Patent Description:
Lithium secondary batteries are widely used in various devices that require charging. In many existing lithium secondary batteries, a powder-dispersed positive electrode (so-called coated electrode) produced by applying a positive electrode mixture containing a positive electrode active material, a conductive agent, a binder, and the like, followed by drying, is employed.

Such powder-dispersed positive electrodes generally contain a relatively large amount (e.g., about <NUM>% by weight) of components (binders and conductive agents) that do not contribute to the capacity of battery, resulting in a low packing density of the positive electrode active material, i.e., lithium complex oxide. Accordingly, the powder-dispersed positive electrode should be greatly improved from the viewpoint of the capacity and charge/discharge efficiency. Some attempts have been made to improve the capacity and charge/discharge efficiency by positive electrodes or layers of positive electrode active material composed of lithium complex oxide sintered plate. In this case, since the positive electrode or the layer of positive electrode active material contains no binder or conductive agent, high capacity and satisfactory charge/discharge efficiency can be expected due to a high packing density of lithium complex oxide. For example, Patent Literature <NUM> (<CIT>) discloses a positive electrode including a positive electrode current collector and a positive electrode active material layer connected to the positive electrode current collector with a conductive bonding layer therebetween. The positive electrode active material layer is composed of a lithium complex oxide sintered plate, and the sintered plate has a thickness of <NUM> or more, a porosity of <NUM> to <NUM>%, and an open pore rate of <NUM>% or more. Further, Patent Literature <NUM> (<CIT>) discloses use of an oriented sintered plate including a plurality of primary grains composed of lithium complex oxide such as lithium cobaltate (LiCoO<NUM>), the plurality of primary grains being oriented at an average orientation angle of over <NUM>° and <NUM>°or less to the plate face in the positive electrode plate, as a positive electrode of a lithium secondary battery including a solid electrolyte.

Further, Patent Literature <NUM> (<CIT>) discloses a non-aqueous electrolyte secondary battery composed of a positive electrode composed of Li<NUM>-XCoO<NUM> (<NUM> ≤ x < <NUM>) supplemented with zirconium, a negative electrode such as lithium, and a non-aqueous electrolytic solution. The surfaces of LiCoO<NUM> particles are covered by ZrO<NUM> or Li<NUM>ZrO<NUM> due to addition of zirconium to be stabilized. As a result, excellent cycle characteristics and excellent storage characteristics are considered to be exhibited without causing decomposition reaction of the electrolytic solution or crystal destruction even at a high potential.

Meanwhile, use of a titanium-containing sintered plate as a negative electrode has been also proposed. For example, Patent Literature <NUM> (<CIT>) discloses a lithium secondary battery using a lithium titanate (Li<NUM>Ti<NUM>O<NUM>) sintered body as a positive electrode or a negative electrode. However, this lithium secondary battery is an all-solid battery having a solid electrolyte layer between a positive electrode and a negative electrode and is not a secondary battery using a non-aqueous electrolytic solution.

In recent years, a small and thin lithium secondary battery with high capacity and high output has been desired. Therefore, it is conceivable to use a lithium complex oxide sintered plate as a positive electrode, expecting high capacity and good charge/discharge efficiency. From the same reason, it is also conceivable to use a titanium-containing sintered plate as a negative electrode. However, when a lithium secondary battery is actually produced using these sintered plates, that is, a ceramic positive electrode plate and a ceramic negative electrode plate, the capacity as expected could not be obtained. In this point, according to the findings of the inventors, a lithium secondary battery with high discharge capacity and excellent charge/discharge cycle performance can be provided by employing a configuration in which a positive electrode layer, a ceramic separator, and a negative electrode layer form one integrated sintered plate as a whole. However, since three layers of a positive electrode layer, a ceramic separator, and a negative electrode layer which have different compositions are bonded together in such a battery of the integrated sintered plate type, the bond strength between layers (between the positive electrode layer and the ceramic separator, and between the ceramic separator and the negative electrode layer) is weak, and the layers easily delaminate due to physical impact (for example, impact given by pinching with tweezers) during battery assembly. Further, when a battery of the integrated sintered plate type is stored in a charged state, metals (typically, cobalt) in the positive electrode active material easily diffuse, and therefore the deterioration in capacity caused by elution of such metals increases, as compared with conventional assembled batteries that are not of the integrated sintered plate type. Therefore, it is desired to suppress the delamination and the deterioration in capacity without impairing the excellent battery performance of batteries of the integrated sintered plate type.

The inventors have now found that, in a lithium secondary battery that is of the integrated sintered plate type in which a positive electrode layer, a ceramic separator, and a negative electrode layer are bonded together, coating the entirety of the integrated sintered plate with a metal oxide layer makes it possible to suppress delamination in the integrated sintered plate due to physical impact during battery assembly and deterioration in capacity due to storage in a charged state.

Accordingly, an object of the present invention is to provide a lithium secondary battery that is of the integrated sintered plate type in which a positive electrode layer, a ceramic separator, and a negative electrode layer are bonded together and that can suppress delamination in the integrated sintered plate due to physical impact during battery assembly and deterioration in capacity due to storage in a charged state.

According to an aspect of the present invention, there is provided a lithium secondary battery comprising:.

<FIG> schematically shows an example of the lithium secondary battery of the present invention. A lithium secondary battery <NUM> shown in <FIG> is in a form of coin-shaped battery, but the present invention is not limited to this and may be a battery in another form such as a thin secondary battery that can be incorporated in a card. The lithium secondary battery <NUM> includes a positive electrode layer <NUM>, a negative electrode layer <NUM>, a ceramic separator <NUM>, an electrolytic solution <NUM>, and an exterior body <NUM>. The positive electrode layer <NUM> is composed of a lithium complex oxide sintered body. The negative electrode layer <NUM> is composed of a titanium-containing sintered body. The ceramic separator <NUM> is interposed between the positive electrode layer <NUM> and the negative electrode layer <NUM>. The positive electrode layer <NUM>, the negative electrode layer <NUM>, and the ceramic separator <NUM> are impregnated with the electrolytic solution <NUM>. The exterior body <NUM> includes a closed space, and the closed space accommodates the positive electrode layer <NUM>, the negative electrode layer <NUM>, the ceramic separator <NUM>, and the electrolytic solution <NUM>. Further, the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> form one integrated sintered plate as a whole, whereby the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> are bonded together. Further, the entirety of the integrated sintered plate is coated with a metal oxide layer (not shown). The phrase "the entirety of the integrated sintered plate is coated with a metal oxide layer" herein means that the integrated sintered plate is coated with a metal oxide layer from the outer surface of the integrated sintered plate to the inner surface of the integrated sintered plate (that is, the surfaces of pores in the layers and the interfaces between the layers of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM>). In this way, in the lithium secondary battery <NUM> of the integrated sintered plate type in which the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> are bonded together, coating the entirety of the integrated sintered plate with a metal oxide layer makes it possible to suppress the delamination in the integrated sintered plate due to physical impact during battery assembly and the deterioration in capacity due to storage in a charged state.

That is, it is conceivable to use a lithium complex oxide sintered plate as a positive electrode, expecting high capacity and good charge/discharge efficiency, as described above. From the same reason, it is also conceivable to use a titanium-containing sintered plate as a negative electrode. However, when a lithium secondary battery is actually produced using these sintered plates, that is, a ceramic positive electrode plate and a ceramic negative electrode plate, the capacity as expected could not be obtained. One of the causes is probably that the positions of the ceramic positive electrode plate and the ceramic negative electrode plate are displaced from each other in the battery assembly process. It is also conceivable to accurately fix the separator to the positive electrode plate and the negative electrode plate so that such a displacement does not occur. However, such a process requires a high level of technology that requires extremely accurate positioning, which therefore causes a reduction in production efficiency and an increase in cost. In addition, the sintered plate that constitutes each electrode plate may be wavy or warped. The presence of such waviness or warpage causes variations in the distance between the positive and negative electrodes, and such variations lead to a reduction in charge/discharge cycle performance.

In contrast, in the lithium secondary battery of the present invention, the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> form one integrated sintered plate as a whole, whereby the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> are bonded together. That is, the three layers of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> are bonded together without resorting to other bonding methods such as adhesives. Here, to "form one integrated sintered plate as a whole" means that green sheets having a three-layer structure composed of a positive electrode green sheet providing the positive electrode layer <NUM>, a separator green sheet providing the ceramic separator <NUM>, and a negative electrode green sheet providing the negative electrode layer <NUM> are fired, so that each layer is sintered. Therefore, if the green sheets with a three-layer structure before firing are punched into a predetermined shape (such as a coin shape and a chip shape) using a punching die, the displacement between the positive electrode layer <NUM> and the negative electrode layer <NUM> in the integrated sintered plate in the final form is supposed not to exist at all. That is, the end face of the positive electrode layer <NUM> and the end face of the negative electrode layer <NUM> are aligned, so that the capacity can be maximized. Alternatively, even if such a displacement is present, the integrated sintered plate is suitable for processing such as laser processing, cutting, and polishing. Therefore, the end face may be finished to minimize or eliminate such a displacement. In any case, the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> are bonded together, as long as they form an integrated sintered plate, the displacement between the positive electrode layer <NUM> and the negative electrode layer <NUM> never occurs afterwards. The high discharge capacity, as expected (that is, close to the theoretical capacity), can be achieved by minimizing or eliminating the displacement between the positive electrode layer <NUM> and the negative electrode layer <NUM>. Further, it is considered that, since the integrated sintered plate has a three-layer structure including the ceramic separator, waviness or warpage is less likely to occur (that is, the flatness is excellent), and therefore variations in the distance between the positive and negative electrodes are less likely to occur (that is, the distance is uniform), as compared with a single positive electrode plate and a single negative electrode plate that are each produced as one sintered plate, thereby contributing to improving the charge/discharge cycle performance. For example, the area displacement ratio between the positive electrode layer <NUM> and the negative electrode layer <NUM> is preferably less than <NUM>%, more preferably less than <NUM>%, further preferably <NUM>%. The area displacement ratio between the positive electrode layer <NUM> and the negative electrode layer <NUM> is defined as a value (%) calculated based on the formula: [(Sp + Sn)/Spn] × <NUM>, wherein the area of the region where the positive electrode layer <NUM> and the negative electrode layer <NUM> overlap each other is referred to as Spn, the area of the region where the positive electrode layer <NUM> protrudes from the negative electrode layer <NUM> is referred to as Sp, and the area of the region where the negative electrode layer <NUM> protrudes from the positive electrode layer <NUM> is referred to as Sn. Further, the lithium secondary battery <NUM> preferably has a ratio of the discharge capacity to the theoretical capacity of <NUM> % or more, more preferably <NUM> % or more, further preferably <NUM>%.

The battery of the integrated sintered plate type thus has a significant advantage. In the battery of this type, however, the three layers of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> which have different compositions are bonded together, as described above, so that the bond strength between the layers is weak, and the layers easily delaminate due to physical impact (for example, impact given by pinching with tweezers) during battery assembly. Further, when a battery of the integrated sintered plate type is stored in a charged state, metal components (typically, cobalt) in the positive electrode active material easily diffuse, so that deterioration in capacity caused by elution of such metals increases. These problems are conveniently overcome by coating the entirety of the integrated sintered plate with a metal oxide layer. Although the mechanism thereof is uncertain, it is considered that, when the entirety of the integrated sintered plate is coated with a metal oxide from the outer surface to the inner surface, the necking between metal oxides increases the bond strength between the layers, and therefore the delamination of the integrated sintered plate due to physical impact during battery assembly is suppressed. Further, it is considered that, when the integrated sintered plate is coated with a metal oxide, the elution of metals (typically, cobalt) from the lithium complex oxide sintered body that is a positive electrode active material can be suppressed (even if metals are eluted, the reaction of the metals with the ceramic separator <NUM> or the negative electrode layer <NUM> is suppressed), and therefore the deterioration in capacity due to storage in a charged state is suppressed.

Meanwhile, with the spread of loT devices in recent years, a small and thin coin-shaped lithium secondary battery with high capacity and high output which can be charged particularly at a constant voltage (CV) has been desired. In this point, the lithium secondary battery <NUM> according to another preferable aspect of the present invention can sufficiently satisfy such requirements. In particular, employment of such respective predetermined sintered plates as a positive electrode and as a negative electrode enables not only heat resistance but also high capacity and high output, particularly, constant-voltage charging and high-speed charging to be achieved. Accordingly, the lithium secondary battery <NUM> of the present invention is preferably used as a battery for loT devices. That is, another preferable aspect of the present invention provides an loT device including a coin-shaped lithium secondary battery. Further, the lithium secondary battery <NUM> of the present invention is suitably used for applications such as smart keys, RFID tags, wearable devices, multifunction solar watches, memory backup power sources, automotive distributed power supplies, and the like, other than loT devices. In this description, the term "loT" is an abbreviation for Internet of Things, and the "loT device" means any device connected to the Internet to exhibit specific functions. However, the lithium secondary battery of the present invention is not limited to such a coin-shaped battery and may be a battery in another form, as described above. For example, the lithium secondary battery may be a thin secondary battery that can be incorporated in a card.

The positive electrode layer <NUM> is composed of a lithium complex oxide sintered body. The fact that the positive electrode layer <NUM> is composed of a sintered body means that the positive electrode layer <NUM> contains no binder or conductive agent. This is because, even if a binder is contained in a green sheet, the binder disappears or burns out during firing. Since the positive electrode layer <NUM> contains no binder, there is an advantage that deterioration of the positive electrode due to the electrolytic solution <NUM> can be avoided. The lithium complex oxide constituting the sintered body is particularly preferably lithium cobaltite (typically, LiCoO<NUM>, which may be hereinafter abbreviated as LCO). Various lithium complex oxide sintered plates or LCO sintered plates are known, and those disclosed in Patent Literature <NUM> (<CIT>) and Patent Literature <NUM> (<CIT>) can be referred to, for example.

According to a preferable aspect of the present invention, the positive electrode layer <NUM>, that is, the lithium complex oxide sintered plate is an oriented positive electrode layer including a plurality of primary grains composed of lithium complex oxide, the plurality of primary grains being oriented at an average orientation angle of over <NUM>° and <NUM>° or less to the layer face of the positive electrode layer. <FIG> shows an example of a SEM image in a cross section perpendicular to the layer face of the oriented positive electrode layer <NUM>, and <FIG> shows an electron backscatter diffraction (EBSD: Electron Backscatter Diffraction) image in a cross section perpendicular to the layer face of the oriented positive electrode layer <NUM>. Further, <FIG> shows an area-based histogram showing the distribution of orientation angles of primary grains <NUM> in the EBSD image shown in <FIG>. In the EBSD image shown in <FIG>, the discontinuity of crystal orientation can be observed. In <FIG>, the orientation angle of each primary grain <NUM> is indicated by the shading of color. A darker color indicates a smaller orientation angle. The orientation angle is a tilt angle formed by plane (<NUM>) of the primary grains <NUM> to the layer face direction. In <FIG> and <FIG>, the points shown in black within the oriented positive electrode layer <NUM> represent pores.

The oriented positive electrode layer <NUM> is an oriented sintered body composed of the plurality of primary grains <NUM> bound to each other. The primary grains <NUM> are each mainly in the form of a plate but may include rectangular, cubic, and spherical grains. The cross-sectional shape of each primary grain <NUM> is not particularly limited and may be a rectangular shape, a polygonal shape other than the rectangular shape, a circular shape, an elliptical shape, or a complex shape other than above.

The primary grains <NUM> are composed of a lithium complex oxide. The lithium complex oxide is an oxide represented by LixMO<NUM> (where <NUM> < x < <NUM> is satisfied, M represents at least one transition metal, and M typically contains one or more of Co, Ni, and Mn). The lithium complex oxide has a layered rock-salt structure. The layered rock-salt structure refers to a crystalline structure in which lithium layers and transition metal layers other than lithium are alternately stacked with oxygen layers interposed therebetween, that is, a crystalline structure in which transition metal ion layers and single lithium layers are alternately stacked with oxide ions therebetween (typically, an α-NaFeO<NUM> structure, i.e., a cubic rock-salt structure in which transition metal and lithium are regularly disposed in the [<NUM>] axis direction). Examples of the lithium complex oxide include LixCoO<NUM> (lithium cobaltate), LixNiO<NUM> (lithium nickelate), LixMnO<NUM> (lithium manganate), LixNiMnO<NUM> (lithium nickel manganate), LixNiCoO<NUM> (lithium nickel cobaltate), LixCoNiMnO<NUM> (lithium cobalt nickel manganate), and LixCoMnO<NUM> (lithium cobalt manganate), particularly preferably LixCoO<NUM> (lithium cobaltate, typically LiCoO<NUM>). The lithium complex oxide may contain one or more elements selected from F, 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. Further, the aforementioned composition may be uniform over the entire positive electrode layer <NUM> or may be unevenly distributed on the surface. The battery performance (e.g., high temperature durability and storage performance) is expected to be improved by containing such elements.

As shown in <FIG> and <FIG>, the average of the orientation angles of the primary grains <NUM>, that is, the average orientation angle is over <NUM>° and <NUM>° or less. This brings various advantages as follows. First, since each primary grain <NUM> lies in a direction inclined from the thickness direction, the adhesion between the primary grains can be improved. As a result, the lithium ion conductivity between a certain primary grain <NUM> and each of other primary grains <NUM> adjacent to the primary grain <NUM> on both sides in the longitudinal direction can be improved, so that the rate characteristic can be improved. Secondly, the rate characteristic can be further improved. This is because, when lithium ions move in and out, the oriented positive electrode layer <NUM> expands and contracts smoothly since the oriented positive electrode layer <NUM> expands and contracts more in the thickness direction than in the layer face direction, as described above, and thus the lithium ions also move in and out smoothly. Thirdly, since the expansion and contraction of the oriented positive electrode layer <NUM> following the inflow and outflow of lithium ions are predominant in the direction perpendicular to the layer face, stress is less likely to occur at the bonding interface between the oriented positive electrode layer <NUM> and the ceramic separator <NUM>, thereby making it easy to maintain good bonding at the interface.

The average orientation angle of the primary grains <NUM> is obtained by the following method. First, three horizontal lines that divide the oriented positive electrode layer <NUM> into four equal parts in the thickness direction and three vertical lines that divide the oriented positive electrode layer <NUM> into four equal parts in the layer face direction are drawn in an EBSD image of a rectangular region of <NUM> × <NUM> observed at a magnification of <NUM> times, as shown in <FIG>. Next, the average orientation angle of the primary grains <NUM> is obtained by arithmetically averaging the orientation angles of all the primary grains <NUM> intersecting at least one of the three horizontal lines and the three vertical lines. The average orientation angle of the primary grains <NUM> is preferably <NUM>° or less, more preferably <NUM>° or less, from the viewpoint of further improving the rate characteristics. From the viewpoint of further improving the rate characteristics, the average orientation angle of the primary grains <NUM> is preferably <NUM>° or more, more preferably <NUM>° or more.

As shown in <FIG>, the orientation angles of the primary grains <NUM> may be widely distributed from <NUM>° to <NUM>°, but most of them are preferably distributed in the region of over <NUM>° and <NUM>° or less. That is, when a cross section of the oriented sintered body constituting the oriented positive electrode layer <NUM> is analyzed by EBSD, the total area of the primary grains <NUM> with an orientation angle of over <NUM>° and <NUM>° or less to the layer face of the oriented positive electrode layer <NUM> (which will be hereinafter referred to as low-angle primary grains) out of the primary grains <NUM> contained in the cross section analyzed is preferably <NUM>% or more, more preferably <NUM>% or more, with respect to the total area of the primary grains <NUM> contained in the cross section (specifically, <NUM> primary grains <NUM> used for calculating the average orientation angle). Thereby, the proportion of the primary grains <NUM> with high mutual adhesion can be increased, so that the rate characteristic can be further improved. Further, the total area of grains with an orientation angle of <NUM>° or less among the low-angle primary grains is more preferably <NUM>% or more with respect to the total area of <NUM> primary grains <NUM> used for calculating the average orientation angle. Further, the total area of grains with an orientation angle of <NUM>° or less among the low-angle primary grains is more preferably <NUM>% or more with respect to the total area of <NUM> primary grains <NUM> used for calculating the average orientation angle.

Since the primary grains <NUM> are each mainly in the form of a plate, the cross section of each primary grain <NUM> extends in a predetermined direction, typically in a substantially rectangular shape, as shown in <FIG> and <FIG>. That is, when the cross section of the oriented sintered body is analyzed by EBSD, the total area of the primary grains <NUM> with an aspect ratio of <NUM> or more in the primary grains <NUM> contained in the cross section analyzed is preferably <NUM>% or more, more preferably <NUM>% or more, with respect to the total area of the primary grains <NUM> contained in the cross section (specifically, <NUM> primary grains <NUM> used for calculating the average orientation angle). Specifically, in the EBSD image as shown in <FIG>, the mutual adhesion between the primary grains <NUM> can be further improved by above, as a result of which the rate characteristic can be further improved. The aspect ratio of each primary grain <NUM> is a value obtained by dividing the maximum Feret diameter of the primary grain <NUM> by the minimum Feret diameter. The maximum Feret diameter is the maximum distance between two parallel straight lines that interpose the primary grain <NUM> therebetween on the EBSD image in observation of the cross section. The minimum Feret diameter is the minimum distance between two parallel straight lines that interpose the primary grain <NUM> therebetween on the EBSD image.

The mean diameter of the plurality of primary grains constituting the oriented sintered body is preferably <NUM> or more. Specifically, the mean diameter of the <NUM> primary grains <NUM> used for calculating the average orientation angle is preferably <NUM> or more, more preferably <NUM> or more, further preferably <NUM> or more. Thereby, since the number of grain boundaries between the primary grains <NUM> in the direction in which lithium ions conduct is reduced, and the lithium ion conductivity as a whole is improved, the rate characteristic can be further improved. The mean diameter of the primary grains <NUM> is a value obtained by arithmetically averaging the equivalent circle diameters of the primary grains <NUM>. An equivalent circle diameter is the diameter of a circle having the same area as each primary grain <NUM> on the EBSD image.

The positive electrode layer <NUM> preferably includes pores. The electrolytic solution can penetrate into the sintered body by the sintered body including pores, particularly open pores, when the sintered body is integrated into a battery as a positive electrode layer. As a result, the lithium ion conductivity can be improved. This is because there are two types of conduction of lithium ions within the sintered body: conduction through constituent grains of the sintered body; and conduction through the electrolytic solution within the pores, and the conduction through the electrolytic solution within the pores is overwhelmingly faster.

The positive electrode layer <NUM>, that is, the lithium complex oxide sintered body preferably has a porosity of <NUM> to <NUM>%, more preferably <NUM> to <NUM>%, further preferably <NUM> to <NUM>%, particularly preferably <NUM> to <NUM>%. The stress relief effect by the pores, the improvement in lithium ion conductivity by internal penetration of the electrolytic solution through the pores, and the increase in capacity can be expected, and the mutual adhesion between the primary grains <NUM> can be further improved, so that the rate characteristics can be further improved. The porosity of the sintered body is calculated by polishing a cross section of the positive electrode layer with CP (cross-section polisher) polishing, thereafter observing the cross section at a magnification of <NUM> times with SEM, and binarizing the SEM image obtained. The average equivalent circle diameter of pores formed inside the oriented sintered body is not particularly limited but is preferably <NUM> or less. The smaller the average equivalent circle diameter of the pores, the mutual adhesion between the primary grains <NUM> can be improved more. As a result, the rate characteristic can be improved more. The average equivalent circle diameter of the pores is a value obtained by arithmetically averaging the equivalent circle diameters of <NUM> pores on the EBSD image. An equivalent circle diameter is the diameter of a circle having the same area as each pore on the EBSD image. Each of the pores formed inside the oriented sintered body is preferably an open pore connected to the outside of the positive electrode layer <NUM>.

The positive electrode layer <NUM>, that is, the lithium complex oxide sintered body preferably has a mean pore diameter of <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>. Within such a range, stress concentration is suppressed from occurring locally in large pores, and the stress is easily released uniformly in the sintered body. Further, the improvement in lithium ion conductivity by internal penetration of the electrolytic solution through the pores can be achieved more effectively.

The positive electrode layer <NUM> preferably has a thickness of <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>. The thickness within such a range can improve the energy density of the lithium secondary battery <NUM> by increasing the capacity of the active material per unit area together with suppressing the deterioration of the battery characteristics (particularly, the increase of the resistance value) due to repeated charging/discharging.

The negative electrode layer <NUM> is composed of a titanium-containing sintered body. The titanium-containing sintered body preferably contains lithium titanate Li<NUM>Ti<NUM>O<NUM> (which will be hereinafter referred to as LTO) or niobium titanium complex oxide Nb<NUM>TiO<NUM>, more preferably LTO. LTO is typically known to have a spinel structure but can have other structures during charging and discharging. For example, the reaction of LTO proceeds in the two-phase coexistence of Li<NUM>Ti<NUM>O<NUM> (spinel structure) and Li<NUM>Ti<NUM>O<NUM> (rock salt structure) during charging and discharging. Accordingly, the structure of LTO is not limited to the spinel structure. LTO may be partially replaced with other elements, and examples of the elements include Nb, Ta, W, Al, and Mg.

The fact that the negative electrode layer <NUM> is composed of a sintered body means that the negative electrode layer <NUM> contains no binder or conductive agent. This is because, even if a binder is contained in a green sheet, the binder disappears or burns out during firing. Since the negative electrode layer contains no binder, high capacity and good charge/discharge efficiency can be achieved by high packing density of the negative electrode active material (for example, LTO or Nb<NUM>TiO<NUM>). The LTO sintered body can be produced according to the method described in Patent Literature <NUM> (<CIT>).

The negative electrode layer <NUM>, that is, the titanium-containing sintered body has a structure that a plurality (namely, a large number) of primary grains are bonded. Accordingly, these primary grains are preferably composed of LTO or Nb<NUM>TiO<NUM>.

The negative electrode layer <NUM> preferably has a thickness of <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>, particularly preferably <NUM> to <NUM>. The larger the thickness of the negative electrode layer <NUM>, a battery with high capacity and high energy density is achieved more easily. The thickness of the negative electrode layer <NUM> is determined by measuring the distance between the two substantially parallel faces of the layer, for example, when the cross section of the negative electrode layer <NUM> is observed by SEM (scanning electron microscopy).

The primary grain size that is the average grain size of the plurality of primary grains forming the negative electrode layer <NUM> is preferably <NUM> or less, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>. Within such a range, the lithium ion conductivity and the electron conductivity are easily compatible with each other, which contributes to improving the rate performance.

The negative electrode layer <NUM> preferably includes pores. The electrolytic solution can penetrate into the sintered body by the sintered body including pores, particularly open pores, when the sintered body is integrated into a battery as a negative electrode layer. As a result, the lithium ion conductivity can be improved. This is because there are two types of conduction of lithium ions within the sintered body: conduction through constituent grains of the sintered body; and conduction through the electrolytic solution within the pores, and the conduction through the electrolytic solution within the pores is overwhelmingly faster.

The negative electrode layer <NUM> preferably has a porosity of <NUM> to <NUM>%, more preferably <NUM> to <NUM>%, further preferably <NUM> to <NUM>%. Within such a range, the lithium ion conductivity and the electron conductivity are easily compatible with each other, which contributes to improving the rate performance.

The negative electrode layer <NUM> preferably has a mean pore diameter of <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>. Within such a range, the lithium ion conductivity and the electron conductivity are easily compatible with each other, which contributes to improving the rate performance.

The ceramic separator <NUM> is a microporous film made of ceramics. The ceramic separator <NUM> is advantageous in that it, of course, has excellent heat resistance and can be produced as one integrated sintered plate together with the positive electrode layer <NUM> and the negative electrode layer <NUM> as a whole. The ceramic contained in the ceramic separator <NUM> is at least one selected from MgO, Al<NUM>O<NUM>, ZrO<NUM>, SiC, Si<NUM>N<NUM>, AlN, and cordierite, more preferably at least one selected from MgO, Al<NUM>O<NUM>, and ZrO<NUM>, further preferably MgO. Use of MgO gives an advantage that the compositional change of the positive electrode layer <NUM>, the negative electrode layer <NUM>, and the ceramic separator <NUM> due to diffusion of the components of the positive electrode layer <NUM> and the negative electrode layer <NUM> into the ceramic separator <NUM> can be suppressed. The ceramic separator <NUM> preferably has a thickness of <NUM> to <NUM>, more preferably <NUM> to <NUM>, further preferably <NUM> to <NUM>, particularly preferably <NUM> to <NUM>. The ceramic separator <NUM> preferably has a porosity of <NUM> to <NUM>%, more preferably <NUM> to <NUM>%.

The ceramic separator <NUM> may contain a glass component for improving the adhesion to the positive electrode layer <NUM> and the negative electrode layer <NUM>. In this case, the content ratio of the glass component in the ceramic separator <NUM> is preferably <NUM> to <NUM> wt%, more preferably <NUM> to <NUM> wt%, further preferably <NUM> to <NUM> wt%, with respect to the total weight of the ceramic separator <NUM>. The glass contains SiO<NUM> in an amount of preferably <NUM> wt% or more, more preferably <NUM> to <NUM> wt%, further preferably <NUM> to <NUM> wt%, particularly preferably <NUM> to <NUM> wt%. The addition of the glass component to the ceramic separator <NUM> is preferably performed by adding glass frit to the raw material powder of the ceramic separator. The glass frit preferably contains at least one or more of Al<NUM>O<NUM>, B<NUM>O<NUM>, and BaO, as a component other than SiO<NUM>. However, if the desired adhesion of the ceramic separator <NUM> to the positive electrode layer <NUM> and the negative electrode layer <NUM> can be ensured, the glass component is not particularly required to be contained in the ceramic separator <NUM>.

As described above, the entirety of the integrated sintered plate composed of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> is coated with a metal oxide layer. That is, the integrated sintered plate is coated with a metal oxide layer from the outer surface of the integrated sintered plate to the inner surface of the integrated sintered plate (that is, the surfaces of pores in the layers and the interfaces between the layers of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM>). The metal oxide layer is not specifically limited, as long as it can suppress the delamination and the deterioration in capacity without impairing the battery performance. The metal oxide layer can be composed of various metal oxides, and such metal oxides may further form complex oxides with lithium contained in the integrated sintered plate. Preferably, the metal oxide layer is composed of an oxide of at least one selected from the group consisting of Zr, Mg, Al, Nb, and Ti and/or a complex oxide of Li and at least one selected from the group consisting of Zr, Mg, Al, Nb, and Ti, more preferably an oxide of Zr and/or a complex oxide of Zr and Li. Further, the content of metals (excluding Li) in the metal oxide layer is preferably <NUM> wt% or less, more preferably <NUM> to <NUM> wt%, further preferably <NUM> to <NUM> wt%, with respect to <NUM> wt% of the integrated sintered plate coated with the metal oxide layer.

The electrolytic solution <NUM> is not specifically limited, and commercially available electrolytic solutions for lithium batteries such as a solution obtained by dissolving a lithium salt (e.g., LiPF<NUM>) in a non-aqueous solvent such as an organic solvent (e.g., a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC), or a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC)) may be used.

In the case of forming a lithium secondary battery having excellent heat resistance, the electrolytic solution <NUM> preferably contains lithium borofluoride (LiBF<NUM>) in a non-aqueous solvent. In this case, the non-aqueous solvent is preferably at least one selected from the group consisting of γ-butyrolactone (GBL), ethylene carbonate (EC) and propylene carbonate (PC), more preferably a mixed solvent composed of EC and GBL, a single solvent composed of PC, a mixed solvent composed of PC and GBL, or a single solvent composed of GBL, particularly preferably a mixed solvent composed of EC and GBL or a single solvent composed of GBL. The non-aqueous solvent has an increased boiling point by containing γ-butyrolactone (GBL), which considerably improves the heat resistance. From such a viewpoint, the volume ratio of EC:GBL in the EC and/or GBL containing non-aqueous solvent is preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio: <NUM> to <NUM>% by volume), more preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio: <NUM> to <NUM>% by volume), further preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio: <NUM> to <NUM>% by volume), particularly preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio: <NUM> to <NUM>% by volume). The lithium borofluoride (LiBF<NUM>) to be dissolved in the non-aqueous solvent is an electrolyte having a high decomposition temperature, which also considerably improves the heat resistance. The LiBF<NUM> concentration in the electrolytic solution <NUM> is preferably <NUM> to <NUM> mol/L, more preferably <NUM> to <NUM> mol/L, further preferably <NUM> to <NUM> mol/L, particularly preferably <NUM> to <NUM> mol/L.

The electrolytic solution <NUM> may further contain at least one selected from vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl ethylene carbonate (VEC), and lithium difluoro(oxalato)borate (LiDFOB), as an additive. Both VC and FEC have excellent heat resistance. Accordingly, a SEI film having excellent heat resistance can be formed on the surface of the negative electrode layer <NUM> by the electrolytic solution <NUM> containing such additives.

Further, a solid electrolyte or a polymer electrolyte may be used instead of the electrolytic solution <NUM> (in other words, a solid electrolyte and a polymer electrolyte can be used as an electrolyte, other than the electrolytic solution <NUM>). In such a case, at least the inside of the pores of the separator <NUM> is preferably impregnated with the electrolyte, as in the case of the electrolytic solution <NUM>. The impregnation method is not specifically limited, but examples thereof include a method of melting the electrolyte and infiltrating it into the pores of the separator <NUM> and a method of pressing the green compact of the electrolyte against the separator <NUM>.

The exterior body <NUM> includes a closed space, and the closed space accommodates the positive electrode layer <NUM>, the negative electrode layer <NUM>, the ceramic separator <NUM>, and the electrolytic solution <NUM>. The exterior body <NUM> may be appropriately selected corresponding to the type of the lithium secondary battery <NUM>. For example, in the case where the lithium secondary battery is in a form of coin-shaped battery as shown in <FIG>, the exterior body <NUM> typically includes the positive electrode can 24a, the negative electrode can 24b, and the gasket 24c, and the positive electrode can 24a and the negative electrode can 24b are crimped via the gasket 24c to form the closed space. The positive electrode can 24a and the negative electrode can 24b can be made of metals such as stainless steel and are not specifically limited. The gasket 24c can be an annular member made of an insulating resin such as polypropylene, polytetrafluoroethylene, and PFA resin and is not particularly limited.

Further, in the case where the lithium secondary battery is in a form of chip battery that can be incorporated in a card, it is preferable that the exterior body is a resin substrate, and the battery elements (that is, the positive electrode layer <NUM>, the negative electrode layer <NUM>, the ceramic separator <NUM>, and the electrolytic solution <NUM>) are embedded in the resin substrate. For example, the battery elements may be sandwiched by a pair of resin films, and it is preferable that the resin films are bonded together by an adhesive, or the resin films are heat-sealed together by hot pressing.

The lithium secondary battery <NUM> preferably further includes a positive electrode current collector <NUM> and/or a negative electrode current collector <NUM>. The positive electrode current collector <NUM> and the negative electrode current collector <NUM> are not specifically limited but are preferably metal foils such as copper foils and aluminum foils. The positive electrode current collector <NUM> is preferably interposed between the positive electrode layer <NUM> and the exterior body <NUM> (e.g., the positive electrode can 24a), and the negative electrode current collector <NUM> is preferably interposed between the negative electrode layer <NUM> and the exterior body <NUM> (e.g., the negative electrode can 24b). Further, a positive side carbon layer <NUM> is preferably provided between the positive electrode layer <NUM> and the positive electrode current collector <NUM> for reducing the contact resistance. Likewise, a negative side carbon layer <NUM> is preferably provided between the negative electrode layer <NUM> and the negative electrode current collector <NUM> for reducing the contact resistance. Both the positive side carbon layer <NUM> and the negative side carbon layer <NUM> are preferably composed of a conductive carbon and may be formed, for example, by applying a conductive carbon paste by screen printing or the like.

The integrated sintered plate with a three-layer structure of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> may be produced by any method but is preferably produced by (<NUM>) preparation green sheets corresponding to the respective three layers and (<NUM>) laminating the green sheets, followed by pressure bonding and firing.

A lithium complex oxide-containing green sheet as a positive electrode green sheet can be prepared, as follows. A raw material powder composed of lithium complex oxide is prepared. The powder preferably comprises pre-synthesized platy particles (e.g., LiCoO<NUM> platy particles) having a composition of LiMO<NUM> (M as described above). The volume-based D50 particle diameter of the raw material powder is preferably <NUM> to <NUM>. For example, the LiCoO<NUM> platy particles can be produced as follows. Co<NUM>O<NUM> powder and Li<NUM>CO<NUM> powder as raw materials are mixed and fired (<NUM> to <NUM>, <NUM> to <NUM> hours) to synthesize LiCoO<NUM> powder. The resultant LiCoO<NUM> powder is milled into a volume-based D50 particle diameter of <NUM> to <NUM> with a pot mill to yield platy LiCoO<NUM> particles capable of conducting lithium ions along the faces of the plate. Such LiCoO<NUM> particles are also produced by a procedure involving grain growth in a green sheet from LiCoO<NUM> powder slurry and crushing the green sheet, or a procedure involving synthesis of platy crystals, such as a flux process, a hydrothermal synthesis process, a single crystal growth process using a melt, and a sol gel process. The resultant LiCoO<NUM> particles are readily cleaved along cleavage planes. The LiCoO<NUM> particles may be cleaved by crushing to produce LiCoO<NUM> platy particles.

The platy particles may be independently used as raw material powder, or a mixed powder of the platy powder and another raw material powder (for example, Co<NUM>O<NUM> particles) may be used as a raw material powder. In the latter case, it is preferred that the platy powder serves as template particles for providing orientation, and another raw material powder (e.g., Co<NUM>O<NUM> particles) serves as matrix particles that can grow along the template particle. In this case, the raw powder is preferably composed of a mixed powder in a ratio of template particles to matrix particles of <NUM>:<NUM> to <NUM>:<NUM>. When the Co<NUM>O<NUM> raw material powder is used as the matrix particles, the volume-based D50 particle diameter of the Co<NUM>O<NUM> raw material powder may be any value, for example, <NUM> to <NUM>, and is preferably smaller than the volume-based D50 particle diameter of LiCoO<NUM> template particles. The matrix particles may also be produced by heating a Co(OH)<NUM> raw material at <NUM> to <NUM> for <NUM> to <NUM> hours. In addition to Co<NUM>O<NUM>, Co(OH)<NUM> particles may be used, or LiCoO<NUM> particles may be used as the matrix particles.

When the raw material powder is composed of <NUM>% of LiCoO<NUM> template particles, or when LiCoO<NUM> particles are used as matrix particles, a large (e.g., <NUM> × <NUM> square) flat LiCoO<NUM> sintered layer can be yielded by firing. Although the mechanism is not clear, since synthesis of LiCoO<NUM> does not proceed in a firing process, a change in volume or local unevenness of the shape probably does not occur.

The raw material powder is mixed with a dispersive medium and any additive (e.g., binder, plasticizer, and dispersant) to form a slurry. A lithium compound (e.g., lithium carbonate) in an excess amount of about <NUM> to <NUM> mol% other than LiMO<NUM> may be added to the slurry to promote grain growth and compensate for a volatile component in a firing process described later. The slurry preferably contains no pore-forming agent. The slurry is defoamed by stirring under reduced pressure, and the viscosity is preferably adjusted into <NUM> to <NUM> cP. The resultant slurry is formed into a sheet to give a green sheet containing lithium complex oxide. The sheet is preferably formed by a forming procedure capable of applying a shear force to platy particles (for example, template particles) in the raw material powder. Through this process, the primary grains can have a mean tilt angle of over <NUM>° and <NUM>° or less to the sheet face. The forming procedure capable of applying a shear force to platy particles suitably includes a doctor blade process. The thickness of the green sheet containing the lithium complex oxide may be appropriately selected so as to give the above desired thickness after firing. Further, one piece of green sheet may be formed into the desired thickness, or a plurality of pieces of green sheets may be laminated to the desired thickness. In particular, in the case where the thickness is over <NUM>, it is preferable to laminate a plurality of pieces of green sheets in view of the construction method.

A titanium-containing green sheet as a negative electrode green sheet may be produced by any method. For example, a LTO-containing green sheet can be prepared, as follows. First, raw material powder (LTO powder) composed of lithium titanate Li<NUM>Ti<NUM>O<NUM> is prepared. Commercially available or newly synthesized LTO powder may be used as the raw material powder. For example, powder obtained by hydrolyzing a mixture of titanium tetraisopropoxy alcohol and isopropoxy lithium may be used, or a mixture containing lithium carbonate, titania, or the like may be fired. The raw material powder preferably has a volume-based D50 particle size of <NUM> to <NUM>, more preferably <NUM> to <NUM>. A larger particle size of the raw material powder tends to increase the size of the pores. Further, in the case where the particle size of the raw material is large, milling (such as pot milling, bead milling, and jet milling) may be performed to a desired particle size. The raw material powder is mixed with a dispersive medium and any additive (e.g., binder, plasticizer, and dispersant) to form a slurry. A lithium compound (e.g., lithium carbonate) in an excess amount of about <NUM> to <NUM> mol% other than LiMO<NUM> may be added to the slurry to promote grain growth and compensate for a volatile component in a firing process described later. The slurry preferably contains no pore-forming agent. The slurry is defoamed by stirring under reduced pressure, and the viscosity is preferably adjusted into <NUM> to <NUM> cP. The resultant slurry is formed into a LTO-containing green sheet. The sheet can be formed by any known process and is preferably formed by a doctor blade process. The thickness of the LTO-containing green sheet may be appropriately selected so as to give the above desired thickness after firing.

A separator green sheet can be prepared, as follows. First, at least one ceramic powder selected from MgO, Al<NUM>O<NUM>, ZrO<NUM>, SiC, Si<NUM>N<NUM>, AIN, and cordierite is prepared. Glass frit may be added to the ceramic powder. The raw material powder preferably has a volume-based D50 particle size of <NUM> to <NUM>, more preferably <NUM> to <NUM>. A larger particle size of the raw material powder tends to increase the size of the pores. Further, in the case where the particle size of the raw material is large, milling (such as pot milling, bead milling, and jet milling) may be performed to a desired particle size. The raw material powder is mixed with a dispersive medium and any additive (e.g., binder, plasticizer, and dispersant) to form a slurry. The slurry is defoamed by stirring under reduced pressure, and the viscosity is preferably adjusted into <NUM> to <NUM> cP. The resultant slurry is formed into a separator green sheet. The sheet can be formed by any known process and is preferably formed by a doctor blade process. The thickness of the separator green sheet may be appropriately selected so as to give the above desired thickness after firing.

Then, the positive electrode green sheet, the separator green sheet, and the negative electrode green sheet are sequentially stacked, and the laminate obtained is pressed so that the green sheets are pressure-bonded together. The pressing may be performed by a known method and is not specifically limited but is preferably performed by CIP (cold isostatic pressing). The pressing pressure is preferably <NUM> to <NUM> kgf/cm<NUM>, more preferably <NUM> to <NUM> kgf/cm<NUM>. The green sheet laminate thus pressure-bonded is preferably punched into a desired shape (such as a coin shape and a chip shape) or size using a punching die. Thereby, the displacement between the positive electrode layer <NUM> and the negative electrode layer <NUM> can be eliminated in the integrated sintered plate in the final form. As a result, the end face of the positive electrode layer <NUM> and the end face of the negative electrode layer <NUM> are aligned, so that the battery capacity can be maximized.

The green sheet laminate obtained is placed on a setter. The setter is made of ceramics, preferably zirconia or magnesia. The setter is preferably embossed. The green sheet disposed on the setter is put into a sheath. The sheath is made of ceramics, preferably alumina. Then, the green sheet in this state is degreased, as needed, and fired to obtain an integrated sintered plate. The degreasing is preferably performed at <NUM> to <NUM> for <NUM> to <NUM> hours. Further, the firing is preferably performed at <NUM> to <NUM> for <NUM> to <NUM> hours, more preferably at <NUM> to <NUM> for <NUM> to <NUM> hours. The heating rate during firing is preferably <NUM> to <NUM>/h, more preferably <NUM> to <NUM>/h. In particular, this heating rate is preferably employed in a temperature rising process from <NUM> to <NUM>, more preferably from <NUM> to <NUM>. Thus, an integrated sintered plate having a three-layer structure of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> is obtained. In the case where the punching process is not performed at the stage of the green sheet laminate, a displacement between the positive electrode layer <NUM> and the negative electrode layer <NUM> can occur in the integrated sintered plate in the final form. In this case, the end face of the integrated sintered plate is preferably finished by a technique such as laser processing, cutting, and polishing, to minimize or eliminate the displacement. As a result, the end face of the positive electrode layer <NUM> and the end face of the negative electrode layer <NUM> are aligned, so that the battery capacity can be maximized.

The entirety of the integrated sintered plate having the three-layer structure of the positive electrode layer <NUM>, the ceramic separator <NUM>, and the negative electrode layer <NUM> used in the lithium secondary battery <NUM> of the present invention is coated with a metal oxide layer. The integrated sintered plate may be coated with the metal oxide layer by any method, but the metal oxide layer is preferably formed, for example, by i) preparing a coating solution containing a metal compound, ii) immersing the integrated sintered plate in the coating solution to allow the coating solution penetrate therein, iii) taking out the integrated sintered body for drying, and iv) heating the integrated sintered body with the metal compound attached to convert the metal compound into a metal oxide. The coating solution prepared in Procedure i) above is not specifically limited as long as it is a solution containing a metal compound capable of forming a metal oxide layer by heating in a solvent (preferably, an organic solvent), but the metal compound is preferably at least one metal compound selected from the group consisting of Zr, Mg, Al, Nb, and Ti, more preferably metal alkoxides. Preferable examples of such a metal compound include metal alkoxides such as zirconium tetra-n-butoxide, magnesium diethoxide, triisopropoxyaluminum, niobium pentaethoxide, and titanium tetraisopropoxide, ammonium carbonate zirconium, tetrakis(ethylmethylamino) zirconium, (tertiary butyl imide) tris(ethylmethylamide) niobium, and tetrakis(dimethylamino) titanium. In Procedure ii) above, the integrated sintered plate immersed in the coating solution is preferably placed under a vacuum or reduced pressure atmosphere, since the coating solution can penetrate into the integrated sintered plate sufficiently and efficiently. In Procedure iii) above, drying may be performed at room temperature or under heating. In Procedure iv) above, the heating is preferably performed at <NUM> to <NUM> for <NUM> to <NUM> hours, more preferably at <NUM> to <NUM> for <NUM> to <NUM> hours. Thus, the integrated sintered plate entirely coated with a metal oxide layer is obtained.

The invention will be illustrated in more detail by the following examples. In the following examples, LiCoO<NUM> will be abbreviated as "LCO", and Li<NUM>Ti<NUM>O<NUM> will be abbreviated as "LTO".

First, Co<NUM>O<NUM> powder (manufactured by SEIDO CHEMICAL INDUSTRY CO. ) and Li<NUM>CO<NUM> powder (manufactured by THE HONJO CHEMICAL CORPORATION) weighed to a molar ratio Li/Co of <NUM> were mixed, and thereafter the mixture was kept at <NUM> for <NUM> hours. The resultant powder was milled into a volume-based D50 of <NUM> with a pot mill to yield powder composed of platy LCO particles. The resultant LCO powder (<NUM> parts by weight), a dispersive medium (toluene:isopropanol = <NUM>:<NUM>) (<NUM> parts by weight), a binder (polyvinyl butyral: Product No. BM-<NUM>, manufactured by SEKISUI CHEMICAL CO. ) (<NUM> parts by weight), a plasticizer (di-<NUM>-ethylhexyl phthalate (DOP), manufactured by Kurogane Kasei Co. ) (<NUM> parts by weight), and a dispersant (product name: RHEODOL SP-O30, manufactured by Kao Corporation) (<NUM> parts by weight) were mixed. The resultant mixture was defoamed by stirring under reduced pressure to prepare a LTO slurry with a viscosity of <NUM> cP. The viscosity was measured with an LVT viscometer manufactured by Brookfield. The slurry prepared was formed into a LCO green sheet onto a PET film by a doctor blade process. The thickness of the LCO green sheet was adjusted to <NUM> after firing.

First, LTO powder (volume-based D50 particle size: <NUM>, manufactured by Sigma-Aldrich <CIT> parts by weight), a dispersive medium (toluene:isopropanol = <NUM>:<NUM>) (<NUM> parts by weight), a binder (polyvinyl butyral: Product No. BM-<NUM>, manufactured by SEKISUI CHEMICAL CO. ) (<NUM> parts by weight), a plasticizer (di-<NUM>-ethylhexyl phthalate (DOP), manufactured by Kurogane Kasei Co. ) (<NUM> parts by weight), and a dispersant (product name: RHEODOL SP-O30, manufactured by Kao Corporation) (<NUM> parts by weight) were mixed. The resultant negative electrode raw material mixture was defoamed by stirring under reduced pressure to prepare a LTO slurry with a viscosity of <NUM> cP. The viscosity was measured with an LVT viscometer manufactured by Brookfield. The slurry prepared was formed into a LTO green sheet onto a PET film by a doctor blade process. The thickness of the LTO green sheet was adjusted to <NUM> after firing.

Magnesium carbonate powder (manufactured by Konoshima Chemical Co. ) was heated at <NUM> for <NUM> hours to obtain MgO powder. The resultant MgO powder and glass frit (CK0199, manufactured by Nippon Frit Co. ) were mixed at a weight ratio of <NUM>:<NUM>. The resultant mixed powder (volume-based D50 particle size: <NUM>) (<NUM> parts by weight), a dispersive medium (toluene:isopropanol = <NUM>:<NUM>) (<NUM> parts by weight), a binder (polyvinyl butyral: Product No. BM-<NUM>, manufactured by SEKISUI CHEMICAL CO. ) (<NUM> parts by weight), a plasticizer (di-<NUM>-ethylhexyl phthalate (DOP), manufactured by Kurogane Kasei Co. ) (<NUM> parts by weight), and a dispersant (product name: RHEODOL SP-O30, manufactured by Kao Corporation) (<NUM> parts by weight) were mixed. The resultant raw material mixture was defoamed by stirring under reduced pressure to prepare a slurry with a viscosity of <NUM> cP. The viscosity was measured with an LVT viscometer manufactured by Brookfield. The slurry prepared was formed into a separator green sheet onto a PET film by a doctor blade process. The thickness of the separator green sheet was adjusted to <NUM> after firing.

The LCO green sheet (positive electrode green sheet), the MgO green sheet (separator green sheet), and the LTO green sheet (negative electrode green sheet) were sequentially stacked, and the resultant laminate was pressed by CIP (cold isostatic pressing) at <NUM> kgf/cm<NUM> so that the green sheets were pressure-bonded together. The laminate thus pressure-bonded was punched into a circular plate with a diameter of <NUM> using a punching die. The resultant laminate in a form of circular plate was degreased at <NUM> for <NUM> hours, then heated to <NUM> at <NUM>/h, and kept for <NUM> minutes to fire, followed by cooling. Thus, one integrated sintered plate (integrated electrode) including three layers of the positive electrode layer (LCO sintered layer) <NUM>, the ceramic separator (MgO separator) <NUM>, and the negative electrode layer (LTO sintered layer) <NUM> was obtained.

First, <NUM> of <NUM>-ethoxyethanol, <NUM> of acetyl acetone, and <NUM> of zirconium tetra-n-butoxide were put into a container, followed by stirring, to give a coating solution. The solution was put into a container, and the integrated sintered plate obtained in Procedure (<NUM>) above was immersed therein. The container was put into a desiccator, followed by vacuuming to -<NUM> kPa and standing for <NUM> minutes. Thereafter, the inside of the desiccator was returned to the atmosphere, and the container containing the integrated sintered plate was taken out. The integrated sintered plate was taken out onto a nonwoven fabric wiper with tweezers, and the coating solution was lightly wiped off, followed by drying at room temperature for <NUM> hours. The integrated sintered plate after drying was placed on an alumina setter and heated in a medium-sized super cantal furnace (available from KYOWA KONETSU KOGYO CO. ) at <NUM> for <NUM> hours. Thus, an integrated sintered plate (integrated electrode) entirely coated with a metal oxide layer (a layer composed of an oxide of Zr or a complex oxide of Zr and Li) was obtained.

The coin-shaped lithium secondary battery <NUM> as schematically shown in <FIG> was produced as follows.

Acetylene black and polyimide amide were weighed to a mass ratio of <NUM>:<NUM> and mixed with an appropriate amount of NMP (N-methyl-<NUM>-pyrrolidone) as a solvent, to prepare a conductive carbon paste as a conductive adhesive. The conductive carbon paste was screen-printed on an aluminum foil as a negative electrode current collector. The integrated sintered body produced in (<NUM>) above was disposed so that the negative electrode layer <NUM> was located within an undried printing pattern (that is, a region coated with the conductive carbon paste), followed by vacuum drying at <NUM> for <NUM> minutes, to produce a structure with the negative electrode layer <NUM> and the negative electrode current collector <NUM> bonded via the negative side carbon layer <NUM>. The negative side carbon layer <NUM> had a thickness of <NUM>.

Acetylene black and polyimide amide were weighed to a mass ratio of <NUM>:<NUM> and mixed with an appropriate amount of NMP (N-methyl-<NUM>-pyrrolidone) as a solvent, to prepare a conductive carbon paste. The conductive carbon paste was screen-printed on an aluminum foil as a positive electrode current collector <NUM>, followed by vacuum drying at <NUM> for <NUM> minutes, to produce a positive electrode current collector <NUM> with a positive side carbon layer <NUM> formed on a surface. The positive side carbon layer <NUM> had a thickness of <NUM>.

The positive electrode current collector <NUM>, the positive side carbon layer <NUM>, the integrated sintered plate (the LCO positive electrode layer <NUM>, the MgO separator <NUM>, and the LTO negative electrode layer <NUM>), the negative side carbon layer <NUM>, and the negative electrode current collector <NUM> were accommodated between the positive electrode can 24a and the negative electrode can 24b, which would form a battery case, so as to be stacked in this order from the positive electrode can 24a toward the negative electrode can 24b, and an electrolytic solution <NUM> was filled therein. Thereafter, the positive electrode can 24a and the negative electrode can 24b were crimped via a gasket 24c to be sealed. Thus, the coin cell-shaped lithium secondary battery <NUM> with a diameter of <NUM> and a thickness of <NUM> was produced. At this time, the electrolytic solution <NUM> was a solution of LiBF4 (<NUM> mol/L) in a mixed organic solvent of ethylene carbonate (EC) and γ-butyrolactone (GBL) at <NUM>:<NUM> (volume ratio).

The one integrated sintered plate including three layers of the LCO sintered layer (positive electrode layer) <NUM>, the LTO sintered layer (negative electrode layer) <NUM>, and the MgO separator (ceramic separator) <NUM> synthesized in Procedure (<NUM>) above and the coin-shaped lithium secondary battery <NUM> manufactured in Procedure (<NUM>) were evaluated for various properties as shown below.

In Procedure (6c) above, the presence or absence of delamination between the layers of the integrated sintered plate (between the positive electrode layer and the ceramic separator, and between the ceramic separator and the negative electrode layer) due to physical impact on each electrode (positive electrode layer or negative electrode layer) by lifting the electrode with vacuum tweezers when assembling the lithium secondary battery <NUM> was checked. Ten battery samples were produced, and delamination was determined to be "present" when at least one layer delaminated.

The capacity retention rate of the lithium secondary battery <NUM> was measured by the following procedures. The lithium secondary battery <NUM> was charged at a constant current of <NUM> mA to a voltage value of <NUM> V and then charged at a constant voltage to a current of <NUM> mA. Then, the battery was discharged at a constant current of <NUM> mA to a voltage value of <NUM> V, to measure a discharge capacity W<NUM>. Thereafter, the battery was stored in a constant temperature bath at <NUM> with the remaining capacity (SOC) of <NUM>% and taken out <NUM> days later to measure a discharge capacity W<NUM>. The W<NUM> measured was divided by W<NUM> and multiplied by <NUM>, thereby calculating a capacity retention rate (%) after storage at <NUM> for <NUM> days with <NUM>% SOC.

An integrated sintered plate and a battery were prepared and evaluated for various properties as in Example A1 except that the coating with a metal oxide layer in Procedure (<NUM>) above was not performed.

An integrated sintered plate and a battery were prepared and evaluated for various properties as in Example A1 except that a coating solution was prepared using magnesium diethoxide instead of zirconium tetra-n-butoxide, and thereby a metal oxide layer composed of Mg oxide or a complex oxide of Mg and Li was formed in Procedure (<NUM>) above.

An integrated sintered plate and a battery were prepared and evaluated for various properties as in Example A1 except that a coating solution was prepared using triisopropoxyaluminum instead of zirconium tetra-n-butoxide, and thereby a metal oxide layer composed of Al oxide or a complex oxide of Al with Li was formed in Procedure (<NUM>) above.

An integrated sintered plate and a battery were prepared and evaluated for various properties as in Example A1 except that a coating solution was prepared using niobium pentaethoxide instead of zirconium tetra-n-butoxide, and thereby a metal oxide layer composed of Nb oxide or a complex oxide of Nb and Li was formed in Procedure (<NUM>) above.

An integrated sintered plate and a battery were prepared and evaluated for various properties as in Example A1 except that a coating solution was prepared using titanium tetraisopropoxide instead of zirconium tetra-n-butoxide, and thereby a metal oxide layer composed of Ti oxide or a complex oxide of Ti and Li was formed in Procedure (<NUM>) above.

Table <NUM> shows the evaluation results for Examples A1 to A6. In any of Examples A1 and A3 to A6, the content of metals (excluding Li) in the metal oxide layer, as calculated based on the change in weight of the integrated sintered plate before and after the coating with the metal oxide layer, was estimated to be <NUM> wt% or less with respect to <NUM> wt% of the integrated sintered plate coated with the metal oxide layer.

Hereinafter, reference examples and a comparative example for demonstrating that batteries using an integrated sintered plate (uncoated with a metal oxide layer) have more excellent performance than assembled batteries without using an integrated sintered plate are shown.

An integrated sintered plate and a battery were prepared and evaluated for various properties, as shown below, as in Example A2 except that <NUM>) the thickness of the LCO green sheet was increased so that the thickness of the positive electrode layer was <NUM>, and <NUM>) the thickness of the LTO green sheet was increased so that the thickness of the negative electrode layer was <NUM>.

The LCO sintered layer was polished with a cross-section polisher (CP) (IB-15000CP, manufactured by JEOL Ltd. ), and the resultant cross section of the positive electrode layer (cross section perpendicular to the layer face of the positive electrode layer) was subjected to the EBSD measurement at a <NUM>-fold field of view (<NUM> × <NUM>) to give an EBSD image. This EBSD measurement was performed using a Schottky field emission scanning electron microscope (model JSM-7800F, manufactured by JEOL Ltd. For all grains identified in the resultant EBSD image, the angles defined by the (<NUM>) planes of the primary grains and the layer face of the positive electrode layer (that is, the tilt of the crystal orientation from the (<NUM>) planes) is determined as a tilt angle. The mean value of the angles was determined as an average orientation angle of the primary grains.

The LCO and LTO sintered layers and the MgO separator were polished with a cross-section polisher (CP) (IB-15000CP, manufactured by JEOL Ltd. ), and the resultant cross-sections were observed with SEM (JSM6390LA, manufactured by JEOL Ltd. ) to determine the thickness of the positive electrode layer, the negative electrode layer, and the separator.

The LCO or LTO sintered layer and the MgO separator were polished with a cross-section polisher (CP) (IB-15000CP, manufactured by JEOL Ltd. ), and the resultant cross section of the positive electrode layer or the negative electrode layer was observed with SEM (JSM6390LA, manufactured by JEOL Ltd. ) at a <NUM>-fold field of view (<NUM> × <NUM>). The SEM image was subjected to an image analysis, the area of all pores was divided by the area of the positive electrode or the negative electrode, and the resultant value was multiplied by <NUM> to calculate the porosity (%).

The mean pore diameters of the LCO or LTO sintered layer were measured by a mercury intrusion method using a mercury porosimeter (Autopore IV <NUM>, manufactured by Shimadzu Corporation).

The area displacement ratio between the positive electrode layer and the negative electrode layer in the battery was calculated. Specifically, the area displacement ratio (%) between the positive and negative electrodes was calculated based on the formula: [(Sp + Sn)/Spn] × <NUM> by measuring the area Spn of the region where the positive electrode layer and the negative electrode layer overlap each other, the area Sp of the region where the positive electrode layer protrudes from the negative electrode layer, and the area Sn of the region where the negative electrode layer protrudes from the positive electrode layer. The areas Spn, Sp, and Sn were measured and calculated by determining the shape from both sides of each sample using a 3D shape measuring machine (VR3000, manufactured by KEYENCE CORPORATION).

The discharge capacity of the battery was measured by the following procedures. That is, the battery was charged at a constant voltage of <NUM> V and then discharged at a discharge rate of <NUM> C to measure the initial capacity, and the resultant initial capacity was employed as a discharge capacity. Then, the discharge capacity was divided by the theoretical capacity and multiplied by <NUM>, to obtain a discharge capacity/theoretical capacity ratio (%).

The theoretical capacity of the battery was calculated by the following procedures. First, the area of each layer of integrated sintered plate was calculated by the shape measurement, and the thickness and porosity of each layer of the integrated sintered plate were calculated from the cross-section SEM, to calculate the effective volumes of the positive electrode layer and the negative electrode layer from the values obtained. The true specific gravity of each constituent of the positive electrode layer and the negative electrode layer was calculated based on JIS standard R1634, and the weight values of the positive electrode layer and the negative electrode layer were calculated. The theoretical capacity value of each of the positive electrode layer and the negative electrode layer was calculated by multiplying the weight of each active material thus obtained by the capacity per weight of the material (described in the battery handbook), and the lower value was employed as the theoretical capacity value of the battery.

The pulse cycle capacity retention rate (constant voltage charge cycle performance) of the battery was measured by the following procedures. First, the battery was charged at a constant voltage of <NUM> V and discharged at a discharge rate of <NUM> C to measure the initial capacity. Then, a total of <NUM> charge/discharge cycles including <NUM> times of charging at a constant voltage of <NUM> V and discharging at a current of <NUM> mA for <NUM> seconds were conducted. Finally, the battery was charged at a constant voltage of <NUM> V and discharged at <NUM> C, to measure the capacity after cycles. The capacity after cycles measured was divided by the initial capacity and multiplied by <NUM>, to determine the pulse cycle capacity retention rate (%).

An integrated sintered plate and a battery were prepared as in Example A2 and evaluated for various properties as in Example B1 except that <NUM>) the thickness of the LCO green sheet was increased so that the thickness of the positive electrode layer was <NUM>, and <NUM>) the thickness of the LTO green sheet was increased so that the thickness of the negative electrode layer was <NUM>.

An integrated sintered plate and a battery were prepared as in Example A2 and evaluated for various properties as in Example B1.

An integrated sintered plate and a battery were prepared and evaluated for the various properties as in Example B4 except that the mean pore diameter of the positive electrode layer was adjusted to <NUM>.

First, Co<NUM>O<NUM> powder (manufactured by SEIDO CHEMICAL INDUSTRY CO. ) and Li<NUM>CO<NUM> powder (manufactured by THE HONJO CHEMICAL CORPORATION) weighed to a molar ratio Li/Co of <NUM> were mixed, and thereafter the mixture was kept at <NUM> for <NUM> hours. The resultant powder was milled and crushed into a volume-based D50 of <NUM> with a pot mill to obtain powder A composed of platy LCO particles. The resultant LCO powder A (<NUM> parts by weight), a dispersive medium (toluene:isopropanol = <NUM>:<NUM>) (<NUM> parts by weight), a binder (polyvinyl butyral: Product No. BM-<NUM>, manufactured by Sekisui Chemical Co. ) (<NUM> parts by weight), a plasticizer (di-<NUM>-ethylhexyl phthalate (DOP), manufactured by Kurogane Kasei Co. ) (<NUM> parts by weight), and a dispersant (product name: RHEODOL SP-O30, manufactured by Kao Corporation) (<NUM> parts by weight) were mixed. The resultant mixture was defoamed by stirring under reduced pressure to prepare an LCO slurry with a viscosity of <NUM> cP. The viscosity was measured with an LVT viscometer manufactured by Brookfield. The slurry prepared was formed into an LCO green sheet onto a PET film by a doctor blade process. The dried thickness of the LCO green sheet was <NUM>.

The LCO green sheet was separated from the PET film, and was cut into a <NUM> square. The cut piece was placed on the center of a bottom magnesia setter (dimensions: <NUM> square, height: <NUM>). A porous magnesia setter as the top setter was placed on the LCO sheet. The LCO sheet disposed between the setters was placed into an alumina sheath of a <NUM> square (manufactured by Nikkato Co. At this time, the alumina sheath was not tightly sealed, and was covered with a lid with a gap of <NUM>. The stack obtained was heated to <NUM> at a heating rate of <NUM>/h and degreased for <NUM> hours, then heated to <NUM> at <NUM>/h, and held for <NUM> hours for firing. After the firing, the fired laminate was cooled to room temperature, and was removed from the alumina sheath. Thus, the LCO sintered plate with a thickness of <NUM> was yielded as a positive electrode plate. The positive electrode plate obtained was cut into a circular shape with a diameter of <NUM> using a laser processing machine, to obtain a positive electrode plate.

First, LTO powder (volume-based D50 particle size <NUM>, manufactured by Sigma-Aldrich Japan) (<NUM> parts by weight), a dispersive medium (toluene:isopropanol = <NUM>:<NUM>) (<NUM> parts by weight), a binder (polyvinyl butyral: Product No. BM-<NUM>, manufactured by SEKISUI CHEMICAL CO. ) (<NUM> parts by weight), a plasticizer (DOP: Di(<NUM>-ethylhexyl)phthalate, manufactured by Kurogane Kasei Co. ) (<NUM> parts by weight), and a dispersant (product name: RHEODOL SP-O30, manufactured by Kao Corporation) (<NUM> parts by weight) were mixed. The resultant negative electrode raw material mixture was defoamed by stirring under reduced pressure to prepare a LTO slurry with a viscosity of <NUM> cP. The viscosity was measured with an LVT viscometer manufactured by Brookfield. The slurry prepared was formed into a LTO green sheet onto a PET film by a doctor blade process. The thickness of the LTO green sheet after drying and firing was adjusted to <NUM>.

The green sheet obtained was cut out into a <NUM>-mm square with a cutter knife and disposed on an embossed zirconia setter. The green sheet on the setter was put into an alumina sheath and kept at <NUM> for <NUM> hours. Thereafter, the temperature was raised at a heating rate of <NUM>/h, to perform firing at <NUM> for <NUM> hours. The LTO sintered plate obtained was cut into a circular shape with a diameter of <NUM> using a laser processing machine, to obtain a negative electrode plate.

Acetylene black and polyimide amide were weighed to a mass ratio of <NUM>:<NUM> and mixed with an appropriate amount of NMP (N-methyl-<NUM>-pyrrolidone) as a solvent, to prepare a conductive carbon paste. The conductive carbon paste was screen-printed on an aluminum foil as a negative electrode current collector. The negative electrode plate produced in (<NUM>) above was disposed within an undried printing pattern (that is, a region coated with the conductive carbon paste), followed by vacuum drying at <NUM> for <NUM> minutes, to produce a negative electrode structure with the negative electrode plate and the negative electrode current collector bonded via a carbon layer. The carbon layer had a thickness of <NUM>.

Acetylene black and polyimide amide were weighed to a mass ratio of <NUM>:<NUM> and mixed with an appropriate amount of NMP (N-methyl-<NUM>-pyrrolidone) as a solvent, to prepare a conductive carbon paste. The conductive carbon paste was screen-printed on an aluminum foil as a positive electrode current collector, followed by vacuum drying at <NUM> for <NUM> minutes, to produce a positive electrode current collector with a carbon layer formed on a surface. The carbon layer had a thickness of <NUM>.

The positive electrode current collector, the carbon layer, the LCO positive electrode plate, a cellulose separator, the LTO negative electrode plate, the carbon layer, and the negative electrode current collector were accommodated between the positive electrode can and the negative electrode can, which would form a battery case, so as to be stacked in this order from the positive electrode can toward the negative electrode can, and an electrolytic solution was filled therein. Thereafter, the positive electrode can and the negative electrode can were crimped via a gasket to be sealed. Thus, the coin cell-shaped lithium secondary battery <NUM> with a diameter of <NUM> and a thickness of <NUM> was produced. At this time, the electrolytic solution was a solution of LiBF<NUM> (<NUM> mol/L) in a mixed organic solvent of ethylene carbonate (EC) and γ-butyrolactone (GBL) at <NUM>:<NUM> (volume ratio).

The LCO sintered plate (positive electrode plate) prepared in Procedure (1b), the LTO sintered plate (negative electrode plate) prepared in Procedure (2b), and the coin-shaped lithium secondary battery manufactured in Procedure (<NUM>) were evaluated for various properties as in Example B1.

Tables <NUM> and <NUM> show the evaluation results for Examples B1 to B6.

Claim 1:
A lithium secondary battery comprising:
a positive electrode layer composed of a lithium complex oxide sintered body;
a negative electrode layer composed of a titanium-containing sintered body;
a ceramic separator interposed between the positive electrode layer and the negative electrode layer;
an electrolyte with which at least the ceramic separator is impregnated; and
an exterior body comprising a closed space, the closed space accommodating the positive electrode layer, the negative electrode layer, the ceramic separator, and the electrolyte,
wherein the positive electrode layer, the ceramic separator, and the negative electrode layer form one integrated sintered plate as a whole, whereby the positive electrode layer, the ceramic separator, and the negative electrode layer are bonded together, and
wherein the entirety of the integrated sintered plate is coated with a metal oxide layer.