JP2013093188A - Secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a secondary battery relaxing a stress due to volume variation of an electrode caused in charging/discharging a battery, even when a sintered body electrode is densified in order to improve capacity of the secondary battery, thereby suppressing deterioration in battery capacity due to breakage of an electrode and a solid electrolyte and destruction of a power generation element.SOLUTION: The secondary battery includes a power generation element including a pair of electrodes bonded via a solid electrolyte layer, and at least one of the pair of electrodes comprises an oxide sintered body. The oxide sintered body has a porosity of 2-10%, has cracks in an inside of the oxide sintered body, and relaxes a stress due to volume variation of an electrode caused in charging/discharging the battery.

Description

  The present invention relates to a secondary battery using a solid electrolyte.

  In recent years, secondary batteries have been used not only for mobile phones and notebook PCs but also as batteries for electric vehicles.

  Conventionally, an electrolyte obtained by impregnating a porous membrane called a separator with a non-aqueous electrolyte solution has been used as an electrolyte of a secondary battery. However, in recent years, a secondary battery using a solid electrolyte from the viewpoint of safety. Has been proposed.

In a secondary battery using a solid electrolyte, since the active material and the electrolyte are electrically joined by contact between the solids, it is considered that a system with a small volume fluctuation accompanying charging / discharging of the active material is preferable. It has been proposed to use Li ₄ Ti ₅ O ₁₂ , which is an active material having a small volume fluctuation due to electric discharge, as a sintered body having a porosity of 10 to 50% for a positive electrode or a negative electrode and to be combined with a solid electrolyte (Patent Document). 1).

JP 2005-340078 A

  However, in the secondary battery described in Patent Document 1, when the densification of the sintered body electrode is further advanced in order to improve the battery capacity, the electrode and the solid electrolyte are caused by the stress caused by the volume change of the electrode accompanying charge / discharge. Is damaged, resulting in a problem that the battery capacity is reduced, and further, the power generation element in which the positive electrode, the solid electrolyte, and the negative electrode are joined is destroyed.

  Even if the sintered body electrode is densified, the present invention relieves the stress caused by the volume change of the electrode that occurs during charging and discharging of the battery, reduces the battery capacity due to the damage of the electrode and the solid electrolyte, and destroys the power generation element. The purpose is to suppress.

  The secondary battery of the present invention includes a power generation element in which a pair of electrodes are joined via a solid electrolyte layer, and at least one of the pair of electrodes is made of an oxide sintered body, and the oxide sintering The body has a porosity of 2 to 10%, and has cracks inside the oxide sintered body.

  According to the present invention, even if the sintered body electrode is densified, the stress caused by the volume fluctuation of the electrode that occurs during charge and discharge of the battery is relaxed by the crack, the battery capacity is reduced due to the damage of the electrode and the solid electrolyte, and the power generation The destruction of elements can be suppressed.

It is a schematic sectional drawing of the secondary battery which is one Embodiment of this invention. It is an electron micrograph of the surface of the oxide sintered compact which introduce | transduced the crack. It is a schematic diagram for demonstrating the measuring method of the maximum width of a crack.

  FIG. 1 is a schematic sectional view of a secondary battery according to an embodiment of the present invention. In the secondary battery of this embodiment, the power generation element 8 in which the positive electrode 1 is formed on one main surface of the solid electrolyte layer 2 and the negative electrode 3 is formed on the other main surface of the solid electrolyte layer 2 as a pair of electrodes is a positive battery. The battery case is housed in a space formed by the case 5 and the negative battery case 7. The positive electrode side battery case 5 and the negative electrode side battery case 7 are caulked through a gasket 6 so that the space in the battery case is kept airtight.

  Further, in order to make good contact between the positive electrode side battery case 5 and the negative electrode side battery case 7, the positive electrode side current collecting layer 4 </ b> P is provided on the surface of the positive electrode 1 facing the positive electrode side battery case 5, and the negative electrode side battery of the negative electrode 3. A negative current collecting layer 4N is formed on the surface facing the case 7 to reduce the contact resistance between the battery case and the power generation element 9.

  As for the electrode used for the secondary battery of this embodiment, at least one of the positive electrode 1 and the negative electrode 3 is made of an oxide sintered body, and the oxide sintered body has a porosity of 2 to 10% and is shown in FIG. Thus, it has a crack in an oxide sintered compact. The oxide sintered body is a sintered body substantially made of an oxide-based active material. Thus, by making the electrode into a sintered body made of an active material, not only can the capacity decrease due to inclusion of a conductive additive, binder, solid electrolyte, etc. not directly involved in power generation be suppressed, The junction area can be greatly increased, the original electron conductivity and ion conductivity of the active material can be effectively utilized, and a secondary battery with high capacity, high energy density and excellent output characteristics can be obtained. . In addition, in order to improve charge / discharge characteristics and sinterability, some of the constituent elements of the active material are replaced with various metal elements, or various metal compounds are added to the active material to produce a sintered body. It doesn't matter. Note that the sintered body substantially made of an oxide-based active material means a sintered body prepared without positively adding other than the oxide-based active material. Impurities resulting from the process may be included.

  Furthermore, by having cracks inside the oxide sintered body, even if the oxide sintered body is densified to a porosity of 2 to 10%, the stress caused by the volume fluctuation of the electrode that occurs during charge / discharge of the battery is cracked. Therefore, the generation of new cracks and the progress of cracks can be suppressed. In addition, the crack may be formed in any of the inside of the crystal grain of an oxide sintered compact and a crystal grain boundary, and may be open to the surface of an oxide sintered compact. The maximum width of the crack is preferably 2 μm or less. When it exceeds 2 μm, there is a concern that particles near the crack may drop off or the strength of the oxide sintered body may be reduced. In addition, when the maximum width of the crack is smaller than 0.1 μm, the stress generated at the time of charging / discharging of the battery cannot be sufficiently relaxed, and a new crack is generated or the crack progresses, and the electrode and the power generation element 4 are destroyed. there is a possibility. For example, when the average particle diameter of the oxide sintered body is 5 to 20 μm, the maximum width of the crack is more preferably 0.3 to 1 μm.

  The porosity of the oxide sintered body is obtained based on the relative density calculated from the density of the oxide sintered body measured by the Archimedes method and the theoretical density of the active material constituting the oxide sintered body. However, it can also be calculated by a well-known method from the micrograph of the cross section of the oxide sintered body, using the area ratio of the pores in the cross section. For example, a part of an electrode that is an oxide sintered body is cut out, a cross section thereof is observed with a scanning electron microscope (SEM), and an image analysis is performed on a 30 μm × 30 μm region of a photograph taken at any five locations. The area ratio of the pores may be calculated and converted into the porosity by a known method. The pores in the oxide sintered body preferably have a diameter of 2 μm or less.

The maximum width of cracks existing inside the oxide sintered body can be confirmed by observing the cross section of the oxidized portion sintered body with a scanning electron microscope (SEM) or the like. For example, a part of an electrode which is an oxide sintered body is cut out, and the presence or absence of cracks is confirmed with a scanning electron microscope (SEM) in a 150 μm × 150 μm region of the cross section. Next, if the width of the crack having the largest gap is measured, for example, at 20 locations in an area of 30 μm × 30 μm, the maximum value is taken as the maximum width W of the crack existing inside the electrode (see FIG. 3). Good. If necessary, the shape of the pores and cracks can be more clearly observed by performing ion etching or cross section polishing (CP) on the cross section of the electrode.

The active material used for the electrode is not particularly limited as long as either the positive electrode 1 or the negative electrode 3 is an oxide sintered body. Examples of the active material used for the positive electrode 1 include lithium titanium composite oxide, lithium cobalt composite oxide, manganese dioxide, lithium nickel composite oxide, lithium nickel cobalt composite oxide, lithium nickel manganese composite oxide, and lithium vanadium composite oxide. Products, vanadium oxide and the like. All of these active materials can form an oxide sintered body. In particular, lithium nickel manganese composite oxide (LiNi _x Mn _y O ₄ [x = 0.1 to 0.5, y = 1.5 to 1.9]) has a high charge / discharge voltage and a large charge / discharge capacity. Therefore, it is an active material particularly suitable for increasing the capacity and energy density of secondary batteries. On the other hand, lithium cobalt composite oxide is excellent from the viewpoint of electron conductivity, and lithium cobalt composite oxide can also be suitably used in applications that require rapid charge / discharge.

  Examples of the active material used for the negative electrode 3 include titanium oxide, tungsten oxide, molybdenum oxide, niobium oxide, vanadium oxide, iron oxide, and the like, and lithium composite oxides composed of these oxides and lithium. All of these active materials can form an oxide sintered body. In particular, lithium titanate, which is a lithium-titanium composite oxide, has a low charge / discharge potential and a large charge / discharge capacity among the oxides. Therefore, when used as an active material for the negative electrode 3, a secondary battery having a high voltage can be formed.

  These active materials are made into an oxide sintered body having a porosity of 2 to 10%, and, for example, by applying the above-described treatment to form cracks inside, the stress due to volume fluctuation accompanying charging / discharging of the positive electrode 1 or the negative electrode 3 can be reduced. Can be relaxed.

Thus, the oxide sintered body that relieves stress by introducing cracks is preferably a lithium titanate sintered body. Since lithium titanate has a small volume fluctuation accompanying charging and discharging, stress can be effectively relieved by introducing cracks. Furthermore, the crystal structure of lithium titanate is preferably a ramsdellite type. The ramsdellite type crystal phase, which is a high-temperature phase, has a high firing temperature, and grain growth results in a larger crystal grain size. Therefore, the stress relaxation effect due to cracks becomes more prominent. In addition, ramsdellite type lithium titanate has excellent cycle characteristics, and lithium ion diffusibility is better than spinel type compounds Li ₄ Ti ₅ O _{12 and the} like. In addition, you may use what substituted the component element of the ramsdellite type lithium titanate by various metal elements, and can also be set as the sintered compact containing various metal elements as a sintering aid. The crystal phase of the oxide sintered body can be confirmed by identifying a diffraction pattern obtained by an X-ray diffraction method (XRD).

  When a lithium titanate sintered body is used as the negative electrode 3, the active material as described above may be used for the positive electrode 1. In this case, when the negative electrode 3 is an oxide sintered body having cracks in the porosity of 2 to 10%, it is possible to realize high capacity and stress relaxation during charge and discharge. In addition, although it is preferable that the positive electrode 1 also has a crack in the inside of porosity 2-10%, the porosity may exceed 10% and it may not have a crack inside.

When a lithium titanate sintered body is used as the positive electrode 1, for example, a carbon material such as graphite, hard carbon, and soft carbon, an alloy capable of inserting and removing Li and Li, and the like are used as the active material of the negative electrode 3. You can also. Also in this case, the positive electrode 1 may be an oxide sintered body having cracks inside with a porosity of 2 to 10%.

  In addition, it is preferable that the thickness of the positive electrode 1 and the negative electrode 3 shall be 20 micrometers-200 micrometers, respectively. Thereby, the absolute amount of the active material necessary for obtaining the battery capacity can be secured, and a secondary battery having good charge / discharge characteristics can be obtained.

  The solid electrolyte layer 2 is required to pass ions and prevent positive and negative electrodes from being short-circuited. The thickness of the solid electrolyte layer 2 is preferably as thin as possible in order to shorten the moving distance as a path for ions, and specifically, the total thickness of the solid electrolyte layer 2 is preferably 10 μm or less, Furthermore, it is good to set it as 3 micrometers or less, More preferably, it is 1 micrometer or less. If the thickness of the solid electrolyte layer 2 is thin, the internal resistance of the solid electrolyte layer 2 is reduced, and battery performance such as output characteristics is improved. Further, if the thickness of the solid electrolyte layer 2 can be reduced, more active material can be packed as compared with the secondary battery having the same volume, so that the capacity is increased and as a result, the energy density is also improved. . However, in order to prevent a short circuit, it is necessary to ensure a minimum thickness that does not cause a short circuit due to dielectric breakdown or pinholes.

  As the solid electrolyte layer 2, for example, a polymer solid electrolyte or an inorganic solid electrolyte can be used. Examples of the polymer solid electrolyte include dry polymer electrolytes such as polyethylene oxide, siloxane polymer, and phosphazene polymer. In the case of using a polymer solid electrolyte, for example, after impregnating a precursor solution of a dry polymer electrolyte in a separator, the precursor solution can be cured by heating and used. As the separator, a resin film with high heat resistance, a glass filter, a ceramic filter, or the like may be used.

As an inorganic solid electrolyte, a glass-based solid electrolyte containing lithium, which is thought to be able to suppress the increase in interfacial resistance by following the change in the shape of the interface accompanying the change in volume of the electrode by the presence of random ion conduction paths. For example, Li _{1 + x} Zr ₂ Si _x P _3-x O ₁₂ , Li _{1 + x} Zr _{2-x / 3} Si _x P _3-x O _{12-2x / 3} (1.5 <x <2.2), Li _{1 + x} M _x Ti _2-x (PO ₄ ) ₃ (M = Al, Sc, Y, or La, 0 <x <2), Li _0.5-3x M _{0.5 + x} TiO ₃ (M = La, Pr, Nd or Sm, 0 <x <1/6), Li ₂ SO ₄ , Li ₄ SiO ₄ , Li ₃ PO ₄ , Li ₄ GeO ₄ , Li ₃ VO ₄ , Li ₂ MoO ₄ , Li ₄ ZrO ₄ , Li ₂ CO _3, Li _{O, LiPON, SiO 2, ZrO} 2, V 2 O 5, P 2 O 5, B 2 O 3, Al 2 O 3, TiO 2, Zn 2 GeO 4, Li 2 S, SiS 2, Li 2 Se, SiSe ₂ , B ₂ S ₃ , P ₂ S ₅ , GeS ₂ , LiI, LiW ₂ O ₇ , LiNbO _{3 and the} like. Among them, lithium phosphate oxynitride (hereinafter also referred to as LIPON) has a high ionic conductivity of about 1 × 10 ⁻⁶ S / cm at room temperature and is known to be electrochemically stable over a wide potential range. It is suitable.

  In the case of using an inorganic solid electrolyte, for example, the first solid electrolyte layer 2P and the second solid electrolyte layer 2N are formed on the surfaces of the positive electrode 1 and the negative electrode 3 by a liquid phase synthesis method or a gas phase synthesis method, respectively. The power generation element 8 is obtained by superimposing and joining the positive electrode 1 and the negative electrode 3 so as to face each other.

  In addition, the first solid electrolyte 2P and the second solid electrolyte 2N may be formed of the same solid electrolyte material or different solid electrolyte materials. However, the first solid electrolyte 2P and the second solid electrolyte 2N may be formed from the viewpoint of firmly bonding the two. The solid electrolyte 2P and the second solid electrolyte 2N are preferably formed of the same solid electrolyte material.

  The positive current collecting layer 4P and the negative current collecting layer 4N are made of conductive carbon ink using a carbon material as a filler, conductive metal ink using a filler such as aluminum, gold, or platinum, ITO glass, tin oxide, etc. A conductive oxide ink using an oxide as a filler can be applied and dried. Moreover, what formed by vapor-depositing metals, such as platinum, aluminum, and titanium, can also be used.

  The secondary battery of the present embodiment has been described above, but the present invention is not limited to the present embodiment, and can be applied to various modifications without departing from the present invention.

  A method for manufacturing the secondary battery of this embodiment will be described.

  First, the positive electrode 1 and the negative electrode 3 are produced. The oxide sintered body is, for example, a commercially available oxide powder or a calcined powder obtained by mixing and calcining them to a mean particle size of about 0.01 to 10 μm by a well-known crushing method such as a ball mill. The raw material powder and a binder such as butyral are mixed by a known method using a dispersant, water added with a plasticizer, or an organic solvent such as toluene as a solvent, if necessary, to prepare a slurry. The slurry is coated on a base film such as a polyethylene terephthalate (PET) film by a known method and dried to produce a green sheet having a desired thickness. At this time, the slurry may be dried and granulated, and a green sheet may be produced by roll pressing, or may be press-formed into a desired shape.

  The obtained green sheet is punched into a desired shape, degreased as necessary, and then fired to obtain an oxide sintered body that becomes the positive electrode 1 and the negative electrode 3. The firing temperature may be appropriately selected according to the sinterability of the oxide that is the raw material powder, for example, 700 to 1100 ° C. for Li—Mn—O-based oxide, and 700 to 1200 ° C. for Li—Co—O-based oxide. In the case of Li-Ti-O-based oxides, an oxide sintered body having a porosity of 2 to 20% can be obtained by adjusting the temperature to 800 to 1250 ° C.

Furthermore, in order to obtain an oxide sintered body having a porosity of 2 to 10%, a raw material powder pulverized to an average particle diameter of 0.1 to 5 μm is used, and a firing temperature is set to 950 for Li-Mn—O-based oxides, for example. ˜1100 ° C., 950 to 1200 ° C. for Li—Co—O based oxide, and 1000 to 1250 ° C. for Li—Ti—O based oxide. Further, the green sheet or the molded body may be subjected to pressure treatment at a pressure of 0.5 to ³ ton / cm ² as necessary, or the surface of the sintered oxide body after polishing may be subjected to processing such as polishing. Good.

Next, cracks are introduced into the oxide sintered body having a porosity of 2 to 10%. For example, when the average particle diameter of the oxide sintered body is 5 to 20 μm, a low-speed large-depth charging process is performed in which the battery is discharged after charging 20% or more of the nominal capacity at a current value of 800 μA / cm ² or less. At this time, if the current value is too large or the charging depth is too large with respect to the area of the oxide sintered body, the maximum width of the introduced crack increases, and particles near the crack fall off or the oxide sintered body If the current value is too small or the charging depth is too small, cracks are not formed, and the stress relaxation effect cannot be obtained. In addition, when introducing cracks in the oxide sintered body having a large average particle diameter, the current value is adjusted to be larger, and in the case of an oxide sintered body having a small average particle diameter, the current value is adjusted to be smaller. Desired cracks can be introduced. Such a low-speed, large-depth charging process may be performed by forming a simple battery using an oxide sintered body, an electrolytic solution, and a Li metal foil, but may be performed after forming a secondary battery. In addition, cracks can be introduced into the oxide sintered body by, for example, subjecting the oxide sintered body to heat treatment at 700 ° C. or higher and then rapidly cooling it.

Next, the first solid electrolyte layer 2P made of lithium phosphate oxynitride glass is formed on one surface of the positive electrode 1 and the negative electrode 3 by a high frequency magnetron sputtering method using, for example, Li ₃ PO ₄ as a target in a nitrogen atmosphere. Two solid electrolyte layers 2N are respectively formed.

  Furthermore, the power generation element 8 is obtained by stacking the positive electrode 1 and the negative electrode 3 so that the first solid electrolyte layer 2P and the second solid electrolyte layer 2N face each other and joining them at 300 to 800 ° C. in a non-oxidizing atmosphere. It is done. In addition, as a counter electrode, for example, metallic lithium is formed on the surface of the solid electrolyte layer 2 formed on the positive electrode 1 or the negative electrode 3 which is an oxide sintered body having a porosity of 2 to 10% by a vacuum evaporation method to generate power. Element 8 may be used.

  The power generation element 8 formed in this way is used by being accommodated in a storage container. As the storage container, any of an outer package and a current collector used in a laminated lithium ion battery or a conventional coin battery can be applied. The shape of the lithium secondary battery of the present invention is not limited to a square shape, a cylindrical shape, a button shape, a coin shape, a flat shape, etc., and an insulating container having a positive electrode terminal and a negative electrode terminal is used. May be.

(1) Production process of oxide sintered body for electrode As the electrode active material, powders of LiNi _0.5 Mn _1.5 O ₄ , Li ₂ Ti ₃ O ₇ and Li ₄ Ti ₅ O ₁₂ were prepared. The powders of Li ₂ Ti ₃ O ₇ and Li ₄ Ti ₅ O ₁₂ were prepared by mixing LiCO ₃ with an average particle diameter of 5 μm and TiO ₂ with an average particle diameter of 1 μm at a predetermined ratio, calcining at 700 ° C., and vibration. What was grind | pulverized to the average particle diameter of 1 micrometer with the mill was used. As LiNi _0.5 Mn _1.5 O ₄ , a commercially available powder having an average particle diameter of 17 μm was pulverized to an average particle diameter of 1 μm with a vibration mill. These raw material powders were mixed by a ball mill together with butyral as a binder using toluene as a solvent to prepare a slurry. The obtained slurry was coated on a polyerylene terephthalate (PET) film and then dried to produce a green sheet having a thickness of 125 μm. The obtained green sheet was pressed at 1 ton / cm ² with a cold isostatic press, then punched out to a diameter of 18 mm and fired at the firing temperature shown in Table 1 to obtain a disk having a diameter of 15 mm and a thickness of 100 μm. Oxide sintered bodies A to G were produced. The LiNi _0.5 Mn _1.5 O ₄ sintered body was subjected to a heat treatment at 700 ° C. for 10 hours after firing.

  Table 1 shows the porosity and average particle diameter of the obtained oxide sintered body. The porosity of the oxide sintered body is determined by observing the ion-etched cross-section with a scanning electron microscope (SEM) and analyzing the area of the pores by image analysis for 30 μm × 30 μm regions of photographs taken at any five locations. The ratio was calculated and converted to porosity. Similarly, the average particle size of the oxide sintered body was calculated by image analysis of a cross-sectional photograph.

  The crystal phase of the oxide sintered body was confirmed by pulverizing the obtained oxide sintered body and measuring a diffraction pattern by an X-ray diffraction method (XRD). In particular, for lithium titanate sintered bodies, the volume ratio of the ramsdellite-type crystal phase was calculated using the (110) diffraction peak intensity of the ramsdelite-type crystal phase and the (111) diffraction peak intensity of the spinel-type crystal phase. The results are shown in Table 1.

(2) Crack Introducing Treatment to Oxide Sintered Body In order to introduce cracks into the B to G oxide sintered bodies, the following treatment was performed. B to G oxide sintered body as one electrode, Li metal foil as a counter electrode, facing each other through a separator impregnated with an electrolyte, and with respect to the area of the surface of the oxide sintered body in contact with the separator Then, constant current charging was performed at 200 μA / cm ² for 180 minutes (condition 1). As the electrolytic solution, a mixed solution of ethylene carbonate (EC) and dimethyl carbonate (DMC) containing 1 mol / L LiPF ₆ was used at a volume ratio of EC: DMC of 7: 3.

About the oxide sintered compact which introduce | transduced the crack, the presence or absence of a crack and the maximum width were confirmed by the following methods. The cross section of the oxide sintered body after the crack introduction treatment was subjected to ion etching processing, and the presence of cracks was confirmed with a scanning electron microscope (SEM) in a region of 150 μm × 150 μm. Among the confirmed cracks, the gap width of the crack having the largest gap width was measured at 20 locations in a 30 μm × 30 μm region, and the maximum value was shown in Table 2 as the maximum width of cracks existing inside the electrode.

  When the crack introduction treatment was performed under the condition 1, the maximum width of the crack was 2 μm or less, and no drop of the structure was observed.

(3) Step of forming solid electrolyte layer on electrode On the surface of oxide sintered body A and oxide sintered bodies B to G subjected to crack introduction treatment, a thickness of 0. Each 5 μm solid electrolyte layer was formed. A lithium phosphate sintered body was used as a target, and the film formation time was 5 hours in a nitrogen atmosphere at a pressure of 5 mtorr.

(4) Production process of power generation element Each oxide sintered body on which the solid electrolyte layer is formed is combined as a positive electrode and a negative electrode as shown in Table 2, and the solid electrolyte layer on the positive electrode side faces the solid electrolyte layer on the negative electrode side. Thus, a power generation element was produced by heating and joining the solid electrolytes at a temperature of 800 ° C. and a pressure of 10 MPa in a nitrogen atmosphere of 0.1 MPa (Sample Nos. 1 to 6, 8). Sample No. For No. 7, a lithium metal titanate sintered solid electrolyte layer on which a solid electrolyte layer was formed was vapor-deposited with Li metal using a stainless steel mask to produce a power generation element. Sample No. None of the electrodes used in No. 8 was subjected to crack introduction treatment.

(5) Step of forming positive and negative current collecting layers Platinum having a diameter of 14 mm was vapor-deposited on the positive electrode and negative electrode surfaces of the obtained power generation element using a stainless steel mask to form a current collecting layer.

(6) Battery assembly process A power generation element in which a current collecting layer is formed is housed between a stainless steel positive battery case and a negative battery case, and the circumference of both battery cases is caulked through a gasket, with a diameter of 20 mm, A coin-type secondary battery having a thickness of 1.6 mm was manufactured.

(7) Evaluation test of secondary battery The performance of the secondary battery obtained by the steps (1) to (6) was confirmed by a charge / discharge test. The test conditions were as follows. However, Sample No. For No. 3, the test was conducted with the upper limit of the charge / discharge voltage set to 4.7V.

Charge / discharge voltage range: upper limit 3.7V, lower limit 1.5V
Charge / discharge current value: 10 μA (constant current charge / discharge)
Measurement temperature: 30 ° C
Table 2 shows the initial charge capacity and the initial discharge capacity.

  As a result of the charge / discharge test, Sample No. 1-7 confirmed that charging / discharging could be repeated. On the other hand, sample No. 1 using a sintered oxide having a porosity of 7% and no crack as a negative electrode. In No. 8, no electromotive force was obtained after the initial charge / discharge.

DESCRIPTION OF SYMBOLS 1 ... Positive electrode 2 ... Solid electrolyte layer 3 ... Negative electrode 4P .. Positive electrode side current collection layer 4N ... Negative electrode side current collection layer 5 ... Positive electrode side battery case 6 ... Gasket 7 ... Negative side battery case 8 ... Power generation element 9 ... Crack W ... Maximum width of crack

Claims (4)

  A power generation element having a pair of electrodes joined via a solid electrolyte layer, at least one of the pair of electrodes is made of an oxide sintered body, and the oxide sintered body has a porosity of 2 to 10 %, And has a crack inside the oxide sintered body.   The secondary battery according to claim 1, wherein a maximum width of the crack is 2 μm or less.   The secondary battery according to claim 1, wherein the oxide sintered body is a sintered body of lithium titanate.   The secondary battery according to claim 3, wherein the crystal structure of the lithium titanate is a ramsdellite type.