Secondary battery system

Abstract

To provide a secondary battery containing sulfide solid electrolyte capable of suppressing the increase in resistance of the secondary battery in the high voltage range.SOLUTION: A disclosed secondary battery system includes a secondary battery and a control unit. The secondary battery includes a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer. The positive electrode active material layer contains a positive electrode active material, a sulfide solid electrolyte, and a fluorine-based liquid material. The control unit controls the charge of the secondary battery so that the positive electrode potential at the end of the charge of the secondary battery is 4.3 V (vs. Li / Li+) or more.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] This application discloses a secondary battery system.

Background Art

[0002] Patent Document 1 discloses using a sulfide solid electrolyte together with a positive electrode active material in the positive electrode active material layer of a secondary battery.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Conventional secondary batteries tend to have an increased resistance in a high voltage region (for example, 4.3 V (vs. Li / Li+) or higher).

Means for Solving the Problems

[0005] As means for solving the above problems, this application discloses the following multiple aspects. <Aspect 1> A secondary battery system including a secondary battery and a control unit, wherein the secondary battery includes a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, the positive electrode active material layer includes a positive electrode active material, a sulfide solid electrolyte, and a fluorine-based liquid material, and the control unit controls charging of the secondary battery such that the positive electrode potential at the charging end potential of the secondary battery is 4.3 V (vs. Li / Li+) or higher. Secondary battery system. <Aspect 2> The secondary battery system according to Aspect 1, wherein the positive electrode active material layer includes perfluoropolyether as the fluorine-based liquid material. Secondary battery system. <Aspect 3> A secondary battery system according to Aspect 1 or 2, wherein the positive electrode active material is a Li-containing oxide. Secondary battery system. <Aspect 4> A secondary battery system according to Aspect 3, wherein the Li-containing oxide contains at least one transition metal element among Mn, Ni, and Co as constituent elements. Secondary battery system.

Effect of the Invention

[0006] According to the secondary battery system of the present disclosure, an increase in the resistance of the secondary battery in a high voltage region is reduced.

Brief Description of the Drawings

[0007]

Figure 1

Modes for Carrying Out the Invention

[0008] Hereinafter, an embodiment of the secondary battery system of the present disclosure will be described, but the secondary battery system of the present disclosure is not limited to the embodiments shown below.

[0009] As shown in FIG. 1, a secondary battery system 100 according to an embodiment includes a secondary battery 10 and a control unit 20. The secondary battery 10 includes a positive electrode active material layer 12, an electrolyte layer 13, and a negative electrode active material layer 14. The positive electrode active material layer 12 includes a positive electrode active material, a sulfide solid electrolyte, and a fluorine-based liquid material. The control unit 20 controls charging of the secondary battery 10 such that the positive electrode potential at the charging end potential of the secondary battery 10 is 4.3 V (vs. Li / Li+) or higher.

[0010] 1. Secondary battery The secondary battery 10 includes a positive electrode active material layer 12, an electrolyte layer 13, and a negative electrode active material layer 14. Further, as shown in FIG. 1, the secondary battery 10 may include a positive electrode current collector 11 that contacts the positive electrode active material layer 12. Further, the secondary battery 10 may include a negative electrode current collector 15 that contacts the negative electrode active material layer 14.

[0011] 1.1 Positive Electrode Active Material Layer The positive electrode active material layer 12 contains a positive electrode active material, a sulfide solid electrolyte, and a fluorine-based liquid material. Further, the positive electrode active material layer 12 may optionally contain a conductive assistant and a binder. Further, the positive electrode active material layer 12 may optionally contain other electrolytes. The content of each component in the positive electrode active material layer 12 may be appropriately determined according to the target battery performance. For example, the positive electrode active material layer 12 may contain 1% by volume or more and 25% by volume or less of the fluorine-based liquid material. The content of the fluorine-based liquid material contained in the positive electrode active material layer 12 is, for example, 1% by volume or more, 3% by volume or more, 5% by volume or more, 7% by volume or more, 8% by volume or more, 9% by volume or more, 10% by volume or more, 11% by volume or more, or 12% by volume or more, and 25% by volume or less, 24% by volume or less, 22% by volume or less, 20% by volume or less, 18% by volume or less, 16% by volume or less, 14% by volume or less, or 12% by volume or less. Incidentally, the volume ratio of the fluorine-based liquid material in the positive electrode active material layer 12 can be measured, for example, as follows. That is, the volume of the positive electrode active material layer 12 is measured in advance using an optical microscope or SEM. The volume of the fluorine-based liquid material contained in the positive electrode active material layer 12 may be specified by washing the positive electrode active material layer 12 with a solvent (a solvent that can dissolve the fluorine-based liquid material and does not dissolve other electrode materials), collecting the filtrate in which the fluorine-based liquid material is dissolved by suction filtration or the like, and analyzing the collected solvent by GC-MS. Alternatively, when the boiling point of the solvent is significantly different from the boiling point of the fluorine-based liquid material, the volume of the fluorine-based liquid material may be directly measured by extracting the fluorine-based liquid material by distillation or the like. Thereby, the volume ratio of the fluorine-based liquid material in the total volume of the positive electrode active material layer 12 measured in advance is calculated. Also, with the total solid content of the positive electrode active material layer 12 as 100% by mass, the content of the positive electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more, and may be less than 100% by mass, 95% by mass or less, or 90% by mass or less. Also, with the total solid content of the positive electrode active material layer 12 as 100% by mass, the content of the sulfide solid electrolyte may be more than 0% by mass, 5% by mass or more, or 10% by mass or more, and may be 60% by mass or less, 50% by mass or less, 40% by mass or less, or 30% by mass or less.Alternatively, taking the entire positive electrode active material layer 12 as 100% by volume, the positive electrode active material, the sulfide solid electrolyte, the fluorine-based liquid material, and optionally other electrolytes, conductive aids, and binders may be contained in a total amount of 85% by volume or more, 90% by volume or more, or 95% by volume or more, and the balance may be voids or other components. The shape of the positive electrode active material layer 12 is not particularly limited, and for example, it may be in the form of a sheet having a substantially flat surface. The thickness of the positive electrode active material layer 12 is not particularly limited, and for example, it may be 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, 1 mm or less, or 500 μm or less.

[0012] 1.1.1 Positive Electrode Active Material The positive electrode active material may be, for example, at least one selected from Li-containing compounds, elemental sulfur, sulfur compounds, etc. In particular, when the positive electrode active material contains a Li-containing compound, a higher effect is more easily obtained. Only one type of positive electrode active material may be used alone, or two or more types may be used in combination. In one embodiment, the positive electrode active material may be a Li-containing oxide. The Li-containing oxide may contain at least one element M, Li, and O. Element M may be, for example, at least one selected from Mn, Ni, Co, Al, Mg, Ca, Sc, V, Cr, Cu, Zn, Ga, Ge, Y, Zr, Sn, Sb, W, Pb, Bi, Fe, and Ti, or at least one selected from the group consisting of Mn, Ni, Co, Al, Fe, and Ti. More specifically, the Li-containing oxide is lithium cobaltate, lithium nickelate, lithium manganate, lithium nickel cobaltate, lithium nickel manganate, lithium cobalt manganate, lithium nickel cobalt manganate (Li 1±α Ni x Co y Mn z O 2±δ (for example, 0 < x < 1, 0 < y < 1, 0 < z < 1, x + y + z = 1), spinel-type lithium compound (Li 1+x Mn 2-x-y M yHetero-element-substituted Li-Mn spinel represented by O4 (where M is at least one selected from Al, Mg, Co, Fe, Ni, and Zn), lithium nickel cobalt aluminate (for example, Li 1±α Ni p Co q Al r O 2±δ (for example, p + q + r = 1)), lithium titanate, lithium metal phosphate (such as LiMPO4, where M is at least one selected from Fe, Mn, Co, and Ni), etc., and may be at least one selected therefrom. In particular, when the Li-containing oxide as the positive electrode active material contains at least one transition metal element among at least Mn, Ni, and Co as constituent elements, higher performance is more easily obtained. Alternatively, when the positive electrode active material contains at least one element among at least Ni, Co, and Al as constituent elements, higher performance is also more easily obtained. The shape of the positive electrode active material is not particularly limited, and for example, it may be in the form of particles. The positive electrode active material particles may be solid particles, hollow particles, or particles having voids. The positive electrode active material particles may be primary particles or secondary particles in which a plurality of primary particles are aggregated. The average particle diameter (D50) of the positive electrode active material particles may be, for example, 1 nm or more and 500 μm or less. The lower limit may be 5 nm or more or 10 nm or more, and the upper limit may be 100 μm or less, 50 μm or less, or 30 μm or less. In the present application, the average particle diameter D50 is the particle diameter (median diameter) at the integrated value of 50% in the volume-based particle size distribution determined by the laser diffraction / scattering method.

[0013] A protective layer containing an ion-conductive oxide may be formed on the surface of the positive electrode active material. Thereby, reactions such as the reaction between the positive electrode active material and the sulfide solid electrolyte are more easily suppressed. Examples of the ion-conductive oxide include Li3BO3, LiBO2, Li2CO3, LiAlO2, Li4SiO4, Li2SiO3, Li3PO4, Li2SO4, Li2TiO3, Li4Ti5O 12, such as Li2Ti2O5, Li2ZrO3, LiNbO3, Li2MoO4, Li2WO4, etc. The ion-conductive oxide may be one in which some elements are substituted by doping elements such as P and B. The coverage rate (area ratio) of the protective layer on the surface of the positive electrode active material may be, for example, 70% or more, 80% or more, or 90% or more. The thickness of the protective layer may be, for example, 0.1 nm or more or 1 nm or more, and may also be 100 nm or less or 20 nm or less.

[0014] 1.1.2 Sulfide solid electrolyte As solid electrolytes having lithium-ion conductivity, any of the known sulfide solid electrolytes can be adopted. The sulfide solid electrolyte may be a glass-based sulfide solid electrolyte (sulfide glass), a glass-ceramics-based sulfide solid electrolyte, or a crystalline sulfide solid electrolyte. When the sulfide solid electrolyte has a crystalline phase, examples of the crystalline phase include a Thio-LISICON type crystalline phase, an LGPS type crystalline phase, and an argyrodite type crystalline phase. The sulfide solid electrolyte may contain, for example, an Li element, an X element (X is at least one of P, As, Sb, Si, Ge, Sn, B, Al, Ga, In), and an S element. In particular, those containing Li, P, and S as constituent elements have high performance. Further, the sulfide solid electrolyte may further contain at least one of an O element and a halogen element. In particular, those containing Li, P, S, and a halogen element as constituent elements have high performance. The composition of the sulfide solid electrolyte is not particularly limited, and examples include xLi2S·(100-x)P2S5 (70≦x≦80), yLiI·zLiBr·(100-y-z)(xLi2S·(1-x)P2S5) (0.7≦x≦0.8, 0≦y≦30, 0≦z≦30), etc. Alternatively, the sulfide solid electrolyte has the general formula: Li 4-x Ge 1-x P xIt may have a composition represented by S4(0 < x < 1). In the above general formula, at least a part of Ge may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, at least a part of P may be substituted with at least one of Sb, Si, Sn, B, Al, Ga, In, Ti, Zr, V, and Nb. In the above general formula, a part of Li may be substituted with at least one of Na, K, Mg, Ca, and Zn. In the above general formula, a part of S may be substituted with a halogen (at least one of F, Cl, Br, and I). Alternatively, the sulfide solid electrolyte is Li 7-a PS 6-a X a (X is at least one of Cl, Br, and I, and a is a number of 0 or more and 2 or less) and may have a composition represented by. a may be 0, or may be greater than 0. In the latter case, a may be 0.1 or more, may be 0.5 or more, and may be 1 or more. Also, a may be 1.8 or less, may be 1.5 or less. The shape of the sulfide solid electrolyte is not particularly limited, and for example, it may be particulate. Only one kind of sulfide solid electrolyte may be used alone, or two or more kinds may be used in combination.

[0015] 1.1.3 Fluorine-based liquid materials The fluorine-based liquid material is a liquid material containing fluorine as a constituent element. According to the findings of the present inventors, in the positive electrode active material layer 12, the fluorine-based liquid material is interposed at the interface between the above positive electrode active material and the sulfide solid electrolyte. That is, the direct contact between the positive electrode active material and the sulfide solid electrolyte is suppressed by the fluorine-based liquid material. As a result, even when the positive electrode potential becomes a high potential, the sulfide solid electrolyte is less likely to be exposed to the high potential, and the decomposition of the sulfide solid electrolyte is suppressed. As a result, even when the positive electrode potential becomes a high potential, the increase in the resistance of the secondary battery 10 is reduced. Further, the fluorine-based liquid material has low reactivity with other battery materials. Therefore, even if the positive electrode active material layer 12 contains a fluorine-based liquid material, the positive electrode active material, the sulfide solid electrolyte, etc. can exist stably. The fluorine-based liquid material may be, for example, an organic fluorine compound in which fluorine is bonded to a carbon chain. For example, the positive electrode active material layer 12 may contain perfluoropolyether as the fluorine-based liquid material. Since perfluoropolyether as the fluorine-based liquid material has an ether bond, it is considered to have a high affinity for the surface of other battery materials. For example, it can be appropriately present in voids between active material materials, voids between sulfide solid electrolyte materials, and voids between the active material material and the sulfide solid electrolyte material. Thereby, the direct contact between the positive electrode active material and the sulfide solid electrolyte is further suppressed. Further, perfluoropolyether, as described above, has low reactivity with other battery materials.

[0016] The positive electrode active material layer 12 may contain perfluoropolyether represented by the following formula (1) as the fluorine-based liquid material. E1-Rf1-R F -O-Rf2-E2(1) [In formula (1), Rf1 and Rf2 are each independently a C1-16 divalent alkylene group which may be substituted by one or more fluorine atoms, E1 and E2 are each independently a monovalent group selected from the group consisting of a fluorine group, a hydrogen group, a hydroxyl group, an aldehyde group, a carboxylic acid group, a C1-10 alkyl ester group, an amide group which may have one or more substituents, and an amino group which may have one or more substituents. R F is a divalent fluoropolyether group.

[0017] In the above formula (1), Rf1 and Rf2 are each independently a C1-16 divalent alkylene group which may be substituted by one or more fluorine atoms. In one embodiment, the "C1-16 divalent alkylene group" in the C1-16 divalent alkylene group which may be substituted by one or more fluorine atoms may be linear or branched, preferably a linear or branched C1-6 alkylalkylene group, particularly a C1-3 alkylene group, and more preferably a linear C1-6 alkylene group, particularly a C1-3 alkylene group. In one embodiment, the "C1-16 divalent alkylene" in the C1-16 divalent alkylene group which may be substituted by one or more fluorine atoms may be linear or branched, preferably a linear or branched C1-6 fluoroalkylene group, particularly a C1-3 fluoroalkylene group, specifically, -CF2CH2-, and -CF2CF2CH2- may also be used, and more preferably a linear C1-6 perfluoroalkylene group, particularly a C1-3 perfluoroalkylene group, specifically, a group selected from the group consisting of -CF2-, -CF2CF2-, and -CF2CF2CF2- may also be used.

[0018] In the above formula (1), E1 and E2 are each independently a monovalent group selected from the group consisting of a fluorine group, a hydrogen group, a hydroxyl group, an aldehyde group, a carboxylic acid group, a C1-10 alkyl ester group, an amide group which may have one or more substituents, and an amino group which may have one or more substituents. As described above, perfluoropolyether has low reactivity with respect to the sulfide solid electrolyte described later. Therefore, even when perfluoropolyether and the sulfide solid electrolyte come into contact with each other, a decrease in ionic conductivity due to alteration or deterioration of the sulfide solid electrolyte hardly occurs. In particular, when the perfluoropolyether has a nonpolar group as a terminal group, the reaction between the perfluoropolyether and the sulfide solid electrolyte is further suppressed, and a higher effect can be expected. In this regard, E1 and E2 are each independently preferably a fluorine group. In one aspect, E1-Rf1 and E2-Rf2 may each independently be a group selected from the group consisting of -CF3, -CF2CF3, and -CF2CF2CF3.

[0019] In the above formula (1), R F is, in each occurrence, independently a divalent fluoropolyether group. R F is preferably of formula (2): -(OC6F 12 ) a -(OC5F 10 ) b -(OC4F8) c -(OC3R Fa 6) d -(OC2F4) e -(OCF2) f - (2) [In formula (2): R Fa is, in each occurrence, independently a hydrogen atom, a fluorine atom or a chlorine atom, a, b, c, d, e and f are each independently an integer from 0 to 200, the sum of a, b, c, d, e and f is 1 or more, the order of existence of each repeating unit enclosed in parentheses with a, b, c, d, e or f attached is arbitrary in the formula, provided that all RFa When it is a hydrogen atom or a chlorine atom, at least one of a, b, c, e, and f is 1 or more. is a group represented by R Fa is preferably a hydrogen atom or a fluorine atom, more preferably a fluorine atom. a, b, c, d, e, and f are preferably each independently an integer from 0 to 100. The sum of a, b, c, d, e, and f is preferably 5 or more, more preferably 10 or more, and may be, for example, 15 or more or 20 or more. The sum of a, b, c, d, e, and f is preferably 200 or less, more preferably 100 or less, still more preferably 60 or less, and may be, for example, 50 or less or 30 or less.

[0020] These repeating units may be linear or branched. R F In, the ratio of d to f (hereinafter referred to as "d / f ratio") may be 0.5 to 4, preferably 0.6 to 3, more preferably 0.7 to 2, and even more preferably 0.8 to 1.4. By setting the d / f ratio to 4 or less, the lubricity and chemical stability are further improved. The smaller the d / f ratio, the more improved the lubricity. On the other hand, by setting the d / f ratio to 0.5 or more, the stability of the compound can be further enhanced. The larger the d / f ratio, the more improved the stability of the fluoropolyether structure. In this case, the value of f is preferably 0.8 or more. In the above, R F The number average molecular weight of the R part is not particularly limited, but is, for example, 500 to 30,000, preferably 1,500 to 30,000, more preferably 2,000 to 10,000. In the present application, the number average molecular weight of R F is 19 a value measured by 19F-NMR.

[0021] 1.1.4 Other components Other electrolytes that can be included in the positive electrode active material layer 12 may be solid electrolytes, liquid electrolytes (electrolyte solutions), or combinations thereof. The solid electrolyte may be an inorganic solid electrolyte or an organic polymer electrolyte. Examples of inorganic solid electrolytes other than sulfide solid electrolytes include oxide solid electrolytes, halide solid electrolytes, complex hydride solid electrolytes, and the like. The electrolyte solution may be, for example, a solution in which a lithium salt is dissolved in a carbonate-based solvent at a predetermined concentration. Examples of carbonate-based solvents include fluoroethylene carbonate (FEC), ethylene carbonate (EC), dimethyl carbonate (DMC), and the like. Examples of lithium salts include lithium amide salts such as LiTFSI and LiFSI, and LiPF6.

[0022] Examples of conductive aids that can be included in the positive electrode active material layer 12 include carbon materials such as vapor-grown carbon fiber (VGCF), acetylene black (AB), ketjen black (KB), carbon nanotube (CNT), and carbon nanofiber (CNF); and metal materials such as nickel, aluminum, and stainless steel. The conductive aid may be, for example, particulate or fibrous, and its size is not particularly limited. Only one type of conductive aid may be used alone, or two or more types may be used in combination.

[0023] Examples of binders that can be included in the positive electrode active material layer 12 include butadiene rubber (BR)-based binders, isobutylene rubber (IIR)-based binders, acrylate butadiene rubber (ABR)-based binders, styrene butadiene rubber (SBR)-based binders, polyvinylidene fluoride (PVdF)-based binders, polytetrafluoroethylene (PTFE)-based binders, polyimide (PI)-based binders, and the like. Only one type of binder may be used alone, or two or more types may be used in combination.

[0024] 1.2 Electrolyte Layer The electrolyte layer 13 is disposed between the positive electrode active material layer 12 and the negative electrode active material layer 14 and can function as a separator. The electrolyte layer 13 contains at least an electrolyte and may further optionally contain a binder or the like. For example, the electrolyte layer 13 may contain the above-mentioned sulfide solid electrolyte. The electrolyte layer 13 may further contain other components such as a dispersant and the above-mentioned fluorine-based liquid material. The content of each component in the electrolyte layer 13 is not particularly limited and may be appropriately determined according to the target battery performance. The shape of the electrolyte layer 13 is not particularly limited. For example, it may be in the form of a sheet having a substantially flat surface. The thickness of the electrolyte layer 13 is not particularly limited. For example, it may be 0.1 μm or more or 1 μm or more, and may also be 2 mm or less or 1 mm or less.

[0025] 1.3 Negative Electrode Active Material Layer The negative electrode active material layer 14 contains at least a negative electrode active material and may further optionally contain an electrolyte, a conductive assistant, a binder, and the like. The negative electrode active material layer 14 may also contain various other additives. As the negative electrode active material, an appropriate one may be selected from known active materials such as silicon-based active materials, carbon-based active materials, and oxide-based active materials. The content of each component in the negative electrode active material layer 14 may be appropriately determined according to the target battery performance. For example, assuming the total solid content of the negative electrode active material layer 14 is 100% by mass, the content of the negative electrode active material may be 40% by mass or more, 50% by mass or more, 60% by mass or more, or 70% by mass or more, and may also be 100% by mass or less, 95% by mass or less, or 90% by mass or less. The shape of the negative electrode active material layer is not particularly limited. For example, it may be a sheet-shaped negative electrode active material layer having a substantially flat surface. The thickness of the negative electrode active material layer 14 is not particularly limited. For example, it may be 0.1 μm or more, 1 μm or more, or 10 μm or more, and may also be 2 mm or less, 1 mm or less, or 500 μm or less.

[0026] 1.4 Other Configurations The positive current collector 11 and the negative current collector 15 of the secondary battery 10 can each adopt any commonly used current collector for a secondary battery. The secondary battery 10 may be one in which each of the above components is housed inside the exterior body. The exterior body can adopt any known exterior body for a battery. Also, a plurality of secondary batteries 10 may be arbitrarily electrically connected and arbitrarily stacked to form a battery pack. In this case, the battery pack may be housed inside a known battery case. The secondary battery 10 may further include obvious components such as necessary terminals. Examples of the shape of the secondary battery 10 include coin type, laminate type, cylindrical type, and square type. The secondary battery 10 can be manufactured by a method as disclosed in, for example, Patent Document 1 (Japanese Patent Application Laid-Open No. 2019-125510), except that a fluorine-based liquid material is included in the positive electrode active material layer 12 together with the above positive electrode active material and sulfide solid electrolyte. That is, each layer constituting the secondary battery 10 may be manufactured by wet forming, dry forming, or the like.

[0027] 2. Control Unit As shown in FIG. 1, the secondary battery system 100 includes a control unit 20. The control unit 20 only needs to be capable of controlling the charge and discharge of the secondary battery 10, and its configuration is not particularly limited. The control unit 20 includes known configurations necessary for controlling the charge and discharge of the secondary battery 10, and may include, for example, a CPU, a RAM, a ROM, etc.

[0028] The control unit 20 controls the charging of the secondary battery 10 such that the positive electrode potential at the charging end potential of the secondary battery 10 becomes 4.3 V (vs. Li / Li+) or higher. In a conventional secondary battery, when charging is performed until the positive electrode potential reaches a high potential of 4.3 V (vs. Li / Li+) or higher, decomposition of the sulfide solid electrolyte or the like occurs, and the resistance of the secondary battery tends to increase. On the other hand, in the secondary battery system 100 according to the present embodiment, as described above, since the positive electrode active material layer 12 of the secondary battery 10 contains a fluorine-based liquid material together with the positive electrode active material and the sulfide solid electrolyte, even when the positive electrode potential of the secondary battery 10 reaches a high potential of 4.3 V (vs. Li / Li+) or higher, decomposition of the sulfide solid electrolyte or the like is likely to be suppressed, and an increase in the resistance of the secondary battery 10 is likely to be reduced.

[0029] The control unit 20 may control the charging of the secondary battery 10 such that the positive electrode potential at the charging end potential of the secondary battery 10 becomes 4.4 V (vs. Li / Li+) or higher, 4.5 V (vs. Li / Li+) or higher, 4.6 V (vs. Li / Li+) or higher, or 4.7 V (vs. Li / Li+) or higher. The upper limit of the charging end potential is not particularly limited. For example, the control unit 20 may control the charging of the secondary battery 10 such that the positive electrode potential at the charging end potential of the secondary battery 10 becomes 4.3 V (vs. Li / Li+) or higher and 5.0 V (vs. Li / Li+) or lower. Note that, as described above, the control unit 20 controls the charging of the secondary battery 10, but may also control the discharging of the secondary battery 10 in addition to the charging of the secondary battery 10. The discharging end potential of the secondary battery 10 may be appropriately determined according to the performance of the secondary battery 10 or the like.

Examples

[0030] Hereinafter, the technology of the present disclosure will be described in more detail while showing examples, but the technology of the present disclosure is not limited to the following examples.

[0031] 1. Fabrication of positive electrode 1.1 Examples 1 to 4, Comparative Examples 7 and 8 In an organic solvent, a binder (PVdF), a conductive assistant (VGCF), a sulfide solid electrolyte (LiI-LiBr-Li2S-P2S5), and a positive electrode active material (LiNi1 / 3 Co 1 / 3 Mn 1 / 3 O₂) and perfluoropolyether (PFPE, perfluoro(polyoxypropylene ethyl ether) 2700 manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) were kneaded using an ultrasonic homogenizer to obtain a positive electrode composite material slurry. The obtained positive electrode composite material slurry was coated on an Al foil and dried to obtain a positive electrode for pressing having a positive electrode active material layer on the Al foil as a positive electrode current collector. Here, the volume ratio of PFPE in the composite material was set to 8% by volume.

[0032] 1.2 Comparative Examples 1 to 6 A positive electrode for pressing was obtained in the same manner as in Examples 1 to 4 and Comparative Examples 7 and 8, except that PFPE was not added to the positive electrode composite material.

[0033] 2. Preparation of the electrolyte layer A binder (PVdF) and a sulfide solid electrolyte (LiI-LiBr-Li₂S-P₂S₅) were added to an organic solvent and kneaded using an ultrasonic homogenizer to obtain an electrolyte composite material slurry. The obtained electrolyte composite material slurry was coated on an Al foil and dried to form a solid electrolyte layer on the Al foil as a base material.

[0034] 3. Preparation of the battery The above positive electrode for pressing was cut out into strip shapes. The cut positive electrode active material layer and the solid electrolyte layer were overlapped and roll-pressed at 165 °C under a pressure of 50 kN / cm, and the Al foil as the base material was peeled off to transfer the solid electrolyte layer onto the surface of the positive electrode active material layer. A Li foil punched out to φ13.00 mm was overlapped on the surface of the solid electrolyte layer to obtain a laminate having a structure of Al foil / positive electrode active material layer / solid electrolyte layer / Li foil. A current extraction tab was attached to the laminate and encapsulated in an aluminum laminate using a vacuum laminator to fabricate an evaluation battery.

[0035] 4. Performance evaluation of the battery Regarding the fabricated battery, charging was performed up to the charge termination voltage (upper limit voltage), and a trickle test (held for one week at each upper limit voltage) was conducted at the upper limit voltage, and the resistance values before and after the trickle test were measured. Specifically, for each battery before and after the trickle test, the voltage was adjusted to 3.7 V, and the resistance value was calculated from the voltage drop after 5 seconds when discharging at a 5C rate. The resistance value after the trickle test with respect to the resistance value before the trickle test was measured, and this was taken as the resistance increase rate. The results are shown in Table 1 below. In Table 1 below, the resistance increase rates for Comparative Examples 2 to 8 and Examples 1 to 4 are shown relative to the resistance increase rate for Comparative Example 1 as the reference (100%).

[0036]

Table 1

[0037] As is clear from the results shown in Table 1, when the positive electrode active material layer contains a sulfide solid electrolyte together with the positive electrode active material and does not contain PFPE (Comparative Examples 1 to 6), when the positive electrode potential in the trickle test is less than 4.3 V (vs. Li / Li+), the resistance of the battery does not increase even after the trickle test (Comparative Examples 1 and 2), whereas when the positive electrode potential in the trickle test is 4.3 V (vs. Li / Li+) or higher, it can be seen that the resistance of the battery increases after the trickle test (Comparative Examples 3 to 6). In Comparative Examples 3 to 6, it is considered that the sulfide solid electrolyte was decomposed due to being exposed to a high potential, and as a result, the resistance of the battery increased.

[0038] On the one hand, when the positive electrode active material layer contains a sulfide solid electrolyte together with the positive electrode active material and contains PFPE (Comparative Examples 7, 8, Examples 1 to 4), when the positive electrode potential in the trickle test is less than 4.3 V (vs. Li / Li+), the advantageous effect due to including PFPE is not particularly exhibited (comparison between Comparative Examples 1, 2 and Comparative Examples 7, 8). On the other hand, when the positive electrode potential in the trickle test is 4.3 V (vs. Li / Li+) or higher, it can be seen that including PFPE can suppress the increase in the resistance of the battery (comparison between Comparative Examples 3 to 6 and Examples 1 to 4). In Examples 1 to 4, PFPE suppresses the direct contact between the positive electrode active material and the sulfide solid electrolyte. Even when the positive electrode potential becomes a high potential, the sulfide solid electrolyte is less likely to be exposed to the high potential, the decomposition of the sulfide solid electrolyte is suppressed, and as a result, it is considered that the increase in the resistance of the battery is reduced even when the positive electrode potential becomes a high potential.

[0039] 5. Supplementary In addition, in the above examples, a specific PFPE was exemplified as the fluorine-based liquid material, but the type of the fluorine-based liquid material is not limited to the PFPE. Also, in the above examples, the case of using specific battery materials (NCM as the positive electrode active material, LiI-LiBr-Li2S-P2S5 as the sulfide solid electrolyte, PVdF as the binder, VGCF as the conductive assistant, and Li foil as the negative electrode active material) was exemplified. However, even when using battery materials other than these, it is considered that the above effects are exhibited by using the fluorine-based liquid material in combination with the positive electrode active material and the sulfide solid electrolyte in the positive electrode active material layer.

[0040] 6. Summary From the above results, it can be said that a secondary battery system having the following configuration can reduce the increase in the resistance of the secondary battery in the high potential region. (1) The secondary battery system includes a secondary battery and a control unit. (2) The secondary battery includes a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer. (3) The positive electrode active material layer contains a positive electrode active material, a sulfide solid electrolyte, and a fluorine-based liquid material. (4) The control unit controls the charging of the secondary battery such that the positive electrode potential at the charging end potential of the secondary battery becomes 4.3 V (vs. Li / Li+) or more.

Explanation of Signs

[0041] 100 Secondary battery system 10 Secondary battery 11 Positive electrode current collector 12 Positive electrode active material layer 13 Electrolyte layer 14 Negative electrode active material layer 15 Negative electrode current collector 20 Control unit

Claims

Claim 1 A secondary battery system comprising a secondary battery and a control unit, wherein the secondary battery includes a positive electrode active material layer, an electrolyte layer, and a negative electrode active material layer, the positive electrode active material layer contains a positive electrode active material, a sulfide solid electrolyte, and a fluorine-based liquid material, the positive electrode active material is a Li-containing oxide, the Li-containing oxide contains at least one transition metal element among Mn, Ni, and Co as constituent elements, the positive electrode active material layer contains perfluoropolyether as the fluorine-based liquid material, and the control unit controls charging of the secondary battery such that the positive electrode potential at the charge termination potential of the secondary battery is 4.3 V (vs. Li / Li+) or higher. A secondary battery system.

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