Honeycomb-type ceramic positive electrode and lithium-ion secondary battery including the same

Abstract

Provided is a ceramic positive electrode that exhibits improved battery characteristics (for example, discharge rate characteristics) in a secondary battery while having a three-dimensional structure that is suited for high capacitance and high output and can internally accommodate a plurality of negative electrodes. This honeycomb-type ceramic positive electrode has a columnar honeycomb structure, has a first end surface, a second end surface parallel to the first end surface, and an outer circumferential side surface perpendicular to the first end surface and the second end surface, and is provided with a plurality of holes extending from the first end surface towards the second end surface. The honeycomb-type ceramic positive electrode is constituted of a lithium composite oxide sintered body in which a plurality of primary particles constituted of a lithium composite oxide are joined, and when a center axis of the columnar honeycomb structure parallel to the outer circumferential side surface or an axis parallel thereto is defined as the z-axis, the plurality of primary particles are oriented in the z-axis direction.

Description

[Technical Field]

[0001] The present invention relates to a honeycomb-type ceramic positive electrode and a lithium-ion secondary battery including the same. [Background technology]

[0002] Lithium-ion secondary batteries are widely used in various devices that require charging. Most existing lithium-ion secondary batteries use a powder-dispersed positive electrode (so-called coated electrode) made by coating and drying a positive electrode mixture containing a positive electrode active material, a conductive additive, a binder, etc.

[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. To address this issue, 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, the positive electrode or positive electrode active material layer does not contain a binder or conductive additive (e.g., conductive carbon), and the increased packing density of the lithium composite oxide is expected to result in high capacity and favorable charge / discharge efficiency. For example, Patent Document 1 (Japanese Patent No. 5587052) discloses a positive electrode for a lithium-ion secondary battery, which includes a positive electrode current collector and a positive electrode active material layer joined to the positive electrode current collector via a conductive adhesive layer. This positive electrode active material layer is said to be made of a lithium composite oxide sintered body plate having a thickness of 30 μm or more, a porosity of 3 to 30%, and an open pore ratio of 70% or more. Patent Document 2 (Japanese Patent No. 6374634) discloses a lithium composite oxide sintered body plate such as lithium cobalt oxide LiCoO2 (hereinafter referred to as LCO) for use in the positive electrode of a lithium ion secondary battery. This lithium composite oxide sintered body plate is said to have a structure in which multiple primary particles having a layered rock salt structure are bonded together, a porosity of 3 to 40%, an average pore diameter of 15 μm or less, an open pore ratio of 70% or more, a thickness of 15 to 200 μm, and a primary particle diameter (average particle diameter of the multiple primary particles) of 20 μm or less. Furthermore, the lithium composite oxide sintered plate is said to have an average angle between the (003) planes of the plurality of primary particles and the plate surface of the lithium composite oxide sintered plate, i.e., an average tilt angle, which is greater than 0° and not greater than 30°.

[0004] Lithium-ion secondary batteries using such sintered lithium composite oxide plates have also been proposed. For example, Patent Document 3 (WO2019 / 187913) discloses a lithium-ion secondary battery including a positive electrode plate that is a sintered lithium composite oxide plate, a negative electrode layer containing carbon, a separator, and an electrolyte, in which the multiple primary particles that make up the sintered lithium composite oxide plate are oriented at an average orientation angle of more than 0° and not more than 30° with respect to the plate surface of the positive electrode plate.

[0005] Meanwhile, secondary batteries equipped with honeycomb electrodes have been proposed. For example, Patent Document 4 (Japanese Patent Publication No. 2020-155334) discloses a secondary battery equipped with a first electrode having a plurality of holes extending from one side to the other, a second electrode inserted into each of these holes, and a separator layer disposed between the first and second electrodes. Patent Document 4 discloses that a secondary battery was fabricated by fabricating a honeycomb negative electrode and inserting a rod-shaped positive electrode coated with a separator layer into the holes of the honeycomb negative electrode. This rod-shaped positive electrode is a powder-dispersed positive electrode fabricated by drying a positive electrode mixture containing a positive electrode active material, a conductive additive, and a binder. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 5587052 [Patent Document 2] Patent No. 6374634 [Patent Document 3] WO2019 / 187913 [Patent Document 4] Japanese Patent Publication No. 2020-155334 Summary of the Invention

[0007] The present inventors have now discovered that by forming a ceramic positive electrode into a columnar honeycomb structure with crystal orientation in a predetermined direction, it is possible to provide a ceramic positive electrode that has a three-dimensional structure suitable for high capacity and high output and can accommodate multiple negative electrodes therein, while also exhibiting improved battery characteristics (e.g., discharge rate characteristics) in secondary batteries.

[0008] Therefore, an object of the present invention is to provide a ceramic positive electrode that has a three-dimensional structure suitable for high capacity and high output and can accommodate multiple negative electrodes therein, while exhibiting improved battery characteristics (e.g., discharge rate characteristics) in secondary batteries.

[0009] According to the present invention, the following aspects are provided. [Aspect 1] A honeycomb ceramic positive electrode having a columnar honeycomb structure, the honeycomb ceramic positive electrode having a first end face, a second end face parallel to the first end face, and an outer peripheral side face perpendicular to the first end face and the second end face, and a plurality of holes extending from the first end face toward the second end face, the honeycomb-type ceramic positive electrode is made of a lithium composite oxide sintered body in which a plurality of primary particles made of a lithium composite oxide are bonded together, When the central axis of the columnar honeycomb structure parallel to the outer peripheral side surface or an axis parallel thereto is defined as the z-axis, the plurality of primary particles are oriented in the z-axis direction. [Aspect 2] 2. The honeycomb-type ceramic positive electrode of claim 1, wherein the plurality of primary particles are oriented at an average orientation angle of more than 0° and less than or equal to 30° with respect to the z-axis. [Aspect 3] 3. The honeycomb ceramic positive electrode according to aspect 1 or 2, wherein the plurality of pores are partitioned by lattice-like partition walls formed of the sintered lithium composite oxide. [Aspect 4] A honeycomb-type ceramic positive electrode according to aspect 3, wherein, when the columnar honeycomb structure is viewed in a plane in the z-axis direction and one direction of the lattice-like partition walls is assigned to the x-axis and the other direction to the y-axis, primary particles constituting the partition walls in the x-axis direction are oriented in the x-axis direction, and primary particles constituting the partition walls in the y-axis direction are oriented in the y-axis direction. [Aspect 5] A honeycomb-shaped ceramic positive electrode according to aspect 4, wherein the plurality of primary particles are oriented at an average orientation angle of more than 0° and not more than 30° with respect to the x-axis or the y-axis for each individual partition wall. [Aspect 6] 6. The honeycomb-type ceramic positive electrode according to any one of aspects 1 to 5, wherein the lithium composite oxide is lithium cobalt oxide. [Aspect 7] A honeycomb ceramic positive electrode according to any one of aspects 1 to 6; a plurality of negative electrodes inserted into the plurality of holes and having ends extending from the first end surface or the second end surface; a separator interposed between the honeycomb ceramic positive electrode and the negative electrode; An electrolyte; a battery container that accommodates the honeycomb ceramic positive electrode, the negative electrode, the separator, and the electrolyte; A lithium-ion secondary battery comprising: [Aspect 8] 8. The lithium-ion secondary battery of claim 7, wherein the negative electrode comprises carbon. [Aspect 9] Aspect 9. The lithium ion secondary battery of aspect 7 or 8, wherein the separator is a ceramic separator. [Aspect 10] A lithium ion secondary battery according to any one of aspects 7 to 9, wherein the ceramic separator comprises at least one selected from the group consisting of MgO, Al2O3, ZrO2, SiC, Si3N4, AlN, and cordierite. [Aspect 11] The lithium ion secondary battery according to any one of aspects 7 to 10, further comprising a positive electrode current collector foil on at least one surface of the first end surface, the second end surface, and the outer peripheral side surface (excluding the surface from which the negative electrode extends). [Aspect 12] 12. The lithium ion secondary battery according to any one of aspects 7 to 11, further comprising a negative electrode current collector foil at an end of the negative electrode extending from the first end face or the second end face. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a schematic perspective view showing an example of a honeycomb ceramic positive electrode according to the present invention. [Figure 2] FIG. 2 is a cross-sectional perspective view for schematically explaining cross sections C1 to C3 for EBSD analysis in a portion constituting the honeycomb ceramic positive electrode according to the present invention. [Figure 3] 1 is an SEM image showing an example of a cross section perpendicular to the layer surface of an oriented positive electrode layer. [Figure 4] 4 is an EBSD image of a cross section of the oriented positive electrode layer shown in FIG. 3. [Figure 5] 5 is a histogram showing the distribution of orientation angles of primary particles in the EBSD image of FIG. 4 on an area basis. [Figure 6A] 1 is a schematic perspective view conceptually showing the configuration of a lithium ion secondary battery according to the present invention. [Figure 6B] 6B is a cross-sectional view of the lithium ion secondary battery shown in FIG. 6A taken along line 6B-6B. DETAILED DESCRIPTION OF THE INVENTION

[0011] Honeycomb ceramic cathode FIG. 1 shows a honeycomb-type ceramic positive electrode 12 according to one embodiment of the present invention. The honeycomb-type ceramic positive electrode 12 has a columnar honeycomb structure. This columnar honeycomb structure has a first end face 12a, a second end face 12b parallel to the first end face 12a, and an outer peripheral side face 12c perpendicular to the first end face 12a and the second end face 12b. The columnar honeycomb structure also has a plurality of holes 12d extending from the first end face 12a toward the second end face 12b. The honeycomb-type ceramic positive electrode 12 is made of a lithium composite oxide sintered body. The lithium composite oxide sintered body is made of a plurality of bonded primary particles made of a lithium composite oxide. When the central axis of the columnar honeycomb structure parallel to the outer peripheral side face 12c or an axis parallel thereto is defined as the z-axis, the plurality of primary particles are oriented in the z-axis direction. By making the ceramic positive electrode 12 into a columnar honeycomb structure with crystal orientation in a predetermined direction in this way, it is possible to provide a ceramic positive electrode 12 that has a three-dimensional structure suitable for high capacity and high output and can accommodate multiple negative electrodes inside, while exhibiting improved battery characteristics (e.g., discharge rate characteristics) in a secondary battery.

[0012] As mentioned above, it is already known that constructing a positive electrode or a positive electrode active material layer using a lithium composite oxide sintered body can improve capacity and charge / discharge efficiency (see, for example, Patent Documents 1 and 2). However, the honeycomb-type ceramic positive electrode 12 of the present invention is a columnar honeycomb structure made of a lithium composite oxide sintered body, with the primary particles oriented at least in the z-axis direction. The z-axis direction corresponds to the length direction of the columnar honeycomb structure, and the orientation of the primary particles in the z-axis direction facilitates the conduction of electrons and lithium ions in the z-axis direction (lengthwise direction), thereby reducing resistance in that direction. Therefore, by constructing a secondary battery by inserting a negative electrode into the holes 12d of the honeycomb-type ceramic positive electrode 12 via a separator, battery characteristics (e.g., discharge rate characteristics) can be improved.

[0013] The honeycomb-type ceramic positive electrode 12 is composed of a lithium composite oxide sintered body in which multiple primary particles composed of lithium composite oxide are bonded together. The fact that the positive electrode 12 is a ceramic or sintered body means that the positive electrode 12 does not contain a binder or conductive additive. This is because even if a binder is contained in the green sheet, the binder will be lost or burned away during firing. Furthermore, the absence of a binder in the ceramic positive electrode 12 has the advantage of preventing deterioration of the positive electrode due to the electrolyte. The lithium composite oxide constituting the sintered body 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 those disclosed in Patent Document 1 (Japanese Patent No. 5587052) and Patent Document 3 (WO2019 / 187913) can be used.

[0014] The honeycomb-type ceramic positive electrode 12 has a columnar honeycomb structure. The outer shape of the columnar honeycomb structure is not particularly limited and may be cylindrical or rectangular. The honeycomb-type ceramic positive electrode 12 preferably has a plurality of pores 12d partitioned by lattice-like partition walls 12e made of a sintered lithium composite oxide.

[0015] In the honeycomb-type ceramic positive electrode 12, when the central axis of the columnar honeycomb structure parallel to the outer peripheral side surface 12c or an axis parallel thereto is defined as the z-axis, multiple primary particles are oriented in the z-axis direction. Specifically, these multiple primary particles are preferably oriented at an average orientation angle of more than 0° and not more than 30° relative to the z-axis. To facilitate understanding of the orientation, the orientation form of the lithium composite oxide sintered body (e.g., partition wall 12e) constituting the honeycomb-type ceramic positive electrode 12 will be described using an oriented positive electrode plate (see Patent Document 3, hereinafter referred to as the "oriented positive electrode layer") made of a lithium composite oxide sintered body as an example. Figure 3 shows an example of a cross-sectional SEM image perpendicular to the layer surface of an oriented positive electrode layer having an average orientation angle of more than 0° and not more than 30° relative to the layer surface of the oriented positive electrode layer, while Figure 3 shows an electron backscatter diffraction (EBSD) image of a cross-section perpendicular to the plate surface of the oriented positive electrode layer. FIG. 4 shows a histogram illustrating the distribution of the orientation angles of the primary particles 11 in the EBSD image of FIG. 3 on an area basis. The EBSD image shown in FIG. 3 allows for 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 layer surface direction. Note that in FIGS. 3 and 4, the black areas within the oriented positive electrode layer represent pores. The orientation morphology of the primary particles in the oriented positive electrode layer shown in FIGS. 3 to 5 directly applies to the lithium composite oxide sintered body (e.g., the partition walls 12e) constituting the honeycomb-type ceramic positive electrode 12. Therefore, in the following description, the lithium composite oxide sintered body (e.g., the partition walls 12e) constituting the honeycomb-type ceramic positive electrode 12 may be referred to as the oriented positive electrode layer. The average orientation angle can be measured according to the procedure described in the Examples below.

[0016] In particular, when the columnar honeycomb structure is viewed in a plan view in the z-axis direction, with one direction of the lattice-like partition walls 12e designated as the x-axis and the other direction as the y-axis, it is preferable that the primary particles constituting the partition walls 12e in the x-axis direction be oriented in the x-axis direction, and the primary particles constituting the partition walls 12e in the y-axis direction be oriented in the y-axis direction. This ensures that the primary particles are oriented in all directions of the columnar honeycomb structure: the x-axis, the y-axis, and the z-axis. This reduces resistance not only in the z-axis direction (the length direction of the columnar honeycomb structure) but also in the x-axis and y-axis directions (i.e., the xy plane direction), which is believed to further promote lithium ion conduction and electron conduction. Furthermore, even when the columnar honeycomb structure is elongated, current collection at the outer peripheral side surface 12c can achieve low resistance. Specifically, it is preferable that the primary particles are oriented at an average orientation angle of more than 0° and not more than 30° with respect to the x-axis or y-axis for each partition wall 12e. That is, it is preferable that the primary particles constituting the partition wall 12e parallel to the x-axis are oriented at an average orientation angle of more than 0° and not more than 30° relative to the x-axis, and that the primary particles constituting the partition wall 12e parallel to the y-axis are oriented at an average orientation angle of more than 0° and not more than 30° relative to the y-axis.

[0017] The honeycomb-type ceramic positive electrode 12 is an oriented sintered body composed of a plurality of primary particles 11 bonded to one another. Each primary particle 11 is mainly plate-shaped, but may also be formed into a rectangular parallelepiped, cubic, spherical, or other shape. The cross-sectional shape of each primary particle 11 is not particularly limited, and may be rectangular, a polygonal shape other than rectangular, circular, elliptical, or any other complex shape.

[0018] Each primary particle 11 is composed of a lithium composite oxide. xAn oxide represented by MO₂ (where 0.05 < x < 1.10, M is at least one kind of transition metal, and M typically includes one or more of Co, Ni, and Mn). 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 in between, that is, a crystal structure in which a transition metal ion layer and a single lithium 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 the 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.

[0019] As shown in Figures 4 and 5, the average value of the orientation angles of the primary particles 11, i.e., the average orientation angle, is greater than 0° and less than or equal to 30°. This brings about the following various advantages. First, because the primary particles 11 lie in a state inclined relative to the thickness direction, the adhesion between the primary particles can be improved. As a result, the lithium ion conductivity between a primary particle 11 and other primary particles 11 adjacent to the primary particle 11 on both sides of the primary particle 11 in the longitudinal direction can be improved, thereby improving the rate characteristics. Second, the rate characteristics can be further improved. This is because, as described above, when lithium ions enter and exit the oriented positive electrode layer, expansion and contraction in the thickness direction predominates over expansion and contraction in the layer surface direction, which smooths the expansion and contraction of the oriented positive electrode layer and, accordingly, smooths the entry and exit of lithium ions.

[0020] 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. 4, three horizontal lines are drawn to divide the oriented positive electrode layer into four equal parts in the thickness direction, and three vertical lines are drawn to divide the oriented positive electrode layer into four equal parts in the layer 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.

[0021] As shown in FIG. 5 , the orientation angles of the primary particles 11 may be distributed widely from 0° to 90°, but preferably the majority of the primary particles 11 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 layer 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 layer surface of the oriented positive electrode layer (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.

[0022] Since each primary particle 11 is mainly plate-shaped, as shown in Figures 3 and 4, 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 4, 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.

[0023] 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.

[0024] The lithium composite oxide sintered body (e.g., partition wall 12e) constituting the honeycomb ceramic positive electrode 12 preferably contains pores. When the sintered body contains pores, particularly open pores, and is incorporated into a battery as a positive electrode plate, the electrolyte can penetrate into the sintered body, thereby improving lithium ion conductivity. This is because lithium ions are conducted in two ways within the sintered body: via the constituent particles of the sintered body and via the electrolyte in the pores; and the latter is overwhelmingly faster.

[0025] The thickness of the partition wall 12e constituting the positive electrode 12 is 70 to 200 μm, preferably 80 to 100 μm, more preferably 80 to 95 μm, and particularly preferably 85 to 95 μm. Within such a range, the active material capacity per unit area is increased, improving the energy density of the lithium-ion secondary battery 10, and also suppressing deterioration of the battery characteristics (particularly an increase in resistance value) due to repeated charge and discharge.

[0026] The honeycomb ceramic positive electrode 12 may be fabricated in a manner similar to the known fabrication methods for oriented positive electrode plates disclosed in Patent Documents 1 to 3, except that instead of forming the electrode into a plate, the electrode is extruded into a honeycomb shape. For example, a clay-like raw material containing plate-shaped particles of a lithium composite oxide such as LiCoO, a dispersion medium, and a binder is extruded through a honeycomb mold, whereby the plate-shaped particles are oriented in the extrusion direction regulated by the honeycomb mold, thereby producing a honeycomb molded body in which the plate-shaped particles are oriented in the x-, y-, and z-axis directions. The resulting honeycomb molded body is then degreased and fired according to a known method, thereby producing a honeycomb ceramic positive electrode 12 composed of an oriented sintered lithium composite oxide.

[0027] Lithium-ion secondary battery 6A and 6B show a lithium-ion secondary battery 10 equipped with a honeycomb-type ceramic positive electrode 12. For ease of explanation, these figures depict only a rectangular prismatic portion cut out from a cylindrical honeycomb structure. The secondary battery 10 includes the honeycomb-type ceramic positive electrode 12, multiple negative electrodes 14, a separator 16, an electrolyte (not shown), and a battery container (not shown). The multiple negative electrodes 14 are inserted into the multiple holes 12d, and the ends of the negative electrodes 14 extend from the first end surface 12a or the second end surface 12b. The separator 16 is interposed between the honeycomb-type ceramic positive electrode 12 and the negative electrode 14.

[0028] The secondary battery 10 may further include a positive electrode current collector foil and / or a negative electrode current collector foil (not shown). In this case, the positive electrode current collector foil is preferably provided on at least one of the first end face 12a, the second end face 12b, and the outer peripheral side face 12c (excluding the face from which the negative electrode 14 extends). On the other hand, the negative electrode current collector foil is preferably provided on the end of the negative electrode 14 extending from the first end face 12a or the second end face 12b.

[0029] The negative electrode 14 preferably contains carbon as a negative electrode active material. Examples of carbon include graphite, pyrolytic carbon, coke, baked resin, mesophase microspheres, and mesophase pitch, with graphite being preferred. The graphite may be either natural or artificial graphite. The negative electrode 14 preferably further contains a binder. Examples of binders include styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), and polytetrafluoroethylene (PTFE), with styrene-butadiene rubber (SBR) or polyvinylidene fluoride (PVDF) being preferred. The size of the negative electrode 14 is not particularly limited as long as it can be inserted into the holes 12d of the honeycomb ceramic positive electrode 12 with the separator 16 interposed therebetween.

[0030] The separator 16 is preferably a ceramic separator. For example, if the surface of the honeycomb ceramic positive electrode 12 (particularly the partition walls 12e) and / or the surface of the negative electrode 14 are coated with a ceramic separator, the lithium-ion secondary battery 10 can be efficiently fabricated simply by inserting the negative electrode 14 into the holes 12d of the honeycomb ceramic positive electrode 12. The ceramic separator 16 is a ceramic microporous film. The ceramic contained in the ceramic separator 16 is preferably at least one selected from MgO, Al2O3, ZrO2, SiC, Si3N4, AlN, and cordierite, more preferably at least one selected from MgO, Al2O3, and ZrO2. The thickness of the ceramic separator 16 is preferably 1 to 40 μm, more preferably 2 to 30 μm, and even more preferably 3 to 20 μm. Alternatively, the separator 16 may be a polymer microporous film. In this case, separator 16 is preferably made of polyolefin, polyimide, polyester (e.g., polyethylene terephthalate (PET)), or cellulose. Examples of polyolefin include polypropylene (PP), polyethylene (PE), and combinations thereof.

[0031] The electrolyte is not particularly limited, and a commercially available electrolyte for lithium batteries may be used, such as a solution in which a lithium salt (e.g., LiPF6) is dissolved in an organic solvent (e.g., a mixed solvent of ethylene carbonate (EC) and methyl ethyl carbonate (MEC), a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), or a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC)).

[0032] The lithium-ion secondary battery 10 including the honeycomb-type ceramic positive electrode 12 may be manufactured by any method. For example, a separator 16 may be formed in advance on the surface of the negative electrode 14, and the negative electrode 14 coated with the separator 16 may be inserted into the holes 12d of the honeycomb-type ceramic positive electrode 12. In this case, the separator 16 may be formed on the surface of the negative electrode 14 by applying a slurry containing ceramic powder (e.g., MgO powder), a binder, a dispersion medium, etc. (e.g., by dip coating) to the negative electrode 14 and drying it. Alternatively, the separator 16 may be formed in advance on the surface of the partition walls 12e of the honeycomb-type ceramic positive electrode 12, and a rod-shaped negative electrode 14 may be inserted into the holes 12d. In this case, instead of inserting the rod-shaped negative electrode 14, the negative electrode 14 may be formed by pouring a slurry containing a negative electrode active material (e.g., graphite slurry) into the holes 12d. At this time, it is preferable from the viewpoint of current collection to form an extended portion of the negative electrode 14 by pouring the slurry in excess so that it overflows from the holes 12d.

[0033] There is no particular limitation on the current collecting structure of the lithium-ion secondary battery 10. For example, the negative electrode 14 may be extended only from the first end face 12a (or the second end face 12b) of the honeycomb-type ceramic positive electrode 12, a negative electrode current collector (e.g., a current collecting foil) may be attached to the extended portion of the negative electrode 14, and a positive electrode current collector (e.g., a current collecting foil) may be attached to the other second end face 12b (or the first end face 12a) and / or the outer peripheral side face 12c of the honeycomb-type ceramic positive electrode 12. In this case, it is preferable that the secondary battery 10 is configured so that the negative electrode 14 does not reach the second end face 12b (or the first end face 12a) where the positive electrode current collector is to be attached. [Example]

[0034] The present invention will be further illustrated by the following examples.

[0035] Example 1 (1) Preparation of the positive electrode A honeycomb ceramic positive electrode was fabricated in the following manner.

[0036] (1a) Preparation of molding raw materials Co3O4 powder (manufactured by Seido Chemical Industry Co., Ltd.) and Li2CO3 powder (manufactured by Honjo Chemical Co., Ltd.) were mixed at a Li / Co molar ratio of 1.01 and then heated to 780 °C for 5 hours. The resulting powder was then crushed and pulverized in a pot mill to a volumetric standard D50 of 0.4 μm to obtain LiCoO2 raw powder. 100 parts by weight of this LiCoO2 raw powder was mixed with 30 parts by weight of a dispersion medium (toluene:isopropanol = 1:1), 10 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) to obtain a clay-like molding material.

[0037] (1b) Molding The obtained molding raw material was extruded to obtain a honeycomb molded body. The honeycomb shape was formed with a wall thickness of 100 μm and a pitch of 2.0 mm. The die area was approximately 20 × 20 mm. The obtained honeycomb formed body was cut into a length of 50 mm.

[0038] (1c) Firing The honeycomb molded body was heated to 600°C at a rate of 200°C / h and degreased for 3 hours, after which it was placed in an alumina sheath (manufactured by Nikkato Corporation). The temperature in the sealed sheath was raised to 920°C at a rate of 200°C / h and held there for 4 hours. The resulting honeycomb structure was densely sintered to obtain a honeycomb-type oriented ceramic positive electrode with a wall thickness of 100 μm and a pitch of 2.0 mm.

[0039] (2) Preparation of the negative electrode 100 parts by weight of artificial graphite (SCMG-CF manufactured by Showa Denko K.K.) and 10 parts by weight of PTFE (Polyflon D-1E manufactured by Daikin Industries, Ltd.) were mixed with 30 parts by weight of isopropanol (manufactured by Fujifilm Wako Pure Chemical Industries), and the mixture was extruded through a die measuring 1.9 × 1.9 mm to obtain a rectangular parallelepiped negative electrode. This was dried under reduced pressure (-95 kPa, 80 °C, 16 h) and cut to a length of 50 mm to produce a rectangular parallelepiped negative electrode.

[0040] (3) Preparation of separator Magnesium carbonate powder (manufactured by Konoshima Chemical Co., Ltd.) was heat-treated at 900°C for 5 hours to obtain MgO powder. 100 parts by weight of this powder was mixed with 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 coating was dip-coated onto the rectangular negative electrode, covering up to 49 mm of the 50 mm length. The electrode was then vacuum-dried (-95 kPa, 100°C, 2 hours) to form a separator film on the surface of the rectangular negative electrode.

[0041] (4) Preparation of conductive adhesive Acetylene black and polyimide amide were weighed out to 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.

[0042] (5) Battery construction A rectangular parallelepiped negative electrode with an MgO separator was inserted into each of the 2.0 mm-pitch holes formed in the honeycomb ceramic positive electrode. The insertion into the honeycomb ceramic positive electrode was limited to the 49 mm portion where the separator was formed. Next, a 10 μm-thick copper foil was attached to the end face of the negative electrode, 1 mm of which protruded from the honeycomb structure, using a conductive adhesive. A 15 μm-thick aluminum foil was attached to the end face of the honeycomb ceramic positive electrode where the negative electrode did not protrude, using a conductive adhesive. This structure was enclosed in a glass cell equipped with a current collector, and an electrolyte was added and sealed to form a battery. The electrolyte was prepared by dissolving LiPF6 at a concentration of 1.0 mol / L in an organic solvent mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 3:7. Furthermore, vinylene carbonate was added as an additive at a concentration of 2 parts by weight.

[0043] (6) Evaluation The honeycomb ceramic positive electrodes and batteries thus produced were evaluated as follows.

[0044] <Average orientation angle of primary particles> The average orientation angles of the primary particles oriented in the x-axis, y-axis, and z-axis directions constituting the honeycomb ceramic positive electrode were measured as follows.

[0045] (i) Average orientation angle for z-axis orientation The honeycomb-type ceramic positive electrode was polished in the direction of arrow A shown in Figure 1 using a cross-section polisher (CP) (JEOL Ltd., IB-15000CP). As shown in Figures 1 and 2, a cross section of the LiCoO2 layer parallel to the exposed xz plane (hereinafter referred to as "cross section C1") was subjected to EBSD measurement at a 1000x field of view (125 μm × 125 μm) to obtain an EBSD image. This EBSD measurement was performed using a Schottky field emission scanning electron microscope (JEOL Ltd., Model JSM-7800F). For all particles identified in the obtained EBSD image, the angle between the (003) plane of the primary particle and the z-axis (or yz-plane) (i.e., the tilt of the crystal orientation from (003)) was calculated as the tilt angle, and the average of these angles was defined as the average orientation angle of the primary particles relative to the z-axis in cross section C1.

[0046] (ii) Average orientation angle with respect to the orientation in the y-axis direction or the x-axis direction The honeycomb ceramic positive electrode was polished in the direction of arrow B shown in Figure 1 using a cross-section polisher (CP) (JEOL Ltd., IB-15000CP). As shown in Figures 1 and 2, a portion of the exposed cross section of the lattice-like LiCoO2 layer parallel to the xy plane, corresponding to the partition wall parallel to the y axis (hereinafter referred to as "cross section C2"), was measured by EBSD in the same manner as above with a 1000x field of view (125 μm × 125 μm) to obtain an EBSD image. For all particles identified in the obtained EBSD image, the angle between the (003) plane of the primary particle and the y axis (or yz plane) (i.e., the tilt of the crystal orientation from (003)) was calculated as the tilt angle, and the average of these angles was taken as the average orientation angle of the primary particles relative to the y axis in cross section C2. In addition, a portion of the cross section of the exposed lattice-like LiCoO2 layer parallel to the x-axis (hereinafter referred to as "cross section C3") corresponding to the partition walls parallel to the x-axis was measured with a 1000x field of view (125 μm × 125 μm) in the same manner as above to obtain an EBSD image. For all particles identified in the obtained EBSD image, the angle between the (003) plane of the primary particle and the x-axis (or xz plane) (i.e., the tilt of the crystal orientation from (003)) was calculated as the tilt angle, and the average of these angles was defined as the average orientation angle of the primary particles with respect to the x-axis in cross section C3.

[0047] <Discharge rate characteristics> The discharge capacity retention rate, as a discharge rate characteristic of the battery, was measured in the voltage range of 3.0 to 4.3 V by the following procedure. Specifically, the battery was charged at a constant current of 0.2 C until the battery voltage reached 4.3 V, followed by constant voltage charging until the current value reached a 0.02 C rate, and then discharged at a constant current of 0.2 C until the battery voltage reached 3.0 V, thereby measuring the 0.2 C discharge capacity. The discharged battery was charged in the same manner as above, and then discharged at a constant current of 0.5 C until the battery voltage reached 3.0 V. The discharged battery was charged again in the same manner as above, and then discharged at a constant current of 1.0 C until the battery voltage reached 3.0 V, thereby measuring the 1.0 C discharge capacity. The 1.0 C discharge capacity was divided by the 0.2 C discharge capacity and multiplied by 100 to calculate the discharge capacity retention rate (%). Rating A: Discharge capacity retention rate is 80% or more Rating B: Discharge capacity retention rate is 70% or more but less than 80% Rating C: Discharge capacity retention rate is less than 70%

[0048] Example 2 (comparison) A battery was produced and evaluated in the same manner as in Example 1, except that a non-oriented honeycomb ceramic positive electrode was produced using LiCoO2 raw material powder produced without pulverization or crushing in Example 1 (1a).

[0049] result The evaluation results obtained in Examples 1 and 2 were as follows:

[0050] [Table 1]

[0051] As shown in Table 1, the battery of Example 1, which used a honeycomb-type ceramic positive electrode oriented at an average orientation angle of 15°, exhibited a superior discharge capacity retention rate to the battery of Example 2 (Comparative Example), which used a non-oriented honeycomb-type ceramic positive electrode. This is thought to be because, in the honeycomb-type ceramic positive electrode used in Example 1, the LiCoO2 particles are oriented in the extrusion direction (z-axis direction) during extrusion molding, and are also oriented in the planar direction (xy-plane direction) perpendicular to the extrusion direction due to the honeycomb lattice structure. As a result, low resistance is realized in three directions: the x-axis direction, the y-axis direction, and the x-axis direction, which promotes lithium ion conduction and electron conduction.

Claims

1. A honeycomb ceramic positive electrode having a columnar honeycomb structure, the honeycomb ceramic positive electrode having a first end face, a second end face parallel to the first end face, and an outer peripheral side face perpendicular to the first end face and the second end face, and a plurality of holes extending from the first end face toward the second end face, the honeycomb-type ceramic positive electrode is made of a lithium composite oxide sintered body in which a plurality of primary particles made of a lithium composite oxide are bonded together, When the central axis of the columnar honeycomb structure parallel to the outer peripheral side surface or an axis parallel thereto is defined as the z-axis, the plurality of primary particles are oriented in the z-axis direction.

2. 2. The honeycomb-type ceramic positive electrode according to claim 1, wherein the primary particles are oriented at an average orientation angle of more than 0° and not more than 30° with respect to the z-axis.

3. 3. The honeycomb ceramic positive electrode according to claim 1, wherein the plurality of pores are partitioned by lattice-shaped partition walls formed of the sintered lithium composite oxide.

4. 4. The honeycomb ceramic positive electrode according to claim 3, wherein, when the columnar honeycomb structure is viewed in a plane in the z-axis direction, one direction of the lattice-like partition walls is assigned to the x-axis and the other direction is assigned to the y-axis, primary particles constituting the partition walls in the x-axis direction are oriented in the x-axis direction, and primary particles constituting the partition walls in the y-axis direction are oriented in the y-axis direction.

5. 5. The honeycomb-type ceramic positive electrode according to claim 4, wherein the primary particles are oriented at an average orientation angle of more than 0° and not more than 30° with respect to the x-axis or the y-axis for each partition wall.

6. 3. The honeycomb ceramic positive electrode according to claim 1, wherein the lithium composite oxide is lithium cobalt oxide.

7. The honeycomb ceramic positive electrode according to claim 1; a plurality of negative electrodes inserted into the plurality of holes and having ends extending from the first end surface or the second end surface; a separator interposed between the honeycomb ceramic positive electrode and the negative electrode; An electrolyte; a battery container that accommodates the honeycomb ceramic positive electrode, the negative electrode, the separator, and the electrolyte; A lithium-ion secondary battery comprising:

8. 8. The lithium ion secondary battery according to claim 7, wherein the negative electrode comprises carbon.

9. 8. The lithium ion secondary battery according to claim 7, wherein the separator is a ceramic separator.

10. The ceramic separator is made of MgO, Al ２ O ３ , ZrO ２ , SiC, Si ３ N ４ 10. The lithium ion secondary battery according to claim 9, comprising at least one selected from the group consisting of AlN and cordierite.

11. The lithium ion secondary battery according to claim 7, further comprising a positive electrode current collector foil on at least one surface of the first end surface, the second end surface, and the outer peripheral side surface (excluding the surface from which the negative electrode extends).

12. The lithium ion secondary battery according to claim 7 , further comprising a negative electrode current collector foil at an end portion of the negative electrode extending from the first end surface or the second end surface.

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