive electrode active material particles. －１ More than 700cm －１ The maximum scattered light intensity I in the range １ And 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range ２ Relative to I ２ / I １ is 0.2 or more and 2 or less, and the molar ratio of phosphorus to manganese measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 or more and 5 or less.

[0007] A secondary battery according to another aspect of the present invention includes a positive electrode that absorbs and releases lithium ions, a negative electrode that absorbs and releases lithium ions, a positive electrode chamber that houses the positive electrode, a negative electrode chamber that houses the negative electrode, a partition wall that is disposed between the positive electrode chamber and the negative electrode chamber and that allows the lithium ions to pass through, a positive electrode electrolyte that is housed in the positive electrode chamber and contains an aqueous solvent, and a negative electrode electrolyte that is housed in the negative electrode chamber and contains an aqueous solvent, wherein the pH of the negative electrode electrolyte is higher than the pH of the positive electrode electrolyte, the positive electrode includes positive electrode active material particles that contain a lithium manganese composite oxide having a spinel crystal structure, and the pH of the positive electrode electrolyte is lower than 9, and the pH of the positive electrode electrolyte is higher than 600 cm as measured by Raman spectroscopy using 532 nm excitation light on the surfaces of the positive electrode active material particles. －１ More than 700cm －１ The maximum scattered light intensity I in the range １ And 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range ２ Relative to I ２ / I １ is 0.2 or more and 2 or less, and the molar ratio of phosphorus to manganese measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 or more and 5 or less.

[0008] According to the present invention, a secondary battery capable of improving charge / discharge characteristics can be provided.

[0009] Fig. 1 is a schematic cross-sectional view showing the configuration of a secondary battery according to a first embodiment of the present invention. Fig. 2 is a schematic cross-sectional view showing the configuration of a secondary battery according to a second embodiment of the present invention. Fig. 3 is a schematic cross-sectional view showing the configuration of a secondary battery according to a first modified example. Fig. 4 is a schematic cross-sectional view showing the configuration of a secondary battery according to a second modified example.

[0010] Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.

[0011] 1. First Embodiment (Secondary Battery) First, a secondary battery according to a first embodiment of the present invention will be described.

[0012] The secondary battery of the present disclosure includes a positive electrode, a negative electrode, and an aqueous electrolyte. The aqueous electrolyte is an electrolyte containing an aqueous solvent. In the present disclosure, the aqueous solvent is an electrolyte containing water molecules (H ２ The secondary battery according to the first embodiment is, for example, a lithium ion secondary battery in which charge / discharge reactions proceed by utilizing the absorption and release of lithium ions in the positive electrode, but is not limited thereto and may be, for example, a lithium metal secondary battery.

[0013] <1-1. Configuration> Fig. 1 is a schematic cross-sectional view showing the configuration of a secondary battery according to a first embodiment of the present invention. As shown in Fig. 1, the 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] The secondary battery of the first embodiment is a so-called single-liquid type secondary battery that includes one type of aqueous electrolyte (electrolyte 40).

[0015] [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.

[0016] 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.

[0017] [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. 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. 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.

[0018] (Positive Electrode Current Collector) The positive electrode current collector 20A is a conductive support member that supports the positive electrode active material layer 20B, and has a pair of surfaces that support the positive electrode active material layer 20B.

[0019] The positive electrode current collector 20A preferably contains at least one conductive material selected from the group consisting of a metal material, a carbon material, and a conductive ceramic material. Specific examples of the metal material used for the positive electrode current collector 20A include titanium, aluminum, and alloys thereof. Specific examples of the conductive ceramic material include indium tin oxide (ITO).

[0020] The material of the positive electrode current collector 20A is preferably insoluble or hardly soluble in the electrolyte solution 40 and corrosion-resistant, and preferably has low reactivity with the positive electrode active material described below. From this viewpoint, the positive electrode current collector 20A preferably contains a metal material. This can suppress deterioration of the positive electrode current collector 20A due to charging and discharging of the secondary battery 1.

[0021] The positive electrode current collector 20A preferably contains titanium as a constituent element, more specifically, it is more preferable that it contains titanium, which further suppresses deterioration of the positive electrode current collector 20A.

[0022] The positive electrode current collector 20A 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.

[0023] 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.

[0024] (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 and a positive electrode conductive agent. The detailed configuration of the positive electrode active material layer 20B will be described later.

[0025] [Negative Electrode] The negative electrode 30 is not particularly limited, but for example, as shown in FIG. 1 , an electrode that is disposed in the internal space S and absorbs and releases cations such as lithium ions can be used. 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.

[0026] (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 type of conductive material such as a metal material, a carbon material, or a conductive ceramic material.

[0027] 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.

[0028] The material of the negative electrode current collector 30A is preferably insoluble or hardly soluble in the electrolyte solution 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 contains a metal material. This suppresses deterioration of the negative electrode current collector 30A due to charging and discharging of the secondary battery 1.

[0029] 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.

[0030] 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.

[0031] (Negative Electrode Active Material Layer) The negative electrode active material layer 30B contains a negative electrode active material that absorbs and releases cations such as lithium ions. However, the negative electrode active material layer 30B may further contain at least one of a negative electrode binder, a negative electrode conductive agent, and the like.

[0032] Specifically, negative electrode active materials that absorb and release lithium ions include titanium oxide, carbon materials, and metal-based materials. Specific examples of titanium oxide include anatase-type titanium oxide, rutile-type titanium oxide, and brookite-type titanium oxide. Specific examples of carbon materials include graphite. Metal-based materials are materials that contain, as a constituent element, at least one of a metal element and a metalloid element that can form an alloy with lithium.

[0033] Furthermore, the negative electrode active material layer 30B may contain, in addition to the negative electrode active material that absorbs and releases lithium ions, a negative electrode active material that absorbs and releases cations other than lithium ions. The type of cations other than lithium ions may be one type or two or more types.

[0034] Specifically, 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.

[0035] 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 K4TiO 12 Specific examples of titanium phosphate oxides include TiP2O7 and NaTi2(PO4)3.

[0036] Niobium-containing compounds include hydrogen niobium compounds and titanium-niobium composite oxides. Specific examples of hydrogen niobium compounds include HNbO 17 Specific examples of titanium-niobium composite oxides include TiNb2O7 and Ti2Nb 10 O 29 And so on.

[0037] The vanadium-containing compound is vanadium oxide, etc. A specific example of the vanadium oxide is vanadium dioxide (VO2).

[0038] The iron-containing compound is an iron hydroxide, etc. A specific example of the iron hydroxide is iron oxyhydroxide (FeOOH).

[0039] Molybdenum-containing compounds include molybdenum oxide and cobalt-molybdenum composite oxides. Specific examples of molybdenum oxides include molybdenum dioxide (MoO2). Specific examples of cobalt-molybdenum composite oxides include CoMoO4.

[0040] Of course, the negative electrode active material layer 30B may contain both one or more types of negative electrode active materials that absorb and release lithium ions and one or more types of negative electrode active materials that absorb and release cations other than lithium ions.

[0041] [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.

[0042] The electrolyte solution 40 includes an aqueous solvent and a solute. The solute includes at least one type of ionic substance that can be ionized in the aqueous solvent. More specifically, the electrolyte solution 40 used in the secondary battery includes alkali metal ions that are occluded by the negative electrode 13 and released by the positive electrode 14.

[0043] The type of aqueous solvent is not particularly limited, and specifically, it can be pure water. The ionic substance can be, for example, an acid, a base, or an electrolyte salt, or two or more of these. The acid can be, for example, oxalic acid (H2C2O4), nitric acid (HNO3), sulfuric acid (H2SO4), hydrochloric acid (HCl), acetic acid (CH3COOH), or citric acid (H3C6H5O7).

[0044] The electrolyte salt is a salt containing a cation and an anion. More specifically, the electrolyte salt is one or more types of metal salts. The type of metal salt is not particularly limited, but may be, for example, an alkali metal salt, an alkaline earth metal salt, a transition metal salt, or other metal salts.

[0045] Examples of alkali metal salts include lithium salts, sodium salts, and potassium salts. The type of lithium salt is not particularly limited, and specific examples include lithium oxalate (Li2C2O4), lithium nitrate (LiNO3), lithium sulfate (Li2SO4), lithium chloride (LiCl), lithium acetate (CH3COOLi), lithium citrate (Li3C6H5O7), lithium hydroxide (LiOH), lithium bis(fluorosulfonyl)imide, and lithium bis(trifluoromethanesulfonyl)imide. The type of sodium salt is not particularly limited, and examples include compounds in which the lithium ions of the above-mentioned lithium salts are replaced with sodium ions. The type of potassium salt is not particularly limited, and examples include compounds in which the lithium ions of the above-mentioned lithium salts are replaced with potassium ions.

[0046] The type of alkaline earth metal salt is not particularly limited, and examples thereof include compounds in which the lithium ions of the above-mentioned lithium salts are replaced with cations of alkaline earth metal elements, more specifically, calcium salts. The type of transition metal salt is not particularly limited, and examples thereof include compounds in which the lithium ions of the above-mentioned lithium salts are replaced with cations of transition metal elements. The type of other metal salt is not particularly limited, and examples thereof include compounds in which the lithium of the above-mentioned lithium salts is replaced with cations of other metal elements.

[0047] The electrolyte solution 40 preferably contains, as anions, one or more of sulfate ions, hydrogen sulfate ions, carbonate ions, hydrogen carbonate ions, phosphate ions, monohydrogen phosphate ions, dihydrogen phosphate ions, and carboxylate ions. This sufficiently suppresses fluctuations in the pH of the electrolyte solution 40, making it easier to sufficiently maintain the pH of the electrolyte solution 40 within the range described below. Examples of carboxylate ions include formate ions, acetate ions, propionate ions, tartrate ions, and citrate ions.

[0048] The electrolyte solution 40 more preferably contains a phosphate anion as the anion. Specific examples of the phosphate anion include a phosphate anion, a monohydrogen phosphate anion, and a dihydrogen phosphate anion. Specific examples of ionic substances containing a phosphate anion as the anion include phosphoric acid, a phosphate-based lithium salt, a sodium phosphate salt, and a potassium phosphate salt. The phosphate-based lithium salt, the sodium phosphate salt, and the potassium phosphate salt are salts containing a phosphate anion as the anion and a lithium ion, a sodium ion, and a potassium ion as the cation, respectively. Specific examples of the phosphate-based lithium salt include lithium phosphate, lithium monohydrogen phosphate, and lithium dihydrogen phosphate. Specific examples of the phosphate-based sodium salt include sodium phosphate, sodium monohydrogen phosphate, and sodium dihydrogen phosphate. Specific examples of the phosphate-based potassium salt include potassium phosphate, potassium monohydrogen phosphate, and potassium dihydrogen phosphate. This allows the coating of the manganese phosphate compound on the positive electrode active material particles, described below, to remain stable in the electrolyte solution 40, thereby further suppressing elution of the positive electrode active material into the electrolyte solution.

[0049] Here, the concentration (mol / kg) of phosphate anions in the electrolytic solution 40 is preferably 0.5 mol / kg or less. This makes it easier for the charge / discharge reaction to proceed stably in the positive electrode 20 in the electrolytic solution 40.

[0050] The pH of the electrolytic solution 40 according to the first embodiment is 9 or less. The pH of the electrolytic solution 40 is preferably 6 or less. This facilitates the progress of charge / discharge reactions in the positive electrode 20. The pH of the electrolytic solution 40 is preferably 3 or more, and more preferably 4 or more. This makes it difficult for the components of the secondary battery, such as the exterior body 10 and the positive electrode current collector 20A, to corrode, thereby improving the electrochemical durability (stability) of the secondary battery.

[0051] The content of the ionic substance, i.e., the concentration (mol / kg) of the electrolyte solution 40 is preferably 0.01 mol / kg or more and 10 mol / kg or less, and more preferably 0.2 mol / kg or more and 4 mol / kg or less. This makes it easier for the charge / discharge reaction to proceed stably in the positive electrode 20 in the electrolyte solution 40.

[0052] Furthermore, the electrolytic solution 40 is 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.

[0053] To determine whether the electrolyte solution 40 is a saturated solution of electrolyte salt, the secondary battery is disassembled and the internal space S is visually inspected to determine whether electrolyte salt has precipitated. In this case, the electrolyte solution 40, the surface of the positive electrode 20, and the inner wall surface of the exterior casing 10 are visually inspected. When the electrolyte solution 40 (liquid) and a precipitate of electrolyte salt (solid) coexist due to the precipitation of electrolyte salt, the electrolyte solution 40 can be determined to be a saturated solution of electrolyte salt. To examine the composition of the precipitate, a surface analysis method such as X-ray photoelectron spectroscopy (XPS) or a composition analysis method such as inductively coupled plasma (ICP) emission spectroscopy may be used.

[0054] The electrolytic solution 40 may be a pH buffer solution. The pH buffer solution may be an aqueous solution containing a weak acid and its conjugate base, or a weak base and its conjugate acid. This sufficiently suppresses fluctuations in the pH of the electrolytic solution 40, making it easier to maintain the pH of the electrolytic solution 40 within the above-mentioned range.

[0055] The electrolytic solution 40 may contain the above-described phosphate anions to serve as a pH buffer solution. In this case, the pH can be adjusted by adjusting the molar ratio of phosphate anions, monohydrogen phosphate anions, and dihydrogen phosphate anions.

[0056] The electrolytic solution 40 may also contain one or more of trishydroxymethylaminomethane, ethylenediaminetetraacetic acid, and the like as a buffer.

[0057] The electrolytic solution 40 may also contain at least one of trishydroxymethylaminomethane and ethylenediaminetetraacetic acid as a buffer.

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

[0059] The positive electrode active material is a substance that absorbs and releases lithium ions and includes a lithium-manganese composite oxide having a spinel crystal structure. In this disclosure, lithium-manganese composite oxide refers to a compound containing lithium and manganese as constituent elements. A specific example of a lithium-manganese composite oxide having a spinel crystal structure is LiMn2O4. This improves charge / discharge characteristics. The constituent elements of the lithium-manganese composite oxide are not limited to lithium, manganese, and oxygen, and may include, for example, a transition metal element other than manganese. The type of transition metal element other than manganese is not particularly limited, and specific examples include nickel, cobalt, and iron. Whether the positive electrode active material contains a lithium-manganese composite oxide having a spinel crystal structure can be determined by X-ray diffraction (XRD). Specifically, if a diffraction peak of a lithium manganese composite oxide having a spinel crystal structure is detected in a diffraction chart obtained by measuring a sample containing a positive electrode active material by XRD, it can be determined that the positive electrode active material contains a lithium manganese composite oxide having a spinel crystal structure.

[0060] The positive electrode active material may contain a substance that absorbs and releases lithium ions other than a lithium-manganese composite oxide having a spinel crystal structure. The type of substance is not particularly limited, but specific examples include lithium composite oxides and lithium phosphate compounds. The lithium composite oxide is an oxide containing lithium and one or more transition metal elements as constituent elements, and has a layered rock salt or spinel crystal structure. The lithium phosphate compound is a phosphate compound containing lithium and one or more transition metal elements as constituent elements, and has an olivine crystal structure. The type of transition metal element is not particularly limited, but specific examples include nickel, cobalt, manganese, and iron.

[0061] Specific examples of lithium composite oxides having a layered rock salt crystal structure include LiNiO2, LiCoO2, and LiCo 0.98 Al 0.01 Mg 0.01 O2, LiNi 0.5 Co 0.2Mn 0.3 O2, LiNi 0.8 Co 0.15 Al 0.05 O2, LiNi 0.33 Co 0.33 Mn 0.33 O2, 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. Specific examples of lithium phosphate compounds having an olivine crystal structure include LiFePO4, LiMnPO4, LiMn 0.5 Fe 0.5 PO4, LiMn 0.7 Fe 0.3 PO4 and LiMn 0.75 Fe 0.25 PO4, etc. The lithium phosphate compound having an olivine crystal structure preferably contains iron as a constituent element, which improves both the charge efficiency and the discharge efficiency.

[0062] 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.

[0063] [Positive Electrode Conductive Agent] The positive electrode conductive agent is a material that improves the conductivity of the positive electrode active material layer 20B and contains at least one type of conductive material. The type of conductive material is not particularly limited, but specific examples include carbon materials, metal materials, conductive ceramic materials, and conductive polymer compounds. It is preferable that the positive electrode conductive agent contains a carbon material, as this sufficiently improves the conductivity of the positive electrode active material layer 20B. Specific examples of carbon materials include graphite, carbon black, acetylene black, and ketjen black.

[0064] The positive electrode active material layer 20B includes positive electrode active material particles. In the present disclosure, the positive electrode active material particles are particles that include a positive electrode active material.

[0065] A first maximum intensity I measured by Raman spectroscopy using 532 nm excitation light on the surface of the positive electrode active material particle. １ and the second maximum intensity I ２ Relative to I ２ / I １ is equal to or greater than 0.2 and equal to or less than 2. Here, the maximum value I of the first intensity １ That is, 600 cm －１ More than 700cm －１ The maximum scattered light intensity I in the range １ It refers to 600 cm －１ More than 700cm －１ The scattered light intensity in the following range represents the intensity of scattered light attributed to the vibration mode of the lithium manganese composite oxide having a spinel-type crystal structure. ２ That is, 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range ２ 950cm －１ More than 1000cm －１ The scattered light intensity in the following range represents the scattered light intensity attributed to the vibration mode of the phosphate ion of the manganese phosphate compound. －１ More than 1000cm －１ If scattered light in the following range is detected, it can be said that a coating made of a manganese phosphate compound is present on the surface of the positive electrode active material particles. Here, the manganese phosphate compound refers to a compound of a phosphate anion and a manganese-containing cation. Ratio I ２ / I １ When the ratio I is in the range of 0.2 or more, it can be said that the coating is formed on the surface of the positive electrode active material particles in a large amount to an extent that the elution of the positive electrode active material into the electrolyte solution 40 can be suppressed. ２ / I １ When the first intensity is in the range of 2 or less, the coating is thin enough not to inhibit the charge / discharge reaction of the positive electrode active material. This improves the charge / discharge characteristics. In the following description, the first intensity maximum value I １ and the second maximum intensity I ２ Relative to I ２ / I １ The intensity ratio I ２ / I １It may be explained as:

[0066] Ratio I ２ / I １ can be measured by the following method. First, the secondary battery to be measured is discharged to a predetermined discharge voltage and then disassembled. The removed positive electrode is washed with water to remove the electrolyte salt. Then, a surface analysis of the positive electrode active material layer 20B is performed by Raman spectroscopy using a Raman spectrometer. The discharge voltage can be appropriately determined based on the potential of the positive electrode active material and the negative electrode active material in a discharged state. To minimize the influence of sample surface irregularities on the measurement results, a confocal Raman microscope is preferably used as the Raman spectrometer. The wavelength of the light source used in Raman spectroscopy is 532 nm, and the laser power density and exposure time are adjusted to values ​​that do not cause sample changes during measurement. The measured spectrum is then subjected to linear background subtraction and detector-derived spike noise removal using analysis software (RAMANviewer, manufactured by Nanophoton), and then measured at 600 cm －１ More than 700cm －１ The maximum value of the scattered light intensity in the following range is defined as the first maximum intensity I １ Measured as 950 cm －１ More than 1000cm －１ The maximum value of the scattered light intensity in the following range is defined as the second maximum intensity I ２ By measuring as the ratio I ２ / I １ can be calculated.

[0067] The P / Mn ratio of the surface of the positive electrode active material particles measured by X-ray photoelectron spectroscopy (XPS) is 0.2 or more and 5 or less. The P / Mn ratio refers to the molar ratio of phosphorus to manganese. In other words, the P / Mn ratio can be considered a parameter indicating the presence of a coating made of a manganese phosphate compound on the surface of the positive electrode active material particles. Therefore, when the P / Mn ratio is 0.2 or more, the coating can be considered to be thick enough to suppress elution of the positive electrode active material into the electrolyte solution 40. When the P / Mn ratio is 5 or less, the coating can be considered to be thin enough not to inhibit the charge / discharge reaction of the positive electrode active material.

[0068] The P / Mn ratio can be measured using the following method. First, the secondary battery to be measured is discharged to a predetermined discharge voltage and then disassembled. The removed positive electrode is washed with water to remove the electrolyte salt. Then, a surface analysis of the positive electrode active material layer 20B is performed using XPS with an XPS device. XPS conditions include irradiating the sample with monochromated Al-kα radiation (1486.6 eV) at a beam size diameter of approximately 100 μm, and measuring the photoelectron spectrum. The photoelectron takeoff angle is set to 45°, and no charge neutralization treatment is performed. Energy correction of the spectrum obtained is then performed using the carbon 1s peak. Specifically, the C1s spectrum of the sample (positive electrode active material layer 20B) is measured from an energy range of 280 eV to 300 eV, and the position of the main peak on the lowest binding energy side is determined to be 284.5 eV. When measuring the C1s spectrum, a wide-area (survey) spectrum may be used in combination as needed to eliminate the influence of peaks from other elements or the possibility of differences in the energy range due to charging, thereby more reliably identifying the C1s spectrum. Then, P2p and Mn2p ３／２ The intensities of the peaks derived from P2p were calculated, and the intensity values ​​were converted into the P / Mn ratio using relative sensitivity factors (RSF). Here, the peak derived from P2p refers to the largest peak in the region of 128 eV to 140 eV, and the peak derived from Mn2p refers to the largest peak in the region of 128 eV to 140 eV. ３／２ The peak derived from refers to the largest peak in the region of 638 eV to 647 eV. ３／２ When other peaks are superimposed on the peaks derived from P2p and Mn2p, waveform separation can be performed using commercially available software (ULVAC-PHI Multipak). ３／２ Alternatively, the intensity of only the peaks derived from P2s and Mn2p may be calculated. １／２ The P / Mn ratio may be calculated based on the intensity of other peaks, such as the peak derived from P2s. Here, the peak derived from P2s refers to the largest peak in the region of 185 eV to 192 eV, and the peak derived from Mn2p １／２ The peak derived from refers to the largest peak in the region of 649 eV or more and 654 eV or less.

[0069] <1-3. Operation> The secondary battery 1 operates as described below.

[0070] 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.

[0071] 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.

[0072] <1-4. Manufacturing Method> When manufacturing the secondary battery 1, the positive electrode 20 and the negative electrode 30 are manufactured and the electrolyte solution 40 is prepared according to the procedure described below as an example, and then the secondary battery 1 is assembled.

[0073] [Fabrication of the Positive Electrode] First, a powder of a positive electrode active material containing a lithium-manganese composite oxide is immersed in an aqueous solution of a salt containing phosphoric acid, such as ammonium phosphate. The positive electrode active material powder is then removed, dried, and calcined to obtain positive electrode active material particles coated with a manganese phosphate compound. The positive electrode active material particles, positive electrode binder, and positive electrode conductor are then mixed together to obtain a mixture. However, the composition of the mixture can be varied 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. The positive electrode active material layer 20B may then 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 way, the positive electrode 20 is produced.

[0074] [Fabrication of Negative Electrode] The negative electrode 30 is fabricated using a procedure similar to that for fabricating the positive electrode 20 described above. Specifically, a negative electrode mixture, in which a negative electrode active material, a negative electrode binder, and a negative cathode conductive agent are mixed together, is poured into a solvent to prepare a paste-like negative electrode mixture slurry. Next, the negative electrode mixture slurry is applied to both surfaces of the negative electrode current collector 30A, excluding the connection terminal portions 30AT, to form a negative electrode active material layer 30B, which is then compression-molded. This completes the fabrication of the negative electrode 30.

[0075] [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.

[0076] [Assembly of 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 and 30AT are pulled out from the inside (internal space S) of the exterior body 10 to the outside so that they are exposed.

[0077] 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.

[0078] 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.

[0079] <1-5. Actions and Effects> As described above, the secondary battery of the first embodiment includes the positive electrode 20 including positive electrode active material particles, the negative electrode 30, and the electrolyte solution 40 including an aqueous solvent. The positive electrode active material particles include a lithium-manganese composite oxide having a spinel-type crystal structure. The pH of the electrolyte solution 40 is less than 9, and the pH of the electrolyte solution 40 is less than 600 cm as measured by Raman spectroscopy using 532 nm excitation light on the surface of the positive electrode active material particles. －１ More than 700cm －１ The maximum scattered light intensity I in the range １ And 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range２ Relative to I ２ / I １ is 0.2 or more and 2 or less. The molar ratio of phosphorus to manganese (P / Mn ratio) measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 or more and 5 or less.

[0080] This means that a coating made of a manganese phosphate compound is present on the surface of the positive electrode active material particles, and the thickness of this coating is large enough to prevent the positive electrode active material from leaching into the electrolyte solution 40, but is thin enough not to inhibit the charge / discharge reactions of the positive electrode active material. Therefore, even if the pH of the electrolyte solution 40 is reduced, the coating can prevent the positive electrode active material from leaching into the electrolyte solution, and the low pH electrolyte solution 40 and the positive electrode active material with reduced leaching can improve the charge / discharge characteristics.

[0081] In a desirable embodiment, the pH of the electrolyte solution 40 is lower than 6. Even in this case, the coating made of the manganese phosphate compound can prevent the positive electrode active material from eluting into the electrolyte solution 40, and the lower pH of the electrolyte solution 40 and the positive electrode active material whose elution is prevented can further improve the charge / discharge characteristics.

[0082] In a preferred embodiment, the electrolyte solution 40 contains at least one phosphate-based anion, which is a group consisting of phosphate anion, monohydrogen phosphate anion, and dihydrogen phosphate anion. The concentration of the phosphate-based anion in the electrolyte solution 40 is 0.5 mol / kg or less. This stabilizes the progress of the charge / discharge reaction in the positive electrode 20 in the electrolyte solution 40 while protecting the coating made of the manganese phosphate compound, thereby further improving the charge / discharge characteristics.

[0083] 2. Second Embodiment (Secondary Battery) Next, a secondary battery according to a second embodiment of the present invention will be described.

[0084] The secondary battery 1A of the second embodiment is a dual-liquid type secondary battery that uses two types of aqueous electrolytes (a cathode electrolyte 61 and an anode electrolyte 62), unlike the secondary battery 1 of the first embodiment, which is a single-liquid type secondary battery that uses one type of aqueous electrolyte (an electrolyte 40).

[0085] 2 is a schematic cross-sectional view showing the configuration of a secondary battery according to a second embodiment of the present invention. The secondary battery 1A according to the second embodiment described here has the same configuration as the secondary battery 1 according to the first embodiment shown in FIG. 1, except for the matters described below.

[0086] As shown in Fig. 2 , secondary battery 1A further includes a partition wall 50. Moreover, secondary battery 1A includes a positive electrode electrolyte 61 and a negative electrode electrolyte 62 instead of the electrolyte 40 of secondary battery 1. In Fig. 2 , the positive electrode electrolyte 61 is lightly hatched, and the negative electrode electrolyte 62 is darkly hatched.

[0087] The exterior body 10 has two spaces (a positive electrode chamber S1 and a negative electrode chamber S2) separated by a partition wall 50.

[0088] 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.

[0089] 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 back and forth between the positive electrode chamber S1 and the negative electrode chamber S2.

[0090] 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 cation exchange membrane that is permeable to lithium ions. The solid electrolyte membrane may be a solid electrolyte membrane that is conductive to lithium ions. This improves the permeability of lithium ions through the partition wall 50.

[0091] 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.

[0092] 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.

[0093] 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.

[0094] The positive electrode electrolyte 61 and the negative electrode electrolyte 62 are both aqueous electrolytes. The positive electrode electrolyte 61 is the same as the electrolyte 40 according to the first embodiment, and therefore a description thereof will be omitted.

[0095] 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.

[0096] 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.

[0097] The negative electrode electrolyte 62 is preferably a saturated solution of an electrolyte salt. This allows the charge / discharge reaction to proceed stably during charge / discharge. The method for determining whether the negative electrode electrolyte 62 is a saturated solution of a lithium salt is the same as the method for determining whether the electrolyte 40 is a saturated solution of a lithium salt.

[0098] 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.

[0099] 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.

[0100] 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.

[0101] The composition of the negative electrode electrolyte 62, such as the type of aqueous solvent and the type and concentration of the ionic substance, can be set arbitrarily.

[0102] For example, the anode 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 the anode electrolyte 62 due to charge and discharge reactions, making it easier to maintain the pH.

[0103] The negative electrode electrolyte 62 may also contain at least one of trishydroxymethylaminomethane, ethylenediaminetetraacetic acid, and the like as a buffer.

[0104] <2-2. Operation> The secondary battery 1A operates as described below.

[0105] When secondary battery 1A is charged, lithium ions are released from positive electrode 20, move to negative electrode 30 through positive electrode electrolyte 61, partition wall 50, and negative electrode electrolyte 62, and are absorbed in negative electrode 30.

[0106] When 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.

[0107] 2-3. Manufacturing Method The manufacturing procedure for the secondary battery 1A is the same as the manufacturing procedure for the secondary battery 1 in the first embodiment described above, except for the points described below.

[0108] 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.

[0109] When assembling a 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.

[0110] 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 secondary battery using two types of aqueous electrolytes (positive electrode electrolyte 61 and negative electrode electrolyte 62) is completed.

[0111] <2-4. Actions and Effects> As described above, the secondary battery 1A of the second embodiment includes a cathode 20 containing cathode active material particles, an anode 30, a cathode chamber S1 that houses the cathode 20, an anode chamber S2 that houses the anode 30, a partition wall 50 that is disposed between the cathode chamber S1 and the anode chamber S2 and allows lithium ions to pass through, a cathode electrolyte 61 that is contained in the cathode chamber S1 and contains an aqueous solvent, and an anode electrolyte 62 that is contained in the anode chamber S2 and contains an aqueous solvent. The pH of the anode electrolyte 62 is higher than the pH of the cathode electrolyte 61. The cathode active material particles contain a lithium-manganese composite oxide having a spinel crystal structure. The pH of the cathode electrolyte 61 is lower than 9, and is measured by Raman spectroscopy using 532 nm excitation light on the surface of the cathode active material particles at 600 cm. －１ More than 700cm－１ The maximum scattered light intensity I in the range １ And 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range ２ Relative to I ２ / I １ is 0.2 or more and 2 or less. The molar ratio of phosphorus to manganese (P / Mn ratio) measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 or more and 5 or less.

[0112] This means that a coating made of a manganese phosphate compound is present on the surface of the positive electrode active material particles, and the thickness of this coating is thin enough not to inhibit the charge / discharge reaction of the positive electrode active material, but thick enough to suppress elution of the positive electrode active material into positive electrode electrolyte 61. Therefore, even if the pH of positive electrode electrolyte 61 is reduced, this coating can suppress elution of the positive electrode active material into the electrolyte, and the low pH positive electrode electrolyte 61 and the positive electrode active material whose elution is suppressed can improve the charge / discharge characteristics.

[0113] In a desirable embodiment, the pH of positive electrode electrolyte 61 is lower than 6. Even in this case, the coating made of a manganese phosphate compound can prevent the positive electrode active material from eluting into positive electrode electrolyte 61, and the charge / discharge characteristics can be further improved by positive electrode electrolyte 61 with a lower pH and the positive electrode active material whose elution is prevented.

[0114] In a desirable embodiment, positive electrode electrolyte 61 contains at least one kind of phosphate-based anion, which is a group consisting of phosphate anion, monohydrogen phosphate anion, and dihydrogen phosphate anion. The concentration of the phosphate-based anion in positive electrode electrolyte 61 is 0.5 mol / kg or less. This makes it possible to stabilize the progress of the charge / discharge reaction at positive electrode 20 in positive electrode electrolyte 61 while protecting the coating made of the manganese phosphate compound, thereby further improving the charge / discharge characteristics.

[0115] 3. 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 types of modifications may be combined with each other.

[0116] [First Modification] Fig. 3 is a schematic cross-sectional view showing the configuration of a secondary battery of a first modification. As shown in Fig. 3, a secondary battery 1B of the first modification further includes a separator 70 and electrolyte layers 81 and 82, which are gel electrolytes. The configuration of the secondary battery 1B shown in Fig. 3 is similar to the configuration of the secondary battery 1 of the first embodiment shown in Fig. 1, except as described below.

[0117] As described above, the separator 70 is disposed between the positive electrode 20 and the negative electrode 30. The electrolyte solution 40 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.

[0118] 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 3, each of the electrolyte layers 81 and 82 is lightly shaded.

[0119] The separator 70 is an insulating porous film that separates the electrolyte layers 81 and 82 from each other and allows cations such as lithium ions to pass through. The separator 70 contains a polymer compound such as polyethylene.

[0120] 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.

[0121] In the secondary battery 1B 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 secondary battery 1 of the first embodiment shown in Fig. 1 can be obtained. In particular, the secondary battery 1B of the second modified example can prevent leakage of the electrolyte.

[0122] [Second Modification] Fig. 4 is a schematic cross-sectional view showing the configuration of a secondary battery of a second modification. In the second embodiment, as shown in Fig. 4, a secondary battery 1C of the second modification differs from the secondary battery 1A of 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 secondary battery 1C shown in Fig. 4 is the same as the configuration of the secondary battery 1A shown in Fig. 2, except as described below.

[0123] 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.

[0124] 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. 4 , 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.

[0125] 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.

[0126] In the secondary battery 1C of the second modified example, 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 secondary battery 1A of the second embodiment shown in Fig. 2 can be obtained. In particular, the secondary battery 1C of the second modified example can prevent leakage of the electrolyte.

[0127] 4. Uses of Secondary Batteries There are no particular limitations on the uses (application examples) of secondary batteries. A secondary battery used as a power source may be a main power source for electronic devices, electric vehicles, and the like, or may be an auxiliary power source. A main power source is a power source that is used preferentially regardless of the presence or absence of other power sources. An auxiliary power source is a power source that is used in place of a main power source, or a power source that can be switched from a main power source.

[0128] Specific examples of uses for secondary batteries are as follows: Electronic devices such as video cameras, digital still cameras, mobile phones, laptop computers, headphone stereos, portable radios, and portable information terminals. Storage devices such as backup power supplies and memory cards. Power tools such as power drills and power saws. Battery packs installed in electronic devices. Medical electronic devices such as pacemakers and hearing aids. Electric vehicles such as electric cars including hybrid cars. Power storage systems such as home or industrial battery systems that store power in preparation for emergencies. In these uses, one secondary battery may be used, or multiple secondary batteries may be used.

[0129] The battery pack may use a single cell or a battery pack. The electric vehicle is a vehicle that operates (travels) using a secondary battery as a driving power source, and may be a hybrid vehicle that also has a driving source other than the secondary battery. In a home power storage system, it is possible to use household electrical appliances by using the power stored in the secondary battery, which is a power storage source.

[0130] Of course, the secondary battery may be used for purposes other than the series of purposes exemplified here.

[0131] An embodiment of the present invention will now be described.

[0132] Examples 1 to 5 and Comparative Examples 1 to 3 After an electrochemical measurement cell was fabricated using the positive electrode 20, the charge / discharge characteristics of the positive electrode 20 were evaluated.

[0133] [Fabrication of Electrochemical Measurement Cells] In Examples 1 to 5, electrochemical measurement cells having substantially the same configuration as the single-liquid type secondary battery (FIG. 1) described in the first embodiment were fabricated by the procedure described below.

[0134] (Preparation of Positive Electrode) First, lithium manganese composite oxide (LiMn ２ O ４ ) was immersed in a 1% by mass aqueous solution of ammonium phosphate, the positive electrode active material powder was removed, dried overnight in a drying oven at 60°C, and then calcined in air at 500°C to obtain positive electrode active material particles having a coating made of a manganese phosphate compound. Here, XRD measurement of the prepared positive electrode active material particles revealed diffraction peaks of a lithium manganese composite oxide having a spinel crystal structure, confirming that the prepared positive electrode active material particles contained a lithium manganese composite oxide having a spinel crystal structure.

[0135] The cathode active material particles prepared above, polymethyl acrylate as a cathode binder, and carbon black as a cathode conductor were mixed together to obtain a cathode mixture. The mixture ratio of the cathode active material particles, the cathode binder, and the cathode conductor was 90:4:6 by mass. Next, a paste-like cathode mixture slurry was prepared by adding the cathode mixture to N-methyl-2-pyrrolidone, an organic solvent. Finally, the cathode mixture slurry was applied to both surfaces of the cathode current collector 20A, excluding the connection terminal portions 20AT, using a coating device. The cathode mixture slurry was then dried to form the cathode active material layer 20B. Here, a 20 μm-thick titanium (Ti) foil was used for the cathode current collector 20A. Thus, the cathode 20 was fabricated.

[0136] Ratio I of the prepared positive electrode ２ / I １was measured by the following method. After washing the prepared positive electrode active material layer with water, a surface analysis of the positive electrode active material layer 20B by Raman spectroscopy was performed using a confocal Raman microscope (RAMANforce, manufactured by Nanophoton Inc.). Here, the wavelength of the light source used in Raman spectroscopy was 532 nm, and the laser power density and exposure time were adjusted to values ​​that would not cause sample changes due to the measurement. Then, the measured spectrum was subjected to linear background subtraction and detector-derived spike noise removal processing using analysis software (RAMANviewer, manufactured by Nanophoton Inc.), and then measured at 600 cm －１ More than 700cm －１ The maximum value of the scattered light intensity in the following range is defined as the first maximum intensity I １ Measured as 950 cm －１ More than 1000cm －１ The maximum value of the scattered light intensity in the following range is defined as the second maximum intensity I ２ By measuring as the ratio I ２ / I １ The obtained values ​​are shown in Table 1.

[0137] The P / Mn ratio of the prepared positive electrode was measured by the following method. After washing the prepared positive electrode active material layer with water, a surface analysis of the positive electrode active material layer 20B was performed by XPS using an XPS apparatus (Versaprobe, manufactured by ULVAC-PHI, Inc.). The XPS conditions included irradiating the sample with monochromated Al-kα radiation (1486.6 eV) at a beam size diameter of approximately 100 μm, and measuring the photoelectron spectrum. Here, the photoelectron takeoff angle was set to 45°, and no charge neutralization treatment was performed. Energy correction of the spectrum obtained was performed using the carbon 1s peak. Specifically, the C1s spectrum of the sample (positive electrode active material layer 20B) was measured, and the position of the main peak on the lowest binding energy side was determined to be 284.5 eV. Then, P2p and Mn2p ３／２ The intensities of the peaks derived from each of the above were calculated, and the intensity values ​​were converted into P / Mn ratios using relative sensitivity factors (RSF). The obtained values ​​are shown in Table 1.

[0138] (Preparation of Electrolyte Solution) In Examples 1 to 5, 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. The solutes shown in Table 1 were used as the electrolyte salt, and the electrolyte solution was prepared to have the concentration shown in Table 1. The pH of the prepared electrolyte solution is shown in Table 1.

[0139] (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 negative electrode 30 (counter electrode). 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 electrolytic solution 40 or nonaqueous electrolytic solution prepared above was supplied to the internal space S. As a result, the electrolytic solution 40 was housed in the internal space S, and the electrochemical measurement cell was completed.

[0140] In Comparative Example 1, an electrochemical measurement cell was prepared in the same manner as in Example 1, except that the positive electrode active material was not immersed in ammonium phosphate, i.e., positive electrode active material particles not formed with a coating made of a manganese phosphate compound were used.

[0141] In Comparative Example 2, an electrochemical measurement cell was fabricated in the same manner as in Example 1, except that an aqueous solution of lithium hydroxide with a concentration of 4.0 mol / kg was used so that the pH of the electrolytic solution 40 would be 12.

[0142] In Comparative Example 3, lithium cobalt oxide (LiCoO ２ An electrochemical measurement cell was fabricated in the same manner as in Example 5, except that the positive electrode active material particles according to Comparative Example 3 did not contain manganese. ２ / I １ and the P / Mn ratio could not be measured.

[0143] [Evaluation of Positive Electrode Characteristics] When the charge / discharge characteristics were evaluated as the operating characteristics of the positive electrode 20, the results shown in Table 1 were obtained.

[0144]

[0145] In measuring the discharge capacity retention rate, the electrochemical measurement cell was charged at a constant current of 1 C to a cutoff potential of 1.2 V (vs. Ag / AgCl) and then discharged at a constant current of 1 C to a cutoff potential of 0.6 V (vs. Ag / AgCl). At this time, the discharge capacity and charge capacity were measured, and the discharge capacity retention rate was calculated based on the formula: discharge capacity retention rate (%) = (discharge capacity / charge capacity) × 100.

[0146] In Comparative Example 3, the cutoff potential was reached quickly during discharge, so that the discharge capacity could not be obtained, and therefore the discharge capacity retention rate could not be measured.

[0147] As shown in Table 1, the ratio I ２ / I １ In Examples 1 to 5, the ratio I ２ / I １ The discharge capacity retention rate was improved compared to Comparative Example 1, in which the ratio I was less than 0.2. ２ / I １ It can be seen that the charge-discharge characteristics can be improved by setting the ratio I to 0.2 or more and 2.0 or less. ２ / I １ It is believed that when the ratio is 0.2 or more and 2.0 or less, a coating made of an appropriate amount of manganese phosphate compound is present on the surface of the positive electrode active material, and the coating improves the charge-discharge characteristics.

[0148] As shown in Table 1, Examples 1 to 5, in which the P / Mn ratio was 0.2 or more and 5.0 or less, showed improved discharge capacity retention rates compared to Comparative Example 1, in which the P / Mn ratio was less than 0.2. Therefore, it is clear that a P / Mn ratio of 0.2 or more and 5.0 or less can improve charge / discharge characteristics. This result is thought to be due to the fact that, when the P / Mn ratio is 0.2 or more and 5.0 or less, a coating made of an appropriate amount of manganese phosphate compound is present on the surface of the positive electrode active material, and this coating improves the charge / discharge characteristics.

[0149] As shown in Table 1, charge and discharge could not be performed in Comparative Example 2, in which the pH of electrolytic solution 40 was 9 or higher, whereas charge and discharge could be performed in Examples 1 to 5, in which the pH of electrolytic solution 40 was less than 9. Therefore, it can be seen that by making the pH of electrolytic solution 40 less than 9, charge and discharge can be performed and the battery can function.

[0150] As shown in Table 1, in Examples 1 to 5, in which a lithium manganese composite oxide (lithium manganese oxide) having a spinel crystal structure was used as the positive electrode active material, the discharge capacity retention rate was improved compared to Comparative Example 3, in which a lithium composite oxide (lithium cobalt oxide) other than the lithium manganese composite oxide was used. This shows that the charge-discharge characteristics can be improved by using a lithium manganese composite oxide having a spinel crystal structure as the positive electrode active material.

[0151] As shown in Table 1, in Example 2, in which the pH of the electrolytic solution was less than 6, the discharge capacity retention rate was improved compared to Example 1, in which the pH of the electrolytic solution was 6 or more. Therefore, it is clear that the charge-discharge characteristics can be improved by making the pH of the electrolytic solution less than 6.

[0152] As shown in Table 1, Example 2, in which the electrolyte solution contained phosphate-based anions, had an improved discharge capacity retention rate compared to Example 1, in which the electrolyte solution did not contain phosphate-based anions. This shows that the charge-discharge characteristics can be improved by including phosphate-based anions in the electrolyte solution.

[0153] As shown in Table 1, Example 3, in which the concentration of phosphate anions in the electrolyte solution was 0.5 mol / kg or less, had a higher discharge capacity retention rate than Example 4, in which the concentration of phosphate anions in the electrolyte solution was greater than 0.5 mol / kg. This shows that the charge-discharge characteristics can be improved by setting the concentration of phosphate anions in the electrolyte solution to 0.5 mol / kg.

[0154] Example 6 After a secondary battery was fabricated using the positive electrode 20, the charge / discharge characteristics of the secondary battery were evaluated.

[0155] [Fabrication of Secondary Battery] The two-liquid type secondary battery described in the second embodiment shown in FIG. 2 was fabricated according to the procedure described below.

[0156] (Fabrication of Positive Electrode) In Example 6, a positive electrode was fabricated under the same conditions as in Example 1, using lithium manganate coated with manganese phosphate as positive electrode active material particles.

[0157] (Fabrication of Negative Electrode) First, anatase titanium oxide as the negative electrode active material, polyvinylidene fluoride as the negative electrode binder, and graphite as the negative electrode conductive agent were mixed in a mass ratio of 89:10:1 to prepare a negative electrode mixture. Next, the negative electrode mixture was added to N-methyl-2-pyrrolidone, an organic solvent, and the solvent was stirred to prepare a paste-like negative electrode mixture slurry. Finally, using a coating device, the negative electrode mixture slurry was applied to both sides of a 20 μm-thick titanium foil negative electrode current collector 30A, excluding the connection terminal portion 30AT, and then dried to form a negative electrode active material layer 30B. This produced the negative electrode 30.

[0158] (Preparation of Positive Electrolyte) The same electrolyte 40 as in Example 1 was used as the positive electrode electrolyte 61 .

[0159] (Preparation of negative electrode electrolyte) Lithium hydroxide (LiOH), which is an alkali ionic substance, was added to pure water, which is an aqueous solvent, and the mixture was stirred to prepare negative electrode electrolyte 62, which is an aqueous electrolyte. Negative electrode electrolyte 62 was prepared to have a solute concentration of 4 mol / kg and a pH of 12.

[0160] (Assembly of 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 drawn 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 in the positive electrode chamber S1, and the negative electrode electrolyte 62 was accommodated in the negative electrode chamber S2, completing a dual-liquid type secondary battery.

[0161] [Evaluation of Secondary Battery Characteristics] In Example 6, the charge / discharge characteristics of the secondary battery were evaluated according to the procedure described below, and the results shown in Table 2 were obtained. Here, the discharge capacity retention rate was measured in the same manner as in Examples 1 to 5.

[0162]

[0163] As shown in Table 2, in Example 6 in which the secondary battery according to the second embodiment was used, the ratio I ２ / I １ was 0.2 or more and 2.0 or less, the P / Mn ratio was 0.2 or more and 5.0 or less, the pH of the positive electrode electrolyte 61 was 9 or more, and lithium manganese oxide was used as the lithium manganese composite oxide having a spinel crystal structure as the positive electrode active material, thereby achieving high charge-discharge efficiency similar to Examples 1 to 5. Therefore, it can be seen that the charge-discharge characteristics can also be improved in the secondary battery according to the second embodiment.

[0164] 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.

[0165] 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.

[0166] REFERENCE SIGNS LIST 1 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 battery comprising: a positive electrode containing positive electrode active material particles; a negative electrode; and an electrolyte containing an aqueous solvent, wherein the positive electrode active material particles contain a lithium manganese composite oxide having a spinel crystal structure; the pH of the electrolyte is less than 9; and a Raman scattering intensity of 600 cm measured on the surface of the positive electrode active material particles by Raman spectroscopy using 532 nm excitation light. －１ More than 700cm －１ The maximum scattered light intensity I in the range １ And 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range ２ Relative to I ２ / I １ is 0.2 or more and 2 or less, and a molar ratio of phosphorus to manganese measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 or more and 5 or less.

2. The secondary battery according to claim 1, wherein the pH of the electrolyte is less than 6.

3. The secondary battery according to claim 1 or 2, wherein the electrolyte solution contains at least one phosphate-based anion selected from the group consisting of phosphate anion, monohydrogen phosphate anion, and dihydrogen phosphate anion, and the concentration of the phosphate-based anion in the electrolyte solution is 0.5 mol / kg or less.

4. A battery comprising: a positive electrode that absorbs and releases lithium ions; a negative electrode that absorbs and releases the lithium ions; a positive electrode chamber that houses the positive electrode; a negative electrode chamber that houses the negative electrode; a partition wall that is arranged between the positive electrode chamber and the negative electrode chamber and that allows the lithium ions to pass; a positive electrode electrolyte that is housed in the positive electrode chamber and contains an aqueous solvent; and a negative electrode electrolyte that is housed in the negative electrode chamber and contains an aqueous solvent, wherein the pH of the negative electrode electrolyte is higher than the pH of the positive electrode electrolyte, the positive electrode includes positive electrode active material particles that contain a lithium manganese composite oxide having a spinel crystal structure, and the pH of the positive electrode electrolyte is lower than 9, and a Raman spectroscopy using 532 nm excitation light on the surface of the positive electrode active material particles at 600 cm －１ More than 700cm －１ The maximum scattered light intensity I in the range １ And 950 cm －１ More than 1000cm －１ The maximum scattered light intensity I in the range ２ Relative to I ２ / I １ is 0.2 or more and 2 or less, and a molar ratio of phosphorus to manganese measured by X-ray photoelectron spectroscopy on the surface of the positive electrode active material particles is 0.2 or more and 5 or less.

5. The secondary battery according to claim 4, wherein the pH of the positive electrode electrolyte is less than 6.

6. The secondary battery according to claim 4 or 5, wherein the positive electrode electrolyte contains at least one phosphate-based anion selected from the group consisting of phosphate anion, monohydrogen phosphate anion, and dihydrogen phosphate anion, and the concentration of the phosphate-based anion in the positive electrode electrolyte is 0.5 mol / kg or less.

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