Method for manufacturing a circuit board assembly

Abstract

Provided is a method for manufacturing a circuit board assembly, capable of effectively suppressing deterioration of battery performance in high-temperature, high-humidity environments, even when employing reflow soldering that may cause such environments. The method includes connecting a lithium-ion secondary battery to a circuit board by reflow soldering, wherein the state of charge (SOC) of the lithium-ion secondary battery at the time of reflow soldering is 30-100%.

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a circuit board assembly. [Background technology]

[0002] Coin-type lithium-ion secondary batteries are widely used in various devices that require charging, and various coin-type lithium-ion secondary batteries have been proposed. For example, Patent Document 1 (JP 2012-209178 A) discloses a coin-type battery in which a positive electrode is disposed on the inner surface of a positive electrode case that also serves as an external terminal, and a negative electrode is disposed on the inner surface of a negative electrode sealing plate that also serves as an external terminal, with the positive electrode and negative electrode facing each other via a separator. In such a coin-type battery, the peripheral portions of the positive electrode case and sealing plate are sealed with a gasket so as to retain an electrolyte inside. Such secondary batteries employ a powder-dispersed positive electrode (a so-called coated electrode) prepared by applying and drying a positive electrode mixture containing a positive electrode active material, a conductive additive, a binder, and the like.

[0003] Generally, powder-dispersed positive electrodes contain relatively large amounts (e.g., about 10 wt %) of components (binders and conductive additives) that do not contribute to capacity, resulting in a low packing density of the lithium composite oxide used as the positive electrode active material. For this reason, powder-dispersed positive electrodes have had significant room for improvement in terms of capacity and charge / discharge efficiency. Therefore, attempts have been made to improve capacity and charge / discharge efficiency by constructing the positive electrode or positive electrode active material layer from a sintered lithium composite oxide plate. In this case, since the positive electrode or positive electrode active material layer does not contain a binder or conductive additive, the packing density of the lithium composite oxide is increased, which is expected to result in high capacity and good charge / discharge efficiency. For example, Patent Document 2 (WO2019 / 221139) discloses a coin-type lithium-ion secondary battery that includes a positive electrode plate that is a lithium composite oxide sintered body plate, a negative electrode plate that is a titanium-containing sintered body plate, a separator, and an electrolyte solution inside an exterior body. It is said that using the sintered body plate as an electrode provides excellent heat resistance that enables reflow soldering. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2012-209178 [Patent Document 2] WO2019 / 221139 Summary of the Invention

[0005] When a lithium-ion secondary battery is mounted on a circuit board by reflow soldering, it is exposed to high temperatures (e.g., 260°C) during reflow heating, which can lead to a problem of battery performance degradation. In particular, batteries that have been reflow soldered can experience performance degradation (e.g., increased battery resistance) if left in a high-temperature, high-humidity environment for an extended period of time. In this regard, Patent Document 2 discloses a lithium-ion secondary battery that can be reflow soldered, but the optimal conditions for reflow soldering are unknown, leaving room for improvement in the manufacturing conditions. Therefore, there is a need for a manufacturing method that can suppress battery performance degradation in a high-temperature, high-humidity environment, even when reflow soldering is performed.

[0006] The present inventors have now discovered that by using a lithium ion secondary battery for reflow soldering at a state of charge (SOC) of 30 to 100%, it is possible to effectively suppress deterioration of battery performance in a high-temperature, high-humidity environment that may occur after reflow soldering.

[0007] Therefore, an object of the present invention is to provide a method for manufacturing a circuit board assembly that employs reflow soldering while effectively suppressing the deterioration of battery performance that can occur in high-temperature, high-humidity environments.

[0008] According to the present invention, the following aspects are provided. [Aspect 1] A method for manufacturing a circuit board assembly, comprising connecting a lithium ion secondary battery to a circuit board by reflow soldering, The method for manufacturing a circuit board assembly, wherein the state of charge (SOC) of the lithium ion secondary battery is 30 to 100% during the reflow soldering. [Aspect 2] 2. The method for producing a circuit board assembly according to aspect 1, wherein the lithium ion secondary battery has a state of charge (SOC) of 35 to 100%. [Aspect 3] 2. The method for producing a circuit board assembly according to aspect 1, wherein the lithium ion secondary battery has a state of charge (SOC) of 70 to 100%. [Aspect 4] A method for producing a circuit board assembly according to any one of aspects 1 to 3, wherein the reflow heating in the reflow soldering is performed at 180 to 270°C. [Aspect 5] The lithium ion secondary battery is a positive electrode layer; a negative electrode layer; a separator interposed between the positive electrode layer and the negative electrode layer; Electrolytes, an exterior body having a sealed space in which the positive electrode layer, the negative electrode layer, the separator, and the electrolyte are accommodated; 5. A method for manufacturing a circuit board assembly according to any one of aspects 1 to 4, comprising: [Aspect 6] A method for manufacturing a circuit board assembly according to aspect 5, wherein the exterior body comprises a positive electrode can, a negative electrode can, and a gasket, and the positive electrode can and the negative electrode can are crimped together via the gasket to form the sealed space. [Aspect 7] The battery further includes a positive electrode terminal joined to an outer surface of the exterior body closer to the positive electrode layer, and a negative electrode terminal joined to an outer surface of the exterior body closer to the negative electrode layer, Aspect 7. The method for manufacturing a circuit board assembly according to aspect 5 or 6, wherein the positive terminal and / or the negative terminal is connected to the circuit board by the reflow solder. [Aspect 8] Aspect 8. The method for manufacturing a circuit board assembly according to any one of aspects 5 to 7, wherein the positive electrode layer is a ceramic positive electrode plate. [Aspect 9] Aspect 9. The method for producing a circuit board assembly according to aspect 8, wherein the ceramic positive electrode plate is a sintered lithium composite oxide plate. [Aspect 10] 10. The method for producing a circuit board assembly according to claim 9, wherein the lithium composite oxide is lithium cobalt oxide. [Aspect 11] Aspect 11. The method for manufacturing a circuit board assembly according to any one of aspects 5 to 10, wherein the negative electrode layer is a ceramic negative electrode plate. [Aspect 12] 12. The method for producing a circuit board assembly according to claim 11, wherein the ceramic negative electrode plate is a titanium-containing sintered body plate. [Aspect 13] Aspect 13. The method for producing a circuit board assembly according to aspect 12, wherein the titanium-containing sintered body comprises lithium titanate or niobium titanium composite oxide. [Aspect 14] 14. The method for manufacturing a circuit board assembly according to any one of aspects 5 to 13, wherein the separator is made of cellulose, polyimide, polyester, or a ceramic selected from the group consisting of MgO, Al2O3, ZrO2, SiC, Si3N4, AlN, and cordierite. [Aspect 15] A method for producing a circuit board assembly according to any one of aspects 5 to 14, wherein the electrolyte is provided in the form of an electrolytic solution, and the electrolytic solution is a solution containing lithium borofluoride (LiBF) in a non-aqueous solvent comprising at least one selected from the group consisting of γ-butyrolactone (GBL), ethylene carbonate (EC), and propylene carbonate (PC). [Brief explanation of the drawings]

[0009] [Figure 1] 1 is a schematic cross-sectional view of an example of a lithium ion secondary battery used in the present invention. [Figure 2] 1 is an SEM image showing an example of a cross section perpendicular to the plate surface of an oriented positive electrode plate. [Figure 3] 3 is an EBSD image of a cross section of the oriented positive electrode plate shown in FIG. 2. [Figure 4]4 is a histogram showing the distribution of orientation angles of primary particles in the EBSD image of FIG. 3 on an area basis. DETAILED DESCRIPTION OF THE INVENTION

[0010] Method for manufacturing a circuit board assembly The present invention relates to a method for manufacturing a circuit board assembly. In this specification, the term "circuit board assembly" refers to a product in which a lithium ion secondary battery (and optionally a device) is mounted on a circuit board. The manufacturing method of the present invention includes connecting a lithium ion secondary battery to a circuit board by reflow soldering. The lithium ion secondary battery has a state of charge (SOC) of 30 to 100% during reflow soldering. By using a lithium ion secondary battery with an SOC of 30 to 100% for reflow soldering in this way, it is possible to effectively suppress deterioration of battery performance in a high-temperature, high-humidity environment that may occur after reflow soldering.

[0011] As mentioned above, when a lithium-ion secondary battery is mounted on a circuit board by reflow soldering, there is a problem in that the battery performance is easily deteriorated due to exposure to high temperatures (e.g., 260°C) during reflow heating. In particular, batteries that have been reflow soldered can experience performance degradation (e.g., increased battery resistance) when placed in a high-temperature, high-humidity environment for an extended period of time. However, this problem is conveniently resolved by the present invention. While the details of the mechanism are unclear, it is believed to be as follows. When a lithium-ion secondary battery is exposed to high temperatures during reflow heating, gas is generated inside the battery due to reactions between the electrodes and the electrolyte (typically, electrolytic solution) and / or moisture, or due to volatilization of the electrolytic solution. This gas increases the battery's internal pressure. In this regard, when the battery's SOC is 30 to 100%, the amount of gas generated during reflow heating increases, increasing the battery's internal pressure. As a result, it is believed that less moisture penetrates into the battery, even in a high-temperature, high-humidity environment. Furthermore, it is believed that the reduced moisture penetration into the battery suppresses moisture-related deterioration of battery performance (e.g., increased battery resistance).

[0012] The SOC of the lithium ion secondary battery during reflow soldering is 30 to 100%, preferably 31 to 100%, more preferably 35 to 100%, and even more preferably 70 to 100%. The SOC may be measured by a known method, but it may also be measured by subjecting the lithium ion secondary battery to initial charge and discharge, setting the discharged state as SOC 0%, and charging as necessary to bring the SOC within the above range. In this sense, the lithium ion secondary battery used in the present invention can be said to have been subjected to at least initial charge and discharge. However, the initial charge and discharge may also be performed by charging and then discharging partway to adjust the SOC to the desired value.

[0013] The lithium ion secondary battery can be connected to the circuit board by a known reflow soldering technique. For example, reflow soldering can be performed by applying or printing solder (e.g., solder paste) to a predetermined position on the circuit board, placing the lithium ion secondary battery at that position, reflow heating in a reflow furnace with a predetermined temperature profile to melt the solder, and cooling the molten solder to solidify it. If the lithium ion secondary battery has a positive terminal and a negative terminal, it is preferable that the positive terminal and / or the negative terminal be connected to the circuit board by reflow soldering.

[0014] Reflow heating in reflow soldering is typically performed at 180 to 270°C, more typically 200 to 260°C. This temperature is the temperature measured for lithium-ion secondary batteries. It is preferable to adopt a heating profile in which the temperature is maintained within the above range for typically 5 to 100 seconds, more typically 10 to 60 seconds, with the maximum temperature reaching 200 to 270°C. Even when exposed to such high temperatures, the manufacturing method of the present invention can effectively suppress deterioration of battery performance.

[0015] Lithium-ion secondary battery FIG. 1 schematically illustrates an example of a lithium-ion secondary battery preferably used in the present invention. The lithium-ion secondary battery 10 shown in FIG. 1 includes a positive electrode layer 12, a negative electrode layer 16, a separator 20, an electrolytic solution 22 (or electrolyte), and an exterior body 24. The separator 20 is interposed between the positive electrode layer 12 and the negative electrode layer 16. The exterior body 24 includes a sealed space, which accommodates the positive electrode layer 12, the negative electrode layer 16, the separator 20, and the electrolytic solution 22. In the lithium-ion secondary battery 10 shown in FIG. 1, the electrolyte is provided in the form of the electrolytic solution 22. However, a solid electrolyte or a polymer electrolyte may be used instead of the electrolytic solution 22. Therefore, in this paragraph and the following description, the term "electrolytic solution" may be interpreted as "electrolyte" as long as no technical inconsistency occurs. In other words, the battery usable in the present invention may be an all-solid-state battery. Furthermore, the lithium ion secondary battery 10 is preferably coin-shaped as shown in FIG. 1, but is not limited to this and may be a lithium ion secondary battery of various shapes.

[0016] According to a preferred embodiment of the present invention, the lithium-ion secondary battery 10 further includes a positive electrode terminal (not shown) bonded to the outer surface of the exterior body 24 closer to the positive electrode layer 12, and a negative electrode terminal (not shown) bonded to the outer surface of the exterior body 24 closer to the negative electrode layer 16. In this case, as described above, the positive electrode terminal and / or the negative electrode terminal are preferably used for reflow soldering to mount the lithium-ion secondary battery 10 on a circuit board. That is, the positive electrode terminal and / or the negative electrode terminal are preferably connected to the circuit board by reflow soldering. The positive electrode terminal is preferably bonded to the positive electrode can 24a of the exterior body 24 by a method such as resistance welding, diffusion welding, or laser welding. Similarly, the negative electrode terminal is preferably bonded to the negative electrode can 24b of the exterior body 24 by a method such as resistance welding, diffusion welding, or laser welding.

[0017] The outer diameter of the lithium ion secondary battery 10 is not particularly limited, but is typically 8 to 25 mm, more typically 9.5 to 22 mm, and even more typically 12.5 to 20 mm.

[0018] The positive electrode layer 12 is a layer containing a positive electrode active material. The positive electrode layer 12 may be a powder-dispersed positive electrode (so-called coated electrode) prepared by applying and drying a positive electrode mixture containing a positive electrode active material (e.g., lithium cobalt oxide), a conductive additive, a binder, and the like. However, the positive electrode layer 12 is preferably a ceramic positive electrode plate, and more preferably a lithium composite oxide sintered body plate. The fact that the positive electrode layer 12 is a ceramic positive electrode plate or a sintered body plate means that the positive electrode layer 12 does not contain a binder or a conductive additive. This is because even if a binder is contained in the green sheet, the binder is lost or burned away during firing. Furthermore, the absence of a binder in the positive electrode layer 12 has the advantage of preventing deterioration of the positive electrode due to the electrolyte solution 22. The lithium composite oxide constituting the sintered body plate is particularly preferably lithium cobalt oxide (typically LiCoO (hereinafter sometimes abbreviated as LCO)). Various lithium composite oxide sintered body plates or LCO sintered body plates are known, and for example, the one disclosed in Patent Document 2 (WO2019 / 221139) can be used.

[0019] According to a preferred embodiment of the present invention, the lithium composite oxide sintered body plate constituting the positive electrode layer 12 includes a plurality of primary particles composed of a lithium composite oxide, and the plurality of primary particles are oriented at an average orientation angle of more than 0° and not more than 30° relative to the plate surface of the positive electrode plate. This constitutes an oriented positive electrode plate. FIG. 2 shows an example of a cross-sectional SEM image perpendicular to the plate surface of the oriented positive electrode plate, while FIG. 3 shows an electron backscatter diffraction (EBSD) image of the cross-section perpendicular to the plate surface of the oriented positive electrode plate. FIG. 4 also shows a histogram showing the area-based distribution of the orientation angles of the primary particles 11 in the EBSD image of FIG. 3. The EBSD image shown in FIG. 3 allows observation of discontinuities in the crystal orientation. In FIG. 3, the orientation angle of each primary particle 11 is indicated by different shades of color, with darker colors indicating smaller orientation angles. The orientation angle is the inclination angle of the (003) plane of each primary particle 11 relative to the plate surface. Note that in FIGS. 2 and 3, the black areas within the oriented positive electrode plate represent pores.

[0020] The positive electrode layer 12, which is an alignment positive electrode plate, is an alignment sintered body composed of a plurality of primary particles 11 bonded to each other. Each primary particle 11 is mainly plate-shaped, but may include those formed in a cuboid shape, a cube shape, a spherical shape, etc. The cross-sectional shape of each primary particle 11 is not particularly limited and may be rectangular, a polygon other than rectangular, circular, elliptical, or a complex shape other than these.

[0021] Each primary particle 11 is composed of a lithium composite oxide. The lithium composite oxide is Li x MO2 (where 0.05 < x < 1.10, M is at least one type of transition metal, and M typically includes one or more of Co, Ni, and Mn), and is an oxide represented by this formula. The lithium composite oxide has a layered rock salt structure. The layered rock salt structure refers to a crystal structure in which lithium layers and transition metal layers other than lithium are alternately stacked with an oxygen layer sandwiched therebetween, that is, a crystal structure in which a transition metal ion layer and a lithium single layer are alternately stacked via oxide ions (typically an α-NaFeO2 type structure, that is, a structure in which transition metals and lithium are regularly arranged in the

[0111] axis direction of a cubic rock salt type structure). Examples of the lithium composite oxide include Li x CoO2 (lithium cobaltate), Li x NiO2 (lithium nickelate), Li x MnO2 (lithium manganate), Li x NiMnO2 (lithium nickel manganate), Li x NiCoO2 (lithium nickel cobaltate), Li x CoNiMnO2 (lithium cobalt nickel manganate), Li x CoMnO2 (lithium cobalt manganate), etc. are mentioned, and particularly preferably Li x CoO2 (lithium cobaltate, typically LiCoO2). The lithium composite oxide may contain one or more elements selected from Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba, Bi, and W.

[0022] As shown in Figures 3 and 4, the average value of the orientation angles of each primary particle 11, i.e., the average orientation angle, is greater than 0° and less than or equal to 30°. This provides the following various advantages. First, because each primary particle 11 lies in a state inclined relative to the thickness direction, the adhesion between each primary particle can be improved. As a result, the lithium ion conductivity between a given primary particle 11 and other primary particles 11 adjacent to that primary particle 11 on both longitudinal sides of that primary particle 11 can be improved, thereby improving the rate characteristics. Second, the rate characteristics can be further improved. This is because, as described above, in an oriented positive electrode plate, expansion and contraction in the thickness direction is more prevalent than in the plate surface direction during the inflow and outflow of lithium ions. This smooths the expansion and contraction of the oriented positive electrode plate, thereby also smoothing the inflow and outflow of lithium ions. Furthermore, the orientation can be expected to have the effect of uniforming the stress applied to the electrode plate to the particles present in the electrode during reflow soldering. Furthermore, this effect is more favorably exhibited in the case of the above-mentioned orientation orientation.

[0023] The average orientation angle of the primary particles 11 is obtained by the following method. First, in an EBSD image of a 95 μm × 125 μm rectangular region observed at 1000x magnification, as shown in FIG. 3, three horizontal lines are drawn to divide the oriented positive electrode plate into four equal parts in the thickness direction, and three vertical lines are drawn to divide the oriented positive electrode plate into four equal parts in the plate surface direction. Next, the average orientation angle of the primary particles 11 is obtained by arithmetically averaging the orientation angles of all the primary particles 11 that intersect with at least one of the three horizontal and three vertical lines. From the viewpoint of further improving the rate characteristics, the average orientation angle of the primary particles 11 is preferably 30° or less, more preferably 25° or less. From the viewpoint of further improving the rate characteristics, the average orientation angle of the primary particles 11 is preferably 2° or more, more preferably 5° or more.

[0024] As shown in FIG. 4 , the orientation angles of the primary particles 11 may be distributed widely from 0° to 90°, but preferably the majority of the orientation angles are distributed in the range of more than 0° and less than or equal to 30°. That is, when the cross section of the oriented sintered body constituting the oriented positive electrode plate is analyzed by EBSD, the total area of ​​the primary particles 11 contained in the analyzed cross section, which have an orientation angle of more than 0° and less than or equal to 30° relative to the plate surface of the oriented positive electrode plate (hereinafter referred to as low-angle primary particles), is preferably 70% or more, more preferably 80% or more, of the total area of ​​the primary particles 11 contained in the cross section (specifically, the 30 primary particles 11 used to calculate the average orientation angle). This increases the proportion of primary particles 11 with high mutual adhesion, thereby further improving the rate characteristics. Furthermore, the total area of ​​the low-angle primary particles with an orientation angle of less than or equal to 20° is more preferably 50% or more of the total area of ​​the 30 primary particles 11 used to calculate the average orientation angle. Furthermore, the total area of ​​the low-angle primary particles having an orientation angle of 10° or less is more preferably 15% or more of the total area of ​​the 30 primary particles 11 used to calculate the average orientation angle.

[0025] Since each primary particle 11 is mainly plate-shaped, as shown in Figures 2 and 3, the cross section of each primary particle 11 extends in a predetermined direction and is typically approximately rectangular. That is, when the cross section of an oriented sintered body is analyzed by EBSD, the total area of ​​the primary particles 11 contained in the analyzed cross section, which have an aspect ratio of 4 or more, is preferably 70% or more, more preferably 80% or more, of the total area of ​​the primary particles 11 contained in the cross section (specifically, the 30 primary particles 11 used to calculate the average orientation angle). Specifically, in an EBSD image such as that shown in Figure 3, this can further improve the mutual adhesion between the primary particles 11, thereby further improving the rate characteristics. The aspect ratio of the primary particles 11 is the value obtained by dividing the maximum Feret diameter of the primary particles 11 by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel lines when the primary particles 11 are sandwiched between the lines on the EBSD image when the cross section is observed. The minimum Feret diameter is the minimum distance between two parallel lines when the primary particle 11 is sandwiched between the lines on the EBSD image.

[0026] The average particle size of the multiple primary particles constituting the oriented sintered body is preferably 5 μm or more. Specifically, the average particle size of the 30 primary particles 11 used to calculate the average orientation angle is preferably 5 μm or more, more preferably 7 μm or more, and even more preferably 12 μm or more. This reduces the number of grain boundaries between the primary particles 11 in the direction of lithium ion conduction, improving overall lithium ion conductivity and further improving rate characteristics. The average particle size of the primary particles 11 is the arithmetic average of the circle-equivalent diameters of each primary particle 11. The circle-equivalent diameter is the diameter of a circle having the same area as each primary particle 11 on an EBSD image.

[0027] The lithium composite oxide sintered body plate constituting the positive electrode layer 12 preferably contains pores. When the sintered body contains pores, particularly open pores, the electrolyte can penetrate into the sintered body when the sintered body is incorporated into a battery as a positive electrode plate, resulting in improved lithium ion conductivity. This is because, of the two types of lithium ion conduction within the sintered body, conduction via the constituent particles of the sintered body and conduction via the electrolyte in the pores, conduction via the electrolyte in the pores is overwhelmingly faster.

[0028] The lithium composite oxide sintered body plate constituting the positive electrode layer 12 preferably has a porosity of 20 to 60%, more preferably 25 to 55%, even more preferably 30 to 50%, and particularly preferably 30 to 45%. The pores are expected to provide a stress relief effect and increase capacity, and the mutual adhesion between the primary particles 11 can be further improved, resulting in improved rate characteristics. The porosity of the sintered body is calculated by polishing the cross section of the positive electrode plate with a cross-section polisher (CP), observing it with an SEM at 1000x magnification, and binarizing the resulting SEM image. The average equivalent circle diameter of each pore formed inside the oriented sintered body is not particularly limited, but is preferably 8 μm or less. The smaller the average equivalent circle diameter of each pore, the more the mutual adhesion between the primary particles 11 can be improved, resulting in further improved rate characteristics. The average equivalent circle diameter of the pores is the arithmetic average of the equivalent circle diameters of 10 pores in an EBSD image. The circle-equivalent diameter is the diameter of a circle having the same area as each pore on an EBSD image. Each pore formed inside the oriented sintered body is preferably an open pore that connects to the outside of the lithium composite oxide sintered body plate.

[0029] The average pore size of the lithium composite oxide sintered body plate constituting the positive electrode layer 12 is preferably 0.1 to 10.0 μm, more preferably 0.2 to 5.0 μm, and even more preferably 0.3 to 3.0 μm. Within the above range, localized stress concentration in large pores is suppressed, and stress within the sintered body is more likely to be released uniformly.

[0030] The thickness of the lithium composite oxide sintered body plate constituting the positive electrode layer 12 is preferably 60 to 600 μm, more preferably 70 to 550 μm, and even more preferably 90 to 500 μm. Within such ranges, the active material capacity per unit area is increased, improving the energy density of the lithium ion secondary battery 10, and deterioration of the battery characteristics (particularly an increase in resistance value) due to repeated charge and discharge can be suppressed.

[0031] The negative electrode layer 16 is a layer containing a negative electrode active material. The negative electrode layer 16 may be a powder-dispersed negative electrode (so-called coated electrode) prepared by applying and drying a negative electrode mixture containing a negative electrode active material, a conductive additive, a binder, etc., but is preferably a ceramic negative electrode plate, and more preferably a titanium-containing sintered body plate. The titanium-containing sintered body plate is preferably a ceramic negative electrode plate, and more preferably a titanium-containing sintered body plate. 12 It is preferable that the battery contains Li4Ti5O (hereinafter referred to as LTO) or niobium titanium composite oxide Nb2TiO7, and more preferably contains LTO. Although LTO is known to typically have a spinel structure, it can also adopt other structures during charge and discharge. For example, LTO can be 12 (spinel structure) and Li7Ti5O 12 The reaction proceeds in the coexistence of two phases, i.e., spinel and rock salt structures. Therefore, LTO is not limited to a spinel structure.

[0032] The fact that the negative electrode layer 16 is a ceramic negative electrode plate or sintered body plate means that the negative electrode layer 16 does not contain a binder or conductive additive. This is because even if a binder is contained in the green sheet, the binder will disappear or be burned away during firing. Because the negative electrode plate does not contain a binder, the packing density of the negative electrode active material (e.g., LTO or NbTiO) is high, resulting in high capacity and good charge / discharge efficiency. LTO sintered body plates can be manufactured according to the method described in Patent Document 2 (WO2019 / 221139).

[0033] The titanium-containing sintered body plate constituting the negative electrode layer 16 has a structure in which a plurality of (i.e., a large number of) primary particles are bonded together. Therefore, it is preferable that these primary particles are composed of LTO or Nb2TiO7.

[0034] The thickness of the titanium-containing sintered body plate constituting the negative electrode layer 16 is preferably 70 to 500 μm, more preferably 85 to 400 μm, and even more preferably 95 to 350 μm. The thicker the LTO sintered body plate, the easier it is to realize a battery with a high capacity and high energy density. The thickness of the titanium-containing sintered body plate can be obtained, for example, by measuring the distance between plate surfaces observed as approximately parallel when the cross section of the titanium-containing sintered body plate is observed with a scanning electron microscope (SEM).

[0035] The primary particle size, which is the average particle size of the multiple primary particles constituting the titanium-containing sintered body plate, is preferably 1.2 μm or less, more preferably 0.02 to 1.2 μm, and even more preferably 0.05 to 0.7 μm. Within this range, it is easy to achieve both lithium ion conductivity and electronic conductivity, which contributes to improving rate performance.

[0036] The titanium-containing sintered body plate constituting the negative electrode layer 16 preferably contains pores. When the sintered body plate contains pores, particularly open pores, it is possible for the electrolyte to penetrate into the sintered body plate when the sintered body plate is incorporated into a battery as a negative electrode plate, thereby improving lithium ion conductivity. This is because, of the two types of lithium ion conduction within the sintered body, conduction via the constituent particles of the sintered body and conduction via the electrolyte in the pores, conduction via the electrolyte in the pores is overwhelmingly faster.

[0037] The porosity of the titanium-containing sintered plate constituting the negative electrode layer 16 is preferably 20 to 60%, more preferably 30 to 55%, and even more preferably 35 to 50%. Within such ranges, it is easy to achieve both lithium ion conductivity and electronic conductivity, which contributes to improving rate performance.

[0038] The average pore size of the titanium-containing sintered plate constituting the negative electrode layer 16 is 0.08 to 5.0 μm, preferably 0.1 to 3.0 μm, and more preferably 0.12 to 1.5 μm. Within this range, it is easy to achieve both lithium ion conductivity and electronic conductivity, which contributes to improving rate performance.

[0039] Separator 20 is preferably made of cellulose, polyolefin, polyimide, polyester (e.g., polyethylene terephthalate (PET)), or ceramic. Cellulose separators are advantageous in that they are inexpensive and have excellent heat resistance. Furthermore, unlike the widely used polyolefin separators, which have poor heat resistance, polyimide, polyester (e.g., polyethylene terephthalate (PET)), or cellulose separators not only have excellent heat resistance but also have excellent wettability with gamma-butyrolactone (GBL), an electrolyte component with excellent heat resistance. Therefore, when using an electrolyte containing GBL, the electrolyte can be sufficiently permeated into the separator (without being repelled). On the other hand, ceramic separators not only have excellent heat resistance, but also have the advantage that they can be manufactured as a single, integrated sintered body together with the positive electrode layer 12 and the negative electrode layer 16. In the case of a ceramic separator, the ceramic constituting the separator is preferably at least one selected from MgO, Al2O3, ZrO2, SiC, Si3N4, AlN, and cordierite, and more preferably at least one selected from MgO, Al2O3, and ZrO2.

[0040] The electrolyte solution 22 is not particularly limited, and commercially available electrolyte solutions for lithium batteries, such as a solution in which a lithium salt is dissolved in a nonaqueous solvent such as an organic solvent, may be used. In particular, an electrolyte solution with excellent heat resistance is preferred, and such an electrolyte solution preferably contains lithium fluoroborate (LiBF) in a nonaqueous solvent. In this case, a preferred nonaqueous solvent is at least one selected from the group consisting of γ-butyrolactone (GBL), ethylene carbonate (EC), and propylene carbonate (PC). More preferred are a mixed solvent of EC and GBL, a single solvent of PC, a mixed solvent of PC and GBL, or a single solvent of GBL. A mixed solvent of EC and GBL or a single solvent of GBL is particularly preferred. The inclusion of γ-butyrolactone (GBL) in the nonaqueous solvent increases the boiling point, significantly improving heat resistance. From this perspective, the volume ratio of EC to GBL in the nonaqueous solvent containing EC and / or GBL is preferably 0:1 to 1:1 (GBL ratio of 50 to 100 volume%), more preferably 0:1 to 1:1.5 (GBL ratio of 60 to 100 volume%), even more preferably 0:1 to 1:2 (GBL ratio of 66.6 to 100 volume%), and particularly preferably 0:1 to 1:3 (GBL ratio of 75 to 100 volume%). Lithium borofluoride (LiBF4) dissolved in the nonaqueous solvent is an electrolyte with a high decomposition temperature, which also significantly improves heat resistance. The concentration of LiBF4 in the electrolytic solution 22 is preferably 0.5 to 2 mol / L, more preferably 0.6 to 1.9 mol / L, even more preferably 0.7 to 1.7 mol / L, and particularly preferably 0.8 to 1.5 mol / L.

[0041] The electrolyte solution 22 may further contain vinylene carbonate (VC) and / or fluoroethylene carbonate (FEC) and / or vinylethylene carbonate (VEC) and / or propane sultone (PS) as additives. Both VC and FEC have excellent heat resistance. Therefore, by including such additives in the electrolyte solution 22, an SEI film with excellent heat resistance can be formed on the surface of the negative electrode layer 16.

[0042] As described above, a solid electrolyte or a polymer electrolyte may be used instead of the electrolytic solution 22 (in other words, a solid electrolyte or a polymer electrolyte may be used as the electrolyte in addition to the electrolytic solution 22). In this case, as in the case of the electrolytic solution 22, it is preferable that the electrolyte is impregnated at least into the pores of the separator 20. The impregnation method is not particularly limited, but examples include a method of melting the electrolyte and infiltrating it into the pores of the separator 20, and a method of pressing a powder compact of the electrolyte against the separator 20. Alternatively, the separator 20 itself may be made of a solid electrolyte.

[0043] The exterior body 24 has a sealed space, and the positive electrode layer 12, the negative electrode layer 16, the separator 20, and the electrolyte solution 22 are accommodated within this sealed space. The exterior body 24 may have a structure commonly used in coin-type batteries (see, for example, Patent Documents 1 and 2), and is not particularly limited. Typically, the exterior body 24 includes a positive electrode can 24a, a negative electrode can 24b, and a gasket 24c, and the positive electrode can 24a and the negative electrode can 24b are crimped together via the gasket 24c to form a sealed space. The positive electrode can 24a and the negative electrode can 24b may be made of a metal such as stainless steel, but are not particularly limited thereto. The gasket 24c may be a ring-shaped member made of an insulating resin such as polypropylene or polytetrafluoroethylene, but are not particularly limited thereto. A gel solution may be applied to the gasket 24c to improve sealing properties. Examples of the type of solution used for the gel solution include heat-curing olefin-based and synthetic rubber-based solutions, but are not limited thereto as long as they are viscous or adhesive after the solvent evaporates. The application method is not limited, and the gasket 24c may be dipped in the solution, or the solution may be applied to the gasket 24c using a dispenser or the like.

[0044] The lithium-ion secondary battery 10 preferably further includes a positive electrode current collector 14 and / or a negative electrode current collector 18. The positive electrode current collector 14 and the negative electrode current collector 18 are not particularly limited, but are preferably metal foils such as copper foil or aluminum foil. The positive electrode current collector 14 is preferably disposed between the positive electrode layer 12 and the positive electrode can 24a, and the negative electrode current collector 18 is preferably disposed between the negative electrode layer 16 and the negative electrode can 24b. Furthermore, a positive electrode-side carbon layer 13 is preferably provided between the positive electrode layer 12 and the positive electrode current collector 14 to reduce contact resistance. Similarly, a negative electrode-side carbon layer 17 is preferably provided between the negative electrode layer 16 and the negative electrode current collector 18 to reduce contact resistance. Both the positive electrode-side carbon layer 13 and the negative electrode-side carbon layer 17 are preferably made of conductive carbon, and may be formed, for example, by applying a conductive carbon paste by screen printing or the like.

[0045] The battery element may be in the form of a unit cell of positive electrode layer 12 / separator 20 / negative electrode layer 16 as shown in Fig. 1, or may be in the form of a multilayer cell having a plurality of unit cells. The multilayer cell is not limited to a flat plate laminate structure in which flat plates or layers are stacked, but may have various laminate structures, including the following examples. Note that in any of the configurations exemplified below, the entire cell laminate may be a single, integrated sintered body. - Folded structure: A laminated structure in which a sheet having a layered configuration including a unit cell and a current collecting layer is folded once or multiple times to form a multilayer (large area) structure. - Wound structure: A laminated structure in which sheets of a layer configuration including a unit cell and a current collecting layer are wound and integrated to form a multilayer (large area) structure. - Multilayer ceramic capacitor (MLCC)-like structure: A multilayer structure (large area) in which a stack unit of current collecting layer / positive electrode layer / ceramic separator layer / negative electrode layer / current collecting layer is repeated in the thickness direction, and multiple positive electrode layers collect current on one side (e.g., the left side) and multiple negative electrode layers collect current on the other side (e.g., the right side).

[0046] Positive electrode plate manufacturing method The lithium composite oxide sintered body plate, which is a preferred form of the positive electrode layer 12, may be produced by any method, but is preferably produced through (a) the preparation of a lithium composite oxide-containing green sheet, (b) the optional preparation of an excess lithium source-containing green sheet, and (c) lamination and firing of the green sheets.

[0047] (a) Preparation of lithium composite oxide-containing green sheet First, a raw powder composed of a lithium composite oxide is prepared. This powder preferably contains synthesized plate-like particles (e.g., LiCoO2 plate-like particles) with a composition LiMO2 (where M is as described above). The volumetric D50 particle size of the raw powder is preferably 0.3 to 30 μm. For example, LiCoO2 plate-like particles can be prepared as follows. First, LiCoO2 powder is synthesized by mixing Co3O4 raw powder and Li2CO3 raw powder and firing (500 to 900°C, 1 to 20 hours). The obtained LiCoO2 powder is pulverized in a pot mill to a volumetric D50 particle size of 0.2 μm to 10 μm, yielding plate-like LiCoO2 particles capable of conducting lithium ions parallel to the plate surface. Such LiCoO2 particles can also be obtained by a method of growing a green sheet using LiCoO2 powder slurry and then crushing it, or by a method of synthesizing plate-like crystals, such as the flux method, hydrothermal synthesis, single crystal growth using a melt, or the sol-gel method. The resulting LiCoO2 particles are in a state where they can be easily cleaved along the cleavage plane. By cleaving the LiCoO2 particles through crushing, LiCoO2 plate-like particles can be produced.

[0048] The plate-like particles may be used alone as the raw material powder, or a mixture of the plate-like powder and other raw material powders (e.g., Co3O4 particles) may be used as the raw material powder. In the latter case, it is preferable to use the plate-like powder as template particles to impart orientation, and the other raw material powders (e.g., Co3O4 particles) as matrix particles that can grow along the template particles. In this case, a powder in which the template particles and the matrix particles are mixed in a ratio of 100:0 to 3:97 is preferably used as the raw material powder. When Co3O4 raw material powder is used as the matrix particles, the volume-based D50 particle size of the Co3O4 raw material powder is not particularly limited and can be, for example, 0.1 to 1.0 μm, but it is preferably smaller than the volume-based D50 particle size of the LiCoO2 template particles. These matrix particles can also be obtained by heat-treating a Co(OH)2 raw material at 500 to 800°C for 1 to 10 hours. In addition to Co3O4, Co(OH)2 particles or LiCoO2 particles may also be used as the matrix particles.

[0049] When the raw material powder is composed of 100% LiCoO2 template particles, or when LiCoO2 particles are used as matrix particles, large (e.g., 90 mm x 90 mm square) and flat LiCoO2 sintered plates can be obtained by sintering. Although the mechanism is unclear, it is expected that volume change or localized unevenness during sintering is unlikely to occur because LiCoO2 is not synthesized during the sintering process.

[0050] The raw material powder is mixed with a dispersion medium and various additives (binder, plasticizer, dispersant, etc.) to form a slurry. A lithium compound other than LiMO2 (e.g., lithium carbonate) may be added to the slurry in an excess amount of approximately 0.5 to 30 mol% to promote grain growth or compensate for volatiles during the firing process described below. It is preferable not to add a pore-forming material to the slurry. The slurry is preferably stirred under reduced pressure to degas, and the viscosity is preferably adjusted to 4000 to 10000 cP. The resulting slurry is formed into a sheet to obtain a lithium composite oxide-containing green sheet. The green sheet thus obtained is an independent sheet-like compact. An independent sheet (sometimes referred to as a "free-standing film") refers to a sheet that can be handled independently from other supports (including flakes with an aspect ratio of 5 or more). In other words, an independent sheet does not include a sheet that is fixed to another support (such as a substrate) and integrated with the support (which is inseparable or difficult to separate). The sheet forming is preferably carried out using a forming method capable of applying shear force to plate-like particles (e.g., template particles) in the raw material powder. This allows the average tilt angle of the primary particles to be greater than 0° and less than 30° relative to the plate surface. A doctor blade method is suitable as a forming method capable of applying shear force to the plate-like particles. The thickness of the lithium composite oxide-containing green sheet may be appropriately set so that it has the desired thickness as described above after sintering.

[0051] (b) Preparation of green sheet containing excess lithium source (optional step) If desired, a green sheet containing an excess lithium source is prepared separately from the lithium composite oxide-containing green sheet. This excess lithium source is preferably a lithium compound other than LiMO2, from which components other than Li are eliminated by firing. A preferred example of such a lithium compound (excess lithium source) is lithium carbonate. The excess lithium source is preferably in powder form, and the volume-based D50 particle size of the excess lithium source powder is preferably 0.1 to 20 μm, more preferably 0.3 to 10 μm. The lithium source powder is then mixed with a dispersion medium and various additives (binder, plasticizer, dispersant, etc.) to form a slurry. The obtained slurry is preferably stirred under reduced pressure to degas and adjusted to a viscosity of 1,000 to 20,000 cP. The obtained slurry is formed into a sheet to obtain a green sheet containing an excess lithium source. The green sheet thus obtained is also an independent sheet-like molded product. Sheet formation can be performed by various well-known methods, but a doctor blade method is preferred. The thickness of the excess lithium source-containing green sheet is preferably set so that the molar ratio (Li / Co ratio) of the Li content in the excess lithium source-containing green sheet to the Co content in the lithium composite oxide-containing green sheet is preferably 0.1 or more, more preferably 0.1 to 1.1.

[0052] (c) Stacking and firing of green sheets A green sheet containing a lithium composite oxide (e.g., a LiCoO2 green sheet) and, if desired, a green sheet containing an excess lithium source (e.g., a Li2CO3 green sheet) are placed in this order on the lower setter, and then the upper setter is placed thereon. The upper and lower setters are made of ceramics, preferably zirconia or magnesia. If the setter is made of magnesia, the pores tend to be small. The upper setter may have a porous structure, a honeycomb structure, or a dense structure. If the upper setter is dense, the pores in the sintered plate tend to be small and the number of pores tends to be large. If necessary, the green sheet containing an excess lithium source is preferably cut to a size such that the molar ratio of the Li content in the green sheet containing the excess lithium source to the Co content in the lithium composite oxide (Li / Co ratio) is preferably 0.1 or more, more preferably 0.1 to 1.1.

[0053] At the stage where the green sheet containing a lithium composite oxide (e.g., a LiCoO2 green sheet) is placed on the lower setter, this green sheet may be degreased as desired and then calcined at 600 to 850° C. for 1 to 10 hours. In this case, the green sheet containing an excess lithium source (e.g., a Li2CO3 green sheet) and the upper setter may be placed in this order on the obtained calcined plate.

[0054] The green sheet and / or calcined plate is then sandwiched between setters, optionally degreased, and then heat-treated (fired) at a medium firing temperature (for example, 700 to 1000°C) to obtain a sintered lithium composite oxide plate. This firing process may be carried out in two stages or in one stage. When firing in two stages, it is preferable that the first firing temperature is lower than the second firing temperature. The sintered plate thus obtained is also in the form of an independent sheet.

[0055] Negative electrode plate manufacturing method The titanium-containing sintered plate, which is a preferred embodiment of the negative electrode layer 16, may be produced by any method. For example, an LTO sintered plate is preferably produced by (a) preparing an LTO-containing green sheet and (b) firing the LTO-containing green sheet.

[0056] (a) Preparation of LTO-containing green sheet First, lithium titanate Li4Ti5O 12A raw material powder (LTO powder) composed of the above is prepared. The raw material powder may be commercially available LTO powder or newly synthesized. For example, a powder obtained by hydrolyzing a mixture of titanium tetraisopropoxyalcohol and isopropoxylithium may be used, or a mixture containing lithium carbonate, titania, etc. may be fired. The volumetric D50 particle size of the raw material powder is preferably 0.05 to 5.0 μm, more preferably 0.1 to 2.0 μm. Larger particle sizes of raw material powders tend to result in larger pores. Furthermore, if the raw material particle size is large, it may be subjected to a pulverization process (e.g., pot mill pulverization, bead mill pulverization, jet mill pulverization, etc.) to obtain the desired particle size. The raw material powder is then mixed with a dispersion medium and various additives (binder, plasticizer, dispersant, etc.) to form a slurry. A lithium compound other than LiMO (e.g., lithium carbonate) may be added to the slurry in an excess amount of approximately 0.5 to 30 mol% to promote particle growth or compensate for volatile content during the firing process described below. It is preferable not to add a pore-forming material to the slurry. The slurry is preferably stirred under reduced pressure to degas and adjusted to a viscosity of 4,000 to 10,000 cP. The resulting slurry is formed into a sheet to obtain an LTO-containing green sheet. The resulting green sheet is an independent sheet-like compact. An independent sheet (sometimes called a "freestanding film") refers to a sheet that can be handled independently from other supports (including flakes with an aspect ratio of 5 or more). In other words, an independent sheet does not include a sheet that is fixed to another support (such as a substrate) and integrated with the support (which is inseparable or difficult to separate). Sheet formation can be performed by various well-known methods, but a doctor blade method is preferred. The thickness of the LTO-containing green sheet can be appropriately set so that it achieves the desired thickness as described above after firing.

[0057] (b) Firing of LTO-containing green sheets The LTO-containing green sheet is placed on a setter. The setter is made of ceramic, preferably zirconia or magnesia. The setter is preferably embossed. The green sheet placed on the setter is then placed in a sheath. The sheath is also made of ceramic, preferably alumina. The sheet is then optionally degreased and fired to obtain an LTO sintered plate. This firing is preferably carried out at 600 to 900°C for 1 to 50 hours, more preferably at 700 to 800°C for 3 to 20 hours. The sintered plate thus obtained is also in the form of an independent sheet. The heating rate during firing is preferably 100 to 1,000°C / h, more preferably 100 to 600°C / h. This heating rate is particularly preferably used during the heating process from 300 to 800°C, more preferably from 400 to 800°C.

[0058] (c) Summary The LTO sintered body plate can be preferably produced as described above. In this preferred production method, it is effective to 1) adjust the particle size distribution of the LTO powder and / or 2) change the heating rate during firing, which are thought to contribute to realizing various properties of the LTO sintered body plate.

[0059] Coating method for integral sintered plate The three-layer integral sintered body plate, consisting of a positive electrode layer, a ceramic separator, and a negative electrode layer, which is preferably used in the lithium-ion secondary battery of the present invention, is preferably entirely coated with a metal oxide layer. By entirely coating the integral sintered body plate with a metal oxide layer, it is possible to prevent delamination of the integral sintered body plate due to physical impact during battery assembly and also to prevent capacity degradation associated with storage in a charged state. Coating the integral sintered body plate with the metal oxide layer may be performed by any method. For example, it is preferable to: i) prepare a coating liquid containing a metal compound; ii) immerse the integral sintered body plate in this coating liquid to allow the coating liquid to penetrate to the interior; iii) remove the integral sintered body and dry it; and iv) heat-treat the integral sintered body with the metal compound attached to convert the metal compound to a metal oxide, thereby forming a metal oxide layer. The coating liquid prepared in step i) above is not particularly limited as long as it contains a metal compound capable of forming a metal oxide layer upon heat treatment in a solvent (preferably an organic solvent). The metal compound is preferably at least one metal compound selected from the group consisting of Zr, Mg, Al, Nb, and Ti, and more preferably a metal alkoxide. Preferred examples of such metal compounds include metal alkoxides such as zirconium tetra-n-butoxide, magnesium diethoxide, triisopropoxyaluminum, niobium pentaethoxide, and titanium tetraisopropoxide. In step ii) above, the integral sintered body plate immersed in the coating liquid is preferably subjected to a vacuum or reduced pressure atmosphere, since this allows the coating liquid to penetrate thoroughly and efficiently into the interior of the integral sintered body plate. Drying in step iii) above can be performed at room temperature, but may also be performed by heating. Heat treatment in step iv) above is preferably performed at 300 to 700°C for 2 to 24 hours, more preferably at 350 to 550°C for 4 to 6 hours. Thus, an integral sintered body plate entirely coated with a metal oxide layer is obtained. [Example]

[0060] The present invention is further illustrated by the following examples.

[0061] Examples 1-4 and 8-11 Coin-type lithium ion secondary batteries for reflow soldering were produced and evaluated as follows.

[0062] (1) Preparation of LCO green sheet (positive electrode green sheet) First, we mixed Co3O4 powder (Coremax) and Li2CO3 powder (Honjo Chemical Co., Ltd.) weighed to a Li / Co molar ratio of 1.01, held the mixture at 780 °C for 5 hours, and then ground the resulting powder to a volumetric D50 of 0.4 μm in a pot mill to obtain a powder of LCO platelet particles. One hundred parts by weight of the resulting LCO powder was mixed with 100 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 10 parts by weight of a binder (polyvinyl butyral, product number BM-2, Sekisui Chemical Co., Ltd.), 4 parts by weight of a plasticizer (DOP: Di(2-ethylhexyl)phthalate, product of Kurogane Kasei Co., Ltd.), 2 parts by weight of a dispersant (Rheodor SP-O30, product of Kao Corporation), and 1 part by weight of ZrO2 (Sigma-Aldrich). The resulting mixture was stirred under reduced pressure to degas and adjusted to a viscosity of 4000 cP to prepare an LCO slurry. The viscosity was measured using a Brookfield LVT viscometer. The prepared slurry was then formed into a sheet on a PET film using a doctor blade method to form an LCO green sheet. The thickness of the LCO green sheet was adjusted to 100 μm after firing.

[0063] (2) Preparation of LTO green sheet (negative electrode green sheet) First, 100 parts by weight of LTO powder (volume-based D50 particle size: 0.06 μm, manufactured by Sigma-Aldrich Japan LLC), 100 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 20 parts by weight of a binder (polyvinyl butyral: product number BM-2, manufactured by Sekisui Chemical Co., Ltd.), 4 parts by weight of a plasticizer (DOP: Di(2-ethylhexyl)phthalate, manufactured by Kurogane Kasei Co., Ltd.), and 2 parts by weight of a dispersant (product name Rheodor SP-O30, manufactured by Kao Corporation) were mixed. The resulting anode raw material mixture was stirred under reduced pressure to degas and the viscosity was adjusted to 4000 cP to prepare an LTO slurry. The viscosity was measured using a Brookfield LVT viscometer. The prepared slurry was then formed into a sheet on a PET film using a doctor blade method to form an LTO green sheet. The LTO green sheet was adjusted to a thickness of 100 μm after firing.

[0064] (3) Preparation of MgO green sheets (separator green sheets) Magnesium carbonate powder (manufactured by Konoshima Chemical Co., Ltd.) was heat-treated at 900°C for 5 hours to obtain MgO powder. The resulting MgO powder was mixed with glass frit (manufactured by Nippon Frit Co., Ltd., CK0199) at a weight ratio of 4:1. 100 parts by weight of the resulting mixed powder (volume-based D50 particle size 0.4 μm), 100 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 20 parts by weight of a binder (polyvinyl butyral, product number BM-2, manufactured by Sekisui Chemical Co., Ltd.), 4 parts by weight of a plasticizer (DOP: Di(2-ethylhexyl)phthalate, manufactured by Kurogane Kasei Co., Ltd.), and 2 parts by weight of a dispersant (product name Rheodor SP-O30, manufactured by Kao Corporation). The resulting raw material mixture was stirred under reduced pressure to degas and the viscosity was adjusted to 4000 cP to prepare a slurry. The viscosity was measured using a Brookfield LVT viscometer. The slurry thus prepared was formed into a sheet on a PET film by a doctor blade method to form a separator green sheet, which had a thickness of 25 μm after firing.

[0065] (4) Lamination, compression and firing Three LCO green sheets (positive electrode green sheets), MgO green sheets (separator green sheets), and two LTO green sheets (negative electrode green sheets) were stacked in this order, and the resulting laminate was subjected to cold isostatic pressing (CIP) at 200 kgf / cm 2 The green sheets were pressed together with a pressure. The laminate thus pressed was then punched into a disk shape with a diameter of 10 mm using a punching die. The obtained disk-shaped laminate was degreased at 600°C for 5 hours, then heated to 800°C at a rate of 1000°C / h and held there for 10 minutes, followed by sintering, and then cooled. In this way, a single integrated sintered plate (integrated electrode) was obtained, including three layers: a positive electrode layer (LCO sintered body layer) 12, a ceramic separator (MgO separator), and a negative electrode layer (LTO sintered body layer).

[0066] (5) Metal oxide layer coating First, 10 g of 2-ethoxyethanol, 0.25 g of acetylacetone, and 1 g of zirconium tetra-n-butoxide were placed in a container and stirred to prepare a coating solution. The integrated sintered body plate obtained in (4) above was immersed in this solution. The container was then placed in a desiccator, evacuated to -95 kPa, and left for 3 minutes. The desiccator was then returned to atmospheric pressure, and the container containing the integrated sintered body plate was removed. The integrated sintered body plate was then placed on a nonwoven wiper with tweezers, gently wiped off the coating solution, and then dried at room temperature for 2 hours. The dried integrated sintered body plate was placed on an alumina setter and heat-treated in a medium-sized Super Kanthal furnace (manufactured by Kyowa Konetsu Kogyo Co., Ltd.) at 400 °C for 5 hours. This resulted in an integrated sintered body plate (integrated electrode) entirely coated with a metal oxide layer (a layer composed of Zr oxide or a composite oxide of Zr and Li).

[0067] (6) Fabrication of lithium secondary batteries (6a) Bonding of the negative electrode layer and negative electrode current collector with conductive carbon paste Acetylene black and polyimide amide were weighed out in a mass ratio of 3:1 and mixed with an appropriate amount of NMP (N-methyl-2-pyrrolidone) as a solvent to prepare a conductive carbon paste as a conductive adhesive. The conductive carbon paste was screen-printed onto aluminum foil as a negative electrode current collector. The integrated sintered body prepared in (5) above was placed so that the negative electrode layer 16 was contained within the undried printed pattern (i.e., the area coated with the conductive carbon paste). This was then vacuum-dried at 60°C for 30 minutes to produce a structure in which the negative electrode layer and negative electrode current collector were bonded via the negative electrode-side carbon layer. The thickness of the negative electrode-side carbon layer was 10 μm.

[0068] (6b) Preparation of a positive electrode current collector with a carbon layer Acetylene black and polyimide amide were weighed out in a mass ratio of 3:1 and mixed with an appropriate amount of NMP (N-methyl-2-pyrrolidone) as a solvent to prepare a conductive carbon paste. The conductive carbon paste was screen-printed onto an aluminum foil positive electrode current collector, and then vacuum-dried at 60°C for 30 minutes to produce a positive electrode current collector with a positive electrode carbon layer formed on the surface. The thickness of the positive electrode carbon layer was 5 μm.

[0069] (6c) Coin cell battery assembly Between the positive and negative cans that would form the battery case, the positive electrode current collector, positive electrode carbon layer, integrated sintered body plate (LCO positive electrode layer), MgO separator and LTO negative electrode layer, negative electrode carbon layer, negative electrode current collector, and wave washer (Misumi) were stacked in this order from the positive can to the negative can. After filling with electrolyte, the positive and negative cans were sealed by crimping with a gasket. In this way, a coin cell-type lithium ion secondary battery with a diameter of 12.5 mm and a thickness of 1.0 mm was produced. The electrolyte used here was a solution prepared by dissolving LiBF4 in a PC organic solvent to a concentration of 1.5 mol / L. The positive electrode terminal was joined to the positive can, and the negative electrode terminal was joined to the negative can by resistance welding. 200 batteries were produced for each example.

[0070] (7) Initial charge / discharge and SOC adjustment The fabricated battery was initially charged and discharged. This initial charge and discharge was performed by constant current (CC) charging at a current value of 0.1 C to a voltage of 2.7 V, followed by constant voltage (CV) charging until the current value reached 0.01 C, and then constant current discharging at a current value of 0.05 C to a voltage of 1.5 V. The discharge capacity at this time was taken as the battery capacity, and the state discharged at 0.05 C was taken as SOC 0%. The battery with SOC 0% was then charged at a constant current of 0.1 C for a capacity calculated by multiplying the charge rate (equivalent to SOC) and battery capacity, thereby adjusting the SOC of the battery to the values ​​shown in Table 1.

[0071] (8) Manufacturing of circuit board assemblies Using a reflow oven (product name: UNI-5016F, manufactured by ANTOM), the battery obtained in (7) above was connected to a circuit board by reflow soldering. This connection was achieved by joining the positive and negative terminals to the circuit board by reflow soldering. The heating (reflow heating) during reflow soldering of connection to the circuit board was performed using a heating profile in which the maximum temperature was 260°C (Examples 1 to 3 and 11), 240°C (Example 8), or 275°C (Examples 9 and 10), and a temperature of 220°C or higher was applied for 60 seconds. Reflow heating using this profile was performed a total of two times (Examples 1 to 3 and 11) or once (Examples 8 to 10).

[0072] (9) High temperature and humidity test The resistance of 100 batteries mounted on the circuit board in (8) above at an SOC of 30% was measured in advance, and the average value was used as the initial resistance value. The resistance value was determined as the resistance value at 10 Hz from AC impedance measurements. The battery-mounted circuit board was stored in a high-temperature, high-humidity environment at a temperature of 85°C and a relative humidity of 85% for 10 days with power on. The resistance of 100 batteries after 10 days of storage was measured at an SOC of 30%, and the average value was used as the post-test resistance value. Batteries whose post-test resistance value increased by 20% or less from the initial resistance value were determined to be good, and a battery was deemed to have passed if the proportion of good batteries out of 100 (i.e., the good product rate) was 85% or more, and otherwise was deemed to have failed.

[0073] Examples 5-7 Batteries were produced and evaluated in the same manner as in Examples 1, 3, and 4, except that: i) glass frit was not added in the preparation of MgO in (3) above (i.e., the ratio of MgO powder to glass frit was 100:0); ii) the thickness of the separator green sheet in (3) above was set so that the thickness after firing would be 12 μm; iii) the lamination, compression bonding, and firing in (4) above were performed as described in (4′) below; iv) in (5) above, instead of using an integrated sintered body plate, the entire positive electrode / separator sintered body plate obtained in (4′) below was coated with a metal oxide layer; v) in (6a) above, the negative electrode sintered body plate produced in (4′) below was placed so that the negative electrode layer 16 was contained within the undried printed pattern; and vi) the coin-shaped battery in (6c) above was assembled as described in (6c′) below. That is, Examples 6, 7, and 8 correspond to Examples 1, 3, and 4, respectively, except for the changes in i) to vi) above.

[0074] (4') Lamination, compression and firing Three LCO green sheets (positive electrode green sheets) and an MgO green sheet (separator green sheet) were stacked in this order, and the resulting laminate was subjected to cold isostatic pressing (CIP) at 200 kgf / cm 2 The green sheets were pressed together with a pressure. The laminate thus pressed was punched into a disk shape with a diameter of 10 mm using a punching die. The obtained disk-shaped laminate was degreased at 600°C for 5 hours, then heated to 900°C at a rate of 1000°C / h and held there for 10 minutes, followed by firing, and then cooled. In this way, a positive electrode / separator sintered body plate consisting of two layers, a positive electrode layer (LCO sintered body layer) 12 and a ceramic separator (MgO separator), was obtained.

[0075] Two LTO green sheets (negative electrode green sheets) were stacked in order, and the resulting laminate was subjected to CIP (cold isostatic pressing) at 200 kgf / cm 2The green sheets were pressed together with a pressure. The laminate thus pressed was then punched out into a disk shape with a diameter of 10 mm using a punching die. The resulting disk-shaped laminate was degreased at 600°C for 5 hours, then heated to 800°C at a rate of 1000°C / h and held there for 10 minutes, after which it was sintered and cooled. In this way, a negative electrode sintered body plate composed of a negative electrode layer (LTO sintered body layer) was obtained.

[0076] (6c') Coin cell battery assembly Between the positive and negative cans that comprise the battery case, the positive electrode current collector, positive electrode carbon layer, positive electrode / separator sintered body plate (LCO positive electrode layer and MgO separator), negative electrode sintered body plate (LTO negative electrode layer), negative electrode carbon layer, negative electrode current collector, and wave washer (Misumi) were stacked in this order from the positive can to the negative can. After filling with electrolyte, the positive and negative cans were sealed by crimping with a gasket. In this way, a coin-cell type lithium-ion secondary battery with a diameter of 12.5 mm and a thickness of 1.0 mm was fabricated. The electrolyte used was a solution of LiBF4 dissolved in a PC organic solvent to a concentration of 1.5 mol / L. The positive electrode terminal was connected to the positive can, and the negative electrode terminal was connected to the negative can by resistance welding. 200 batteries were fabricated in this manner.

[0077] result The manufacturing conditions and the evaluation results of the high-temperature, high-humidity test for the batteries produced in Examples 1 to 11 are shown in Table 1.

[0078] [Table 1]

Claims

1. A method for manufacturing a circuit board assembly, comprising connecting a lithium ion secondary battery to a circuit board by reflow soldering, the state of charge (SOC) of the lithium ion secondary battery at the time of the reflow soldering is 30 to 100%; The lithium ion secondary battery is a positive electrode layer that is a sintered lithium composite oxide plate; a negative electrode layer that is a titanium-containing sintered body plate; a separator interposed between the positive electrode layer and the negative electrode layer; Electrolytes, an exterior body having a sealed space in which the positive electrode layer, the negative electrode layer, the separator, and the electrolyte are accommodated; A method for manufacturing a circuit board assembly, comprising:

2. The method for manufacturing a circuit board assembly according to claim 1, wherein the state of charge (SOC) of the lithium ion secondary battery is 35 to 100%.

3. The method for manufacturing a circuit board assembly according to claim 1, wherein the state of charge (SOC) of the lithium ion secondary battery is 70 to 100%.

4. The method for manufacturing a circuit board assembly according to any one of claims 1 to 3, wherein the reflow heating in the reflow soldering is carried out at 180 to 270°C.

5. The method for manufacturing a circuit board assembly according to claim 1 , wherein the exterior body comprises a positive electrode can, a negative electrode can, and a gasket, and the positive electrode can and the negative electrode can are crimped together via the gasket to form the sealed space.

6. The battery further includes a positive electrode terminal joined to an outer surface of the exterior body closer to the positive electrode layer, and a negative electrode terminal joined to an outer surface of the exterior body closer to the negative electrode layer, The method for manufacturing a circuit board assembly according to claim 1 , wherein the positive terminal and / or the negative terminal is connected to the circuit board by the reflow soldering.

7. The method for manufacturing a circuit board assembly according to claim 1 , wherein the lithium composite oxide is lithium cobalt oxide.

8. The method for manufacturing a circuit board assembly according to claim 1 , wherein the titanium-containing sintered body comprises lithium titanate or niobium titanium composite oxide.

9. The separator is made of cellulose, polyimide, polyester, or MgO, Al ２ O ３ , ZrO ２ , SiC, Si ３ N ４ 10. The method of claim 1, wherein the substrate is made of a ceramic selected from the group consisting of AlN and cordierite.

10. The electrolyte is provided in the form of an electrolytic solution, and the electrolytic solution contains lithium fluoroborate (LiBF) in a non-aqueous solvent comprising at least one selected from the group consisting of γ-butyrolactone (GBL), ethylene carbonate (EC), and propylene carbonate (PC). ４ 2. The method for manufacturing a circuit board assembly according to claim 1, wherein the liquid contains:

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