Patent Publication Number: US-9425454-B2

Title: Lithium secondary battery and cathode of the lithium secondary battery with conductive bonding layer

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a cathode of a lithium secondary battery and to a lithium secondary battery having the cathode. 
     2. Description of the Related Art 
     The cathode active material layer of a lithium secondary battery (may be referred to as a “lithium ion secondary battery”) is widely known to be formed through kneading a kneaded product of a lithium composite oxide (lithium transition metal oxide) powder with additives such as a binder and a conducting agent and molding the kneaded product. For example, Japanese Patent Application Laid-Open (kokai) No. Hei 5-226004 discloses such a technique. Hereinafter, such a configuration is referred to as a “powder-dispersion type cathode.” 
     Since the powder-dispersion type cathode contains a large amount (e.g., about 10 wt. %) of a binder, which is not a capacity-enhancing ingredient, the relative amount of the lithium composite oxide serving as a cathode active material contained in the cathode is small. Therefore, the capacity and charge-discharge efficiency of such a powder-dispersion type cathode are not satisfactory and to be further improved. 
     To overcome this drawback, efforts have been made to improve the capacity and charge-discharge efficiency through forming the cathode or the cathode active material layer from a sintered lithium composite oxide sheet. For example, Japanese Patent Application Laid-Open (kokai) Nos. Hei 8-180904 and 2001-143687 disclose such a technique. According to this technique, the cathode or the cathode active material layer contains no binder. Therefore, the lithium composite oxide filling density increases, conceivably leading to high capacity and excellent charge-discharge efficiency. 
     In the case where the cathode or the cathode active material layer is formed from a sintered lithium composite oxide sheet, when the sintered sheet has a low lithium composite oxide filling ratio (i.e., high voidage), the effect of enhancing the performance of the aforementioned powder-dispersion type cathode; in particular, capacity-enhancing effect, is unsatisfactory. In fact, cathodes formed from a sintered lithium composite oxide sheet disclosed in Japanese Patent Application Laid-Open (kokai) Nos. Hei 8-180904 and 2001-143687 have a low filling ratio (a voidage of 15 to 60%), which is unsatisfactory in terms of capacity. 
     Meanwhile, when the sintered sheets has an excessively high lithium composite oxide filling ratio, high capacity is attained, but the cyclic characteristic (i.e., capacity retention performance after repetition of charge-discharge cycles) is known to be problematically impaired. The impairment in cyclic characteristic is caused also when the sintered sheet has a thickness of about 10 μm. Particularly when the sintered sheet has a thickness as large as 30 μm or more, the impairment is considerably severe. 
     In order to elucidate the cause for the impairment, sintered lithium composite oxide sheets tested in an experiment example where the cyclic characteristic had been impaired were previously observed under an electron microscope. Through observation, cracks were found to be generated in the sintered sheets. The cracks were generated at the grain boundary, where the boundary between regions which are adjacent to each other and have different crystal orientations (hereinafter, the cracks are referred to as “grain boundary cracks”). Furthermore, the sintered lithium composite oxide sheets tested in the experiment example were observed under an electron microscope at the interface between the conductive bonding layer (disposed between the cathode collector and the sintered sheet) and the sintered sheet. Through the observation, separation (void formation) was found to occur at the interface (hereinafter referred to as “bonding interface separation”). 
     The grain boundary cracking is thought to be caused by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles, wherein the crystal lattice expansion/contraction includes the expansion/contraction of volume and the expansion/contraction not involving change in volume. Similarly, the bonding interface separation is thought to be caused by the tensile or shear stress generated between the interface and the conductive bonding layer associated with the morphological change of the sintered lithium composite oxide sheet by crystal lattice expansion/contraction. Thus, such grain boundary cracking or bonding interface separation, which results in generation of an electrically isolated portion (i.e., a portion which does not contribute to capacity) through breakage of an electrical conduction path in the sintered lithium composite oxide sheet, to thereby reduce capacity, is thought to be a possible cause for deterioration in cycle characteristic. 
     SUMMARY OF THE INVENTION 
     The present invention has conceived in order to solve the aforementioned problems. Thus, an object of the present invention is to provide a lithium secondary battery which has high capacity while maintaining excellent cycle characteristic. Another object of the invention is to provide a cathode of the lithium secondary battery. 
     &lt;Configuration&gt; 
     The lithium secondary battery cathode (hereinafter may be referred to simply as “cathode”) of the present invention comprises a cathode collector formed of a conductive substance, and a cathode active material layer formed of a sintered lithium composite oxide sheet. The cathode active material layer is bonded to the cathode collector by the mediation of a conductive bonding layer (conductive adhesive layer). In addition to the cathode, the lithium secondary battery of the present invention further includes an anode containing as an anode active material a carbonaceous material or a lithium-occluding substance, and an electrolyte. The electrolyte is disposed so as to intervene the space between the cathode and the anode. A characteristic feature of the present invention resides in that the cathode active material layer has a thickness of 30 μm or more, a mean pore size of 0.1 to 5 μm, and a voidage of 3% or more and less than 15%. 
     As used herein, the term “lithium composite oxide” refers to an oxide represented by Li x MO 2  (0.05&lt;x&lt;1.10, M is at least one transition metal (typically, M includes one or more species of Co, Ni, and Mn)) and generally having a layered rock salt structure. As used herein, the term “layered rock salt structure” refers to a crystal structure in which lithium layers and layers of a transition metal other than lithium are arranged in alternating layers with an oxygen layer therebetween; i.e., a crystal structure in which transition metal ion layers and lithium layers are arranged in alternating layers via oxide ions (typically, α-NaFeO 2  type structure: cubic rock salt type structure in which transition metal and lithium are arrayed orderly in the direction of the [111] axis). 
     In the case where the sintered lithium composite oxide sheet has a layered rock salt structure, the ratio of intensity of diffraction by the (003) plane to intensity of diffraction by the (104) plane, [003]/[104], as obtained through X-ray diffraction, is preferably 2 or less, more preferably 1 or less, still more preferably 0.5 or less. One possible reason for the improvement of cycle characteristic when the peak intensity ratio [003]/[104] is 2 or less is as follow. 
     The crystal lattice expansion/contraction (expansion/contraction in volume) associated with charge-discharge cycles occurs to the highest extent in a direction normal to the (003) plane (i.e., [003] direction). Therefore, cracking caused by crystal lattice expansion/contraction associated with charge-discharge cycles readily occurs in a direction parallel to the (003) plane. Furthermore, the (003) plane is a closest packing plane in terms of oxygen and does not allow lithium ions and electron to enter and exit the crystal lattice. Thus, the (003) plane is a chemically and electrochemically inert plane. 
     Meanwhile, as described above, when the peak intensity ratio [003]/[104] is 2 or less, there is reduced the ratio of the (003) plane which is developed to a plate surface of the sintered lithium composite oxide sheet or to the bonding interface between the sheet and the cathode collector, or the ratio of the (003) plane which is oriented in the direction parallel to the plate surface in the sintered lithium composite oxide sheet. When the decrease in ratio of the (003) plane developed to the bonding interface, the bonding strength at the bonding interface increased, to thereby prevent separation of the sintered sheet. In addition, grain boundary cracking occurring in a direction parallel to the plate surface, which would impair inter alia the capacity, can be effectively prevented. Thus, the cycle characteristic is improved. 
     Generally, the sintered lithium composite oxide “sheet” may be formed so as to have a ratio w/t of 3 or more, wherein t represents the thickness of the sheet, and w represents the minimum size in the plate surface direction (direction orthogonal to the thickness direction). 
     The term “thickness direction” refers to the direction which defines the “thickness” t of the sintered lithium composite oxide sheet; typically, a direction parallel with the vertical direction in a state in which the sintered lithium composite oxide sheet is placed stably on a horizontal surface. The term “thickness” refers to the size of the sintered lithium composite oxide sheet as measured in the “thickness direction.” 
     The term “plate surface” refers to a surface of the sintered lithium composite oxide sheet which is orthogonal to the “thickness direction” of the sheet. Since the “plate surface” of the sintered lithium composite oxide sheet is the widest surface on the sheet, the “plate surface” may be referred to as the “principal surface.” The term “plate surface direction” refers to a direction parallel with the “plate surface” (i.e., in-plane direction). Thus, the “thickness” of the sintered lithium composite oxide sheet is equivalent to the shortest distance between the generally parallel two plate surfaces of the sheet. The aforementioned “thickness direction” is the direction which defines the shortest distance between the two “plate surfaces.” 
     Specifically, the thickness t of the sintered lithium composite oxide sheet is determined by, for example, measuring the distance between the generally parallel plate surfaces as observed in cross section under an SEM (scanning electron microscope). The minimum size in the plate surface direction “w” is determined by, for example, taking a plan view of the sintered lithium composite oxide sheet under an SEM, and measuring the diameter of the inscribed circle of the contour of the plan view. 
     The “mean pore size” is the mean diameter of the pores in the sintered lithium composite oxide sheet. The “diameter” is generally the diameter of an imaginary sphere under the assumption that each pore is reduced to a sphere having the same volume or cross-sectional area as that of the pore. According to the present invention, the “mean value” is preferably calculated on the number basis. The mean pore size may be obtained through, for example, a widely known method such as image processing of a cross-sectional SEM (scanning electron microscope) image or the mercury penetration method. 
     As used herein, the term “voidage” refers to the volume proportion of voids (pores: including open pores and closed pores) in the sintered lithium composite oxide sheet of the present invention. “Voidage” may also be referred to as “porosity.” “Voidage” is calculated from, for example, bulk density and true density of the sintered lithium composite oxide sheet. 
     When the sintered lithium composite oxide sheet has a structure in which numerous primary particles (crystal particles) have been bound to one another, the primary particles preferably have a size (primary particle size) of 5 μm or less. One possible reason therefor is as follows. Generally, the smaller the primary particle size, the more the grain boundaries. As the number of grain boundaries increases, the internal stress generated by crystal lattice expansion/contraction associated with charge-discharge cycles is more favorably scattered. Even when cracking occurs, propagation of cracking is effectively prevented in the presence of a large number of grain boundaries. Thus, the cycle characteristic is enhanced. 
     The cathode collector may be disposed on at least one of the two plate surfaces of the sintered lithium composite oxide sheet. In other words, the cathode collector may be disposed on only one of the two plate surfaces of the sintered lithium composite oxide sheet. Alternatively, the cathode collector may be disposed on each surface (each of the two plate surfaces) of the sintered lithium composite oxide sheet. 
     In the case where the cathode collector is disposed on each plate surface of the sintered lithium composite oxide sheet, the cathode collector disposed on one plate surface may have a thickness greater than the other cathode collector so as to sustain the sintered lithium composite oxide sheet, and the cathode collector disposed on the other plate surface may have such a structure as not inhibit intercalation/deintercalation of lithium ions into/from the sintered lithium composite oxide sheet (e.g., a mesh-like structure or a porous structure). 
     The conductive bonding layer may be formed from, for example, a material containing a conductive powder (metallic powder and/or a powder of a conductive carbonaceous material such as acetylene black or graphite) and a binder. The conductive bonding layer may be formed so as to have a thickness of, for example, 0.1 to 20 μm. In order to enhance electroconductivity, each surface of the sintered lithium composite oxide sheet, or one surface thereof to be bonded to the conductive bonding layer may be provided with a conductive thin film (thin film formed of a metal such as Au, Pt, Ag, Al, or Cu, or carbon) through sputtering or a similar technique. 
     In bonding the cathode active material layer to the cathode collector by the mediation of the conductive bonding layer, one cathode active material layer (including such a layer piece whose shape has been modified so as to adapt to the cathode collector) formed of the sintered lithium composite oxide sheet may be bonded to one cathode collector. Alternatively, the cathode active material layer to be bonded to the cathode collector may be formed of a plurality of sintered lithium composite oxide sheets which are one-dimensionally or two-dimensionally arranged as viewed from the top. In other words, the cathode may be formed by dividing the produced sintered lithium composite oxide sheet through, for example, cutting, to a plurality of fragments, disposing the fragments one-dimensionally or two-dimensionally as viewed from the top, and boding the cathode collector to the thus-disposed fragments. Yet alternatively, two or more cathode active material layers may be disposed. In other words, an additional cathode active material layer may be disposed on the cathode active material layer bonded to the cathode collector. In this case, the two cathode active material layers may be bonded by the mediation of a conductive bonding layer. 
     &lt;Action and Effect&gt; 
     In the aforementioned configuration, as described above, the sintered lithium composite oxide sheet includes pores having a specific size at a specific ratio. Therefore, the stress generated by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles is favorably (uniformly) released by the pores. As a result, grain boundary cracking associated with repeated charge-discharge cycles can be prevented to the possible extent. In addition, by virtue of the pores (open pores) present at the interface with the conductive bonding layer, the bonding strength is enhanced. As a result, deformation of the sintered lithium composite oxide sheet caused by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles can be prevented, whereby the aforementioned bonding interface separation is favorably prevented. Thus, according to the present invention, the capacity of the cell employing the cathode can be enhanced, while favorable cycle characteristic is maintained. 
     Meanwhile, when the mean pore size is in excess of 5 μm, relatively large pores are provided. Such larger pores are generally not in the form of complete sphere but in the form of deformed sphere. Therefore, stress concentration is likely to occur locally in the larger pores, and the effect of uniformly releasing the stress in the sintered sheet is difficult to attain. When the mean pore size is less than 0.1 μm or the voidage is less than 3%, the stress releasing effect by the pores is insufficient. Thus, in both cases, grain boundary cracking is not satisfactorily prevented. Furthermore, in both cases, the effect of enhancing the bonding strength to the conductive bonding layer is impeded, and boding interface separation is not satisfactorily prevented. 
     For example, lithium cobaltate expands its volume upon charge (i.e., deintercalation of lithium ions), whereas lithium nickelate expands its volume upon discharge (i.e., intercalation of lithium ions). Therefore, volume expansion/contraction upon charge-discharge cycles can be apparently reduced to zero by appropriately adjusting the compositional proportion of cobalt or nickel. However, even in this case, lattice length changes. Specifically, Li(CO 0.5 Ni 0.5 )O 2  expands in a c-axis direction and contracts in an a-axis direction. 
     Therefore, the present invention is very effectively applied to the sintered sheet having the following composition: a lithium composite oxide having a layered rock salt structure (e.g., lithium cobaltate Li p CoO 2  [wherein 1≦p≦1.1], lithium nickelate LiNiO 2 , lithium manganate Li 2 MnO 3 , lithium nickel manganate Li p (Ni 0.5 , Mn 0.5 )O 2 , a solid solution thereof represented by the formula Li p (CO x , Ni y , Mn z )O 2  [wherein 0.97≦p≦1.07, x+y+z=1], Li p (CO x , Ni y , Al z )O 2  [wherein 0.97≦p≦1.07, x+y+z=1, 0&lt;x≦0.25, 0.6≦y≦0.9, 0&lt;z≦0.1], or a solid solution of Li 2 MnO 3  and LiMO 2  (M is a transition metal such as Co or Ni)). The sintered sheet of the present invention may contain one or more elements selected from among, for example, Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba, and Bi, so long as any of the aforementioned formulas is satisfied. 
     The present invention is particularly effectively applied to the case of a composition exhibiting increased volume expansion/contraction; for example, Li p (Co x , Ni y , Mn z )O 2  wherein the proportion by mole of nickel is 0.75 or more or the proportion by mole of cobalt is 0.9 or more, or Li p (Co x , Ni y , Al)O 2  wherein the proportion by mole of nickel is 0.7 or more. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a sectional view of the schematic configuration of an embodiment of the lithium secondary battery of the present invention; 
         FIG. 1B  is an enlarged sectional view of the cathode shown in  FIG. 1A ; 
         FIG. 2  is a sectional view schematically showing the structure of a coin-cell-type lithium secondary battery employed for evaluating cell characteristics; 
         FIGS. 3A and 3B  are graphs showing cyclic characteristics of LiCoO 2  ceramic sheets of the embodiments and comparative examples; 
         FIGS. 4A to 4C  are photographs of LiCoO 2  ceramic sheets of the embodiments and comparative examples, taken under a scanning electron microscope; 
         FIG. 5  shows graphs showing rate characteristics of LiCoO 2  ceramic sheets of the embodiment in which voids have been provided; 
         FIG. 6  is a side sectional view of the configuration of a modification of the cathode shown in  FIG. 1B ; 
         FIG. 7  is a sectional view of the schematic configuration of another embodiment of the lithium secondary battery of the present invention; 
         FIG. 8  is a sectional view of the schematic configuration of another embodiment of the lithium secondary battery of the present invention; and 
         FIG. 9  is a sectional view of the configuration of another modification of the cathode shown in  FIG. 1B . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will next be described with reference to examples and comparative examples. The following description of the embodiments is nothing more than the specific description of mere example embodiments of the present invention to the possible extent in order to fulfill description requirements (descriptive requirement and enabling requirement) of specifications required by law. Thus, as will be described later, naturally, the present invention is not limited to the specific configurations of embodiments and examples to be described below. Modifications that can be made to the embodiments and examples are collectively described herein principally at the end, since insertion thereof into the description of the embodiments would disturb understanding of consistent description of the embodiments. 
     &lt;Configuration of Lithium Secondary Battery&gt; 
       FIG. 1A  is a sectional view of the schematic configuration of an lithium secondary battery  10  to which an embodiment of the present invention is applied. Referring to  FIG. 1A , the lithium secondary battery  10  of the embodiment has a cell casing  11 , a separator  12 , an electrolyte  13 , an anode  14 , and a cathode  15 . 
     The separator  12  is provided so as to halve the interior of the cell casing  11  into the anode  14  section and the cathode  15  section. That is, the anode  14  and the cathode  15  are provided within the cell casing  11  in such a manner as to face each other with the separator  12  located therebetween. The cell casing  11  accommodates the electrolyte  13 . 
     The electrolyte  13  which may be in the present invention is preferably a liquid electrolyte from the viewpoints, for example, electrical characteristics and easy handlability. The liquid electrolyte which may be employed in the present invention is preferably a nonaqueous-solvent-type which is prepared by dissolving an electrolyte salt such as a lithium salt in a nonaqueous solvent such as an organic solvent. The electrolyte  13  may be a polymer electrolyte, a gel electrolyte, an organic solid electrolyte, or an inorganic solid electrolyte, and no problem arises when such an electrolyte is employed. In a preferred mode, the electrolyte is charged into open pores provided in the cathode active material layer (i.e., a cathode active material layer  15   b  in  FIG. 1B  given hereinbelow) or penetrate the cathode active material layer, form the viewpoint of activating intercalation/deintercalation of lithium ions into/from the cathode active material. 
     No particular limitation is imposed on the solvent for the nonaqueous electrolytic solution. Examples of the solvent include chain esters, such as dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, and methyl propione carbonate; cyclic esters having high dielectric constant, such as ethylene carbonate, propylene carbonate, butylene carbonate, and vinylene carbonate; and mixed solvents of a chain ester and a cyclic ester. Among them, a mixed solvent containing a chain ester serving as a main solvent with a cyclic ester is particularly preferred. 
     Examples of the electrolyte salt to be dissolved in the above-mentioned nonaqueous solvent include LiClO 4 , LiPF 6 , LiBF 4 , LiAsF 6 , LiSbF 6 , LiCF 3 SO 3 , LiC 4 F 9 SO 3 , LiCF 3 CO 2 , Li 2 C 2 F 4 (SO 3 ) 2 , LiN(RfSO 2 )(Rf′SO 2 ), LiC(RfSO 2 ) 3 , LiC n F 2n+1 SO 3  (n≧2), and LiN(RfOSO 2 ) 2  [Rf and Rf′ each represent a fluoroalkyl group]. They may be used singly or in combination of two or more species. 
     Among the above-mentioned electrolyte salts, a fluorine-containing organic lithium salt having a carbon number of 2 or greater is particularly preferred. This is because the fluorine-containing organic lithium salt is high in anionic property and readily undergoes electrolytic dissociation, and is thus readily dissolvable in the above-mentioned solvent. No particular limitation is imposed on the electrolyte salt concentration of the nonaqueous electrolytic solution (electrolyte  13 ). However, for example, the concentration is 0.3 mol/L to 1.7 mol/L, more preferably 0.4 mol/L to 1.5 mol/L. 
     Any anode active material may be used in the anode  14 , so long as the material can occlude and release lithium ions. For example, there may be used carbonaceous materials (e.g., graphite, pyrolytic carbon, coke, glassy carbon, a fired product of organic polymer compound, mesocarbon microbeads, carbon fiber, and activated carbon). Further, a portion of graphite may be replaced with a metal which can be alloyed with, for example, lithium or an oxide. Also, there may be used, as the anode active material, metallic lithium; an alloy which contains metallic lithium and another element (e.g., silicon, tin, or indium); an oxide of silicon, tin, or the like which can perform charge and discharge at low electric potential near that at which lithium does; and a lithium-occluding material such as a lithium cobalt nitride (e.g., Li 2.6 CO 0.4 N). 
     When graphite is employed as the anode active material, the voltage at full charge can be considered to be about 0.1 V (vs. lithium). Thus, the electric potential of the cathode  15  can be conveniently calculated as a cell voltage plus 0.1 V. Therefore, since the electric potential of the cathode  15  at charging is readily controlled, graphite is preferred. 
       FIG. 1B  is an enlarged sectional view of the cathode  15  shown in  FIG. 1A . Referring to  FIG. 1B , the cathode  15  includes a cathode collector  15   a  and a cathode active material layer  15   b . The cathode collector  15   a  is formed of a conductive material such as a metal (e.g., aluminum foil) and bonded to the cathode active material layer  15   b  via conductive bonding layer  15   c . The cathode active material layer  15   b  is composed of a sintered lithium composite oxide sheet and has a thickness of 30 μm or more (specifically 30 to 200 μm), a mean pore size of 0.1 to 5 μm, and a voidage of 3% or higher and less than 15%. 
     &lt;Summary of Method for Producing a Sintered Lithium Composite Oxide Sheet Employed in Cathode Active Material Layer&gt; 
     The sintered lithium composite oxide sheet employed in cathode active material layer  15   b  shown in  FIG. 1B  is readily and reliably formed through, for example, the following production method. 
     1. Preparation of Raw Material Particles 
     Particles of a transition metal (e.g., Co, Ni, or Mn) compound containing no lithium compound are employed as raw material particles. The raw material particles may be appropriately pulverized and classified. In consideration of a composition of interest, a plurality of types of raw material particles may be appropriately mixed together. The raw material particles may be mixed with a low-melting-point oxide (e.g., boron oxide, bismuth oxide, or antimony oxide), a low-melting-point chloride (e.g., sodium chloride or potassium chloride), or a low-melting-point glass material (e.g., borosilicate glass) in an amount of 0.001 to 30 wt. % for the purpose of promoting grain growth. 
     To the raw material particles, an additive for forming pores having a size and voidage of interest as mentioned above; i.e., a pore-forming material may be added, followed by uniformly mixing. The pore-forming material is preferably a particulate or fibrous substance which decomposes (vaporizes or is carbonized) in the subsequent calcination step. Specific examples of preferably employed pore-forming materials include theobromine, graphite, and organic synthetic resins such as nylon, phenolic resin, poly(methyl methacrylate), polyethylene, poly(ethylene terephthalate), and foamable resin, in the form of particle or fiber. Needless to say, if no such a pore-forming material is added, the pores having a size and voidage of interest as mentioned above can be formed through appropriately tuning the particle size of the raw material particles, the firing temperature employed in the calcination step, etc. 
     2. Raw Material Particle Sheet Forming Step 
     Raw material particles or a mixture thereof is subjected to a sheet forming step, to thereby form an “independent” sheet-like compact. As used herein, “independent” sheet (may be referred to as “self-standing film”) refers to a sheet which is independent of another support and can be handled by itself. The term “independent sheet-like compact” also means thin tablets having an aspect ratio of 5 or higher. Therefore, “independent” sheet does not include a sheet bonded to and integrated with another support (e.g., substrate) (i.e., impossible or difficult to separate from the support). 
     Sheet forming may be carried out through any well known technique. Specifically, sheet forming may be performed by means of, for example, a doctor blade-type sheet forming machine (doctor blade method), a drum dryer, a disk dryer, or a screen printing machine. The thickness of a sheet-like compact is appropriately determined so as to attain the aforementioned preferred thickness after firing. 
     3. Compact Calcination Step 
     The sheet-like compact formed through the aforementioned forming step is fired at a relatively low temperature (e.g., 700 to 1,200° C.) before the below-described lithium incorporation step. This calcination step forms a porous sheet-like intermediate fired compact containing numerous pores of relatively large size. This calcination step is carried out in air in a state where, for example, the above-formed sheet-like compact is placed on a zirconia-made embossed setter. 
     4. Lithium Incorporation Step 
     A lithium compound is applied to the intermediate fired compact obtained through the aforementioned calcination step, followed by thermal treatment, whereby lithium is incorporated into the intermediate fired compact. Thus, there is produced an “independent” sintered lithium composite oxide sheet for a cathode active material layer. The lithium incorporation step is carried out in, for example, a crucible. 
     Examples of the employable lithium compound include lithium salts such as lithium carbonate, lithium nitrate, lithium peroxide, lithium hydroxide, lithium acetate, lithium chloride, lithium oxalate, and lithium citrate; and lithium alkoxides such as lithium methoxide and lithium ethoxide. The lithium compound is added so that, in the sheet-like sintered compact represented by the formula Li x MO 2 , the ratio by mole of lithium to M (i.e., Li/M) is 1 or more. 
     When lithium is incorporated into the porous sheet-like intermediate fired compact yielded in the aforementioned calcination step, the pore size of the intermediate fired compact is decreases, whereby the aforementioned pores having a size and voidage of interest are formed. 
     &lt;Evaluation Method and Results&gt; 
     For the evaluation of cell characteristics, a coin cell having the same shape as that of a type CR2032 cell was fabricated in the following manner.  FIG. 2  is a sectional view schematically showing the structure of a coin-cell-type lithium secondary battery  10 . 
     A cathode active material layer  15   b  was formed from the produced “independent” sintered lithium composite oxide sheet, and the layer was worked into pieces having a φ of about 5 to about 10 mm. Separately, a mixture of acetylene black and PVdF (1:0.6 by mass) was mixed with an appropriate amount of NMP serving as a solvent, to thereby prepare a paste. The paste was applied onto a cathode collector  15   a  formed of aluminum foil through screen printing. The sintered plate was placed such that the plate was located inside the printed pattern in an undried state and was dried at 100° C. for 30 minutes, to thereby complete bonding, whereby a cathode  15  was produced. The thickness of the conductive bonding layer  15   c  was adjusted to 10 to 20 μm. 
     In the space between a cathode can  111  and an anode can  112  forming a cell casing  11 , there was placed a stacked body in which a cathode  15  (a cathode collector  15   a  facing opposite the cathode can  111 ), a separator  12 , and an anode  14  (metallic lithium plate) were stacked in this order. The cell casing was filled with a liquid electrolyte  13  and sealed with a gasket  113 , to thereby fabricate the coin-cell-type lithium secondary battery  10 . The liquid electrolyte  13  was prepared by dissolving LiPF 6  in an equivolume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) serving as an organic solvent to a concentration of 1 mol/L. 
     The thus-fabricated coin cell was evaluated in terms of cycle characteristic (percent capacity maintenance). 
     Cycle Characteristic (Percent Capacity Maintenance): 
     The fabricated cell was subjected to cyclic charge-discharge at a test temperature of 20° C. The cyclic charge-discharge repeats: (1) charge at 1 C rate of constant current and constant voltage until 4.2 V is reached, and (2) discharge at 1 C rate of constant current until 3.0 V is reached. By dividing the discharge capacity of the cell as measured after the cyclic charge-discharge by the initial discharge capacity of the cell and being multiplied by 100, percent capacity maintenance (%) was obtained. 
     Rate Characteristic: 
     The above-fabricated cell was evaluated through the following charge and discharge operations at a test temperature of 20° C. One cycle consists of the following charge and discharge operations: constant-current charge is carried out at 0.1 C rate of current until the cell voltage becomes 4.2 V; subsequently, constant-voltage charge is carried out under a current condition of maintaining the cell voltage at 4.2 V, until the current drops to 1/20, followed by 10 minutes rest; and then, constant-current discharge is carried out at a specific rate of current until the cell voltage becomes 3.0 V. 
       FIGS. 3A and 3B  are graphs showing cyclic characteristics of LiCoO 2  ceramic sheets of the embodiments and comparative examples. In  FIG. 3A , the graph shows the cyclic characteristics of LiCoO 2  ceramic sheets of comparative examples in which no void has been provided, while the graph of  FIG. 3B  shows the cyclic characteristics of LiCoO 2  ceramic sheets (thickness: 30 μm) according to the embodiment in which voids have been provided. 
     As shown in the graph of  FIG. 3A , the cyclic characteristics of the LiCoO 2  ceramic sheets in which no void had been provided were impaired as the thickness increased. In particular, an LiCoO 2  ceramic sheet having a thickness of 30 μm exhibited considerably impaired cycle characteristic. In contrast, as shown in the graph of  FIG. 3B , a void-provided LiCoO 2  ceramic sheet having a thickness of 30 μm was found to exhibit remarkably improved cycle characteristic. 
       FIGS. 4A to 4C  are photographs of LiCoO 2  ceramic sheets of the embodiments and comparative examples, taken under a scanning electron microscope. In  FIG. 4A , there are given photographs of LiCoO 2  ceramic sheets of comparative examples in which no void has been provided. Each of the SEM photographs on the left side shows a corresponding ceramic sheet removed through decomposition of the as-produced coin cell, and each of the SEM photographs on the right side shows a corresponding ceramic sheet removed through decomposition of the coin cell after 50 charge-discharge cycles.  FIG. 4B  is a SEM photograph of a cross-section of an LiCoO 2  ceramic sheet of a comparative example in which no void has been provided.  FIG. 4C  is a SEM photograph of a cross-section of an LiCoO 2  ceramic sheet of the embodiment in which voids have been provided (voidage: 8%). 
     As shown in  FIG. 4A , in LiCoO 2  ceramic sheets of comparative examples in which no void had been provided, grain boundary cracking was observed after 50 charge-discharge cycles. This grain boundary cracking is thought to be caused by internal stress generated by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles. In addition, after 50 charge-discharge cycles, a ≧50% of the area of the conductive bonding layer was found to be separated from the interface between the bonding layer and the ceramic sheet during peeling from the collector of the LiCoO 2  ceramic sheet of the comparative example in which no void had been provided. The deterioration in cycle characteristic of the LiCoO 2  ceramic sheets in which no void has been provided is thought to be caused by such grain boundary cracking and bonding interface separation of the conductive bonding layer, which result in generation of an electrically isolated portion (i.e., a portion which does not contribute to capacity) through breakage of an electrical conduction path in each LiCoO 2  ceramic sheet, to thereby reduce capacity. 
     Thus, in order to relax internal stress generated by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles and to enhance bonding strength of the conductive bonding layer and the ceramic sheet, the present inventors have provided voids in the LiCoO 2  ceramic sheet at a voidage falling within the aforementioned range. Through provision of such voids, as described above, remarkable improvement of the cycle characteristic was observed.  FIG. 5  shows graphs showing rate characteristics of LiCoO 2  ceramic sheets of the embodiments in which voids have been provided. As shown in  FIG. 5 , through provision of voids in the ceramic sheet, the cycle characteristic and rate characteristic of the cell employing the ceramic sheet were found to be improved. 
     EXAMPLES 
     The present invention will next be described in more detail by way of specific examples of sintered lithium composite oxide sheets serving as the aforementioned cathode active material layer  15   b . Evaluations of the sheets are also described. 
     Example 1 
     Specific Example 1 
     Cobalt-Based Composition 
     In Experiment Examples 1-1 to 1-9, LiCoO 2  ceramic sheets were produced under the conditions shown in Table 1. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Material 
                 Intermediate 
                 Pore-forming material 
               
               
                   
                 particle size 
                 firing temp. 
                 shape/amount 
               
               
                   
                 (μm) 
                 (° C.) 
                 (wt. parts) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Exp. Ex. 1-1 
                 0.3 
                 1,100 
                 0 
               
               
                 Exp. Ex. 1-2 
                 1.0 
                 1,100 
                 0 
               
               
                 Exp. Ex. 1-3 
                 0.5 
                 1,000 
                  fibrous/5 
               
               
                 Exp. Ex. 1-4 
                 1.0 
                 1,000 
                 spherical/5  
               
               
                 Exp. Ex. 1-5 
                 1.0 
                 900 
                 spherical/10 
               
               
                 Exp. Ex. 1-6 
                 0.5 
                 1,100 
                 0 
               
               
                 Exp. Ex. 1-7 
                 0.1 
                 1,200 
                 0 
               
               
                 Exp. Ex. 1-8 
                 3.0 
                 1,100 
                 0 
               
               
                 Exp. Ex. 1-9 
               
               
                   
               
            
           
         
       
     
     The detailed production conditions employed in Experiment Examples 1-1, 1-2, and 1-6 to 1-8 shown in Table 1 are as follows. 
     (1) Preparation of Slurry 
     CO 3 O 4  powder (particle size: 1 to 5 μm; product of Seido Kagaku Kogyo Co., Ltd.) was crushed and classified to prepare CO 3 O 4  raw material powder. The CO 3 O 4  raw material powder (100 parts by weight) was mixed with a dispersion medium (toluene:isopropanol=1:1) (100 parts by weight), a binder (polyvinyl butyral: product No. “BM-2,” product of Sekisui Chemical Co. Ltd.) (10 parts by weight), a plasticizer (DOP: di(2-ethylhexyl)phthalate, product of Kurogane Kasei Co., Ltd.) (4 parts by weight), and a dispersant (trade name “RHEODOL SP-O30,” product of Kao Corporation) (2 parts by weight). The resultant mixture was stirred under reduced pressure for defoaming, and the viscosity thereof was adjusted to 3,000 to 4,000 cP. The viscosity was measured by means of an LVT-type viscometer (product of Brookfield) in Specific Example 1 and other Examples. 
     (2) Sheet Forming 
     The thus-prepared slurry was formed into a sheet-like compact on a PET film through the doctor blade method so that the thickness of the compact as measured after drying was adjusted to 30 μm. 
     (3) Calcination 
     The sheet-like compact was removed from the PET film and was cut into square pieces (70 mm×70 mm) by means of a cutter. Each piece was placed at the center of the aforementioned zirconia-made setter (90 mm×90 mm, height: 1 mm) having an embossed (protrusion: 300 μm) surface. The piece was heated in air at a specific temperature (firing temperature for yielding an intermediate fired product) for 5 hours. Thereafter, the piece was cooled at 200 degrees (° C.)/h. Then, a portion of the piece which was not fused to the setter was removed from the furnace. 
     (4) Lithium Incorporation 
     LiNO 3  powder (product of Kanto Chemical Co., Inc.) was applied to the thus-obtained CO 3 O 4  ceramic sheet so that the ratio by mole of Li/Co was 1.2, and the mixture was thermally treated in a crucible (atmospheric conditions) at 840° C. for 20 hours, to thereby produce an “independent” LiCoO 2  ceramic sheet having a thickness of 30 μm for the cathode active material layer  15   b.    
     The detailed production conditions employed in Experiment Examples 1-3 to 1-5 shown in Table 1 are as follows. 
     A slurry was prepared through the same procedure employed in, for example, Experiment Example 1-1, except that a pore-forming material was added. As the pore-forming material, a fibrous material (Celish PC10S, product of Daicel FineChem LTd.) or a spherical material (Nylon powder, product of Sumitomo Environ Science) was employed. The thus-prepared slurry was formed into a sheet on a PET film in the same manner as employed in, for example, Experiment Example 1-1. 
     The sheet-like compact was removed from the PET film and was cut into square pieces (70 mm×70 mm) by means of a cutter. Each piece was placed at the center of the aforementioned zirconia-made setter and heated in air at 900° C. (firing temperature for yielding an intermediate fired product) for 10 hours. Thereafter, the piece was cooled at 200 degrees (° C.)/h. Then, a portion of the piece which was not fused to the setter was removed from the furnace. Lithium was incorporated into the thus-obtained CO 3 O 4  ceramic sheet in the same manner as employed in, for example, Experiment Example 1-1, to thereby produce an “independent” LiCoO 2  ceramic sheet having a thickness of 30 μm for the cathode active material layer. 
     The detailed production conditions employed in Experiment Example 1-9 shown in Table 1 are as follows. 
     Specifically, Li 2 CO 3  and CO 3 O 4  were mixed together so that the Li/Co ratio was adjusted to 1.0, and the mixture was heated in air at 800° C. for 24 hours, to thereby yield an LiCoO 2  powder. The powder was subjected to further crushing, to thereby adjust the mean particle size to 5.0 μm. The thus-obtained powder was formed into a sheet in the same manner as employed in Experiment Examples 1-1, 1-2, and 1-6 to 1-8. Each sheet was placed at the center of the aforementioned zirconia-made setter and heated in air at 900° C. for 10 hours, and then cooled at 200 degrees (° C.)/h. Then, a portion of the sheet which was not fused to the setter was removed from the furnace, to thereby produce an “independent” LiCoO 2  ceramic sheet. 
     Table 2 shows the evaluation results of Experiment Examples shown in Table 1. In Table 2, the term “percent capacity maintenance” was obtained from the capacity after completion of 50 charge-discharge cycles. The “voidage” was obtained by dividing the bulk density of each sintered sheet (measured through the Archimedes&#39; method) by the true density thereof (measured by means of a picnometer), to thereby obtain the relative density, and subtracting the relative density from 1. In the measurement of bulk density, the tested sintered sheet was boiled in water in order to sufficiently remove air remaining in the pores. When the sample had a small pore size, the pores of the sample were impregnated in advance with water by means of a vacuum impregnation apparatus (CitaVac, product of Struers), and the thus-treated sample was subjected to boiling treatment. The “mean pore size” was measured through the mercury penetration method by means of a mercury penetration micropore distribution analyzer “AutoPore IV9510,” product of Shimadzu Corporation. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. Ex. 1-1 
                 3.1 
                 0.1 
                 89 
               
               
                   
                 Exp. Ex. 1-2 
                 3 
                 5 
                 85 
               
               
                   
                 Exp. Ex. 1-3 
                 7.8 
                 0.3 
                 91 
               
               
                   
                 Exp. Ex. 1-4 
                 8 
                 5 
                 88 
               
               
                   
                 Exp. Ex. 1-5 
                 14.7 
                 5 
                 92 
               
               
                   
                 Exp. Ex. 1-6 
                 2.5 
                 3 
                 35 
               
               
                   
                 Exp. Ex. 1-7 
                 2.9 
                 0.05 
                 60 
               
               
                   
                 Exp. Ex. 1-8 
                 3 
                 7 
                 50 
               
               
                   
                 Exp. Ex. 1-9 
                 22 
                 6 
                 89 
               
               
                   
                   
               
            
           
         
       
     
     As is clear from Tables 1 and 2, the sintered sheets of Experiment Examples 1-1 to 1-5, which had a mean pore size of 0.1 to 5 μm and a voidage of 3% or higher and lower than 15%, exhibited excellent percent capacity maintenance, even after completion of 50 charge-discharge cycles. In contrast, the sintered sheets of Experiment Examples 1-6 and 1-7, which had a voidage lower than the lower limit of the aforementioned range, and the sintered sheets of Experiment Examples 1-7 and 1-8, which had a mean pore size falling outside the aforementioned range, exhibited considerably impaired cyclic characteristic (i.e., percent capacity maintenance after completion of 50 charge-discharge cycles), as compared with those of Experiment Examples 1-1 to 1-5. One possible reason therefor is as follows. 
     The sintered sheets of Experiment Examples 1-1 to 1-5 each include pores having a specific pore size at a specific proportion. Therefore, the stress generated by crystal lattice expansion/contraction associated with intercalation and deintercalation of lithium ions in charge-discharge cycles is favorably (uniformly) released. As a result, grain boundary cracking associated with charge-discharge cycles can be prevented to the possible extent. In addition, since the bonding strength between the ceramic sheet for the cathode active material layer and the conductive bonding layer is enhanced, the aforementioned bonding interface separation is effectively prevented. Thus, according to the sintered sheets of Experiment Examples 1-1 to 1-5, the capacity of the cell employing each sintered sheet can be enhanced, while favorable cycle characteristic is maintained. 
     Meanwhile, in Experiment Example 1-8, in which the mean pore size is in excess of 5 μm, relatively large pores are provided. Such larger pores are generally not in the form of complete sphere but in the form of deformed sphere. Therefore, stress concentration is likely to occur locally in the larger pores, and the effect of uniformly releasing the stress in the sintered sheet is difficult to attain. In Experiment Examples 1-6 and 1-7, in which the mean pore size is less than 0.1 μm or the voidage is less than 3%, the stress releasing effect by the pores is insufficient. Thus, in both cases, grain boundary cracking is not satisfactorily prevented. Furthermore, in both cases, the effect of enhancing the bonding strength between the ceramic sheet for the cathode active material layer and the conductive bonding layer is impeded, and boding interface separation is not satisfactorily prevented. 
     Meanwhile, if voids are performed without forming an intermediate fired product or incorporation of lithium into the intermediate, or without adding a pore-forming material, the voidage excessively increases as shown in Experiment Example 1-9, failing to attain high capacity. That is, conceivably, although apparently excellent cycle characteristic is attained, a drop in percent capacity maintenance is not clearly observed due to excessively low initial capacity caused by excessively low filling ratio. The same consideration may be applicable to the fact that high percent capacity maintenance is attained, although the mean pore size is greater than the upper limit of the aforementioned specific range. 
     Next, the ratio of intensity of X-ray diffraction by the (003) plane to intensity of X-ray diffraction by the (104) plane; i.e., the peak intensity ratio [003]/[104] was evaluated. XRD (X-ray diffraction) was carried out though the following procedure. 
     Specifically, a ceramic sheet for the cathode active material layer, which had been worked to a φ of about 5 to about 10 mm, was placed on a sampler holder for XRD. By means of an XRD apparatus (RINT-TTR III, product of Rigaku Corp.), the surface of the ceramic sheet for the cathode active material layer was irradiated with X-ray so as to measure an XRD profile, thereby obtaining the ratio of intensity (peak height) of diffraction by the (003) plane to intensity (peak height) of diffraction by the (104) plane, [003]/[104]. Thus, according to the above-mentioned method, there is obtained a profile of diffraction by crystal faces present in parallel with crystal faces of the plate surface; i.e., a profile of diffraction by crystal faces oriented in a plate surface direction. 
     In order to evaluate the influence of the peak intensity ratio [003]/[104], sintered sheet samples of Experiment Examples 1-10 to 1-13, which had the same voidage and pore size but varied orientation degrees, were produced. In Experiment Examples 1-10 to 1-13, the same raw material particle size and firing temperature for yielding an intermediate fired product as employed in Experiment Example 1-4 were employed, whereby the same voidage and pore size (voidage: 8%, mean pore size; 5 μm) as employed in Experiment Example 1-4 were realized. However, lithium introduction conditions (i.e., type and amount of lithium compound, and treatment temperature: see, Table 3) were appropriately modified, to thereby vary the orientation degree. Table 4 shows the results. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                   
                 Li/M 
                 Li incorporation 
               
               
                   
                 Li compd. 
                 mole ratio 
                 temp. (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. Ex. 1-10 
                 Li 2 O 2   
                 1 
                 775 
               
               
                   
                 Exp. Ex. 1-11 
                 LiNO 3   
                 1 
                 800 
               
               
                   
                 Exp. Ex. 1-4 
                 LiNO 3   
                 1.2 
                 840 
               
               
                   
                 Exp. Ex. 1-12 
                 Li 2 CO 3   
                 5 
                 870 
               
               
                   
                 Exp. Ex. 1-13 
                 Li 2 CO 3   
                 10 
                 900 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                   
                   
                 [003]/[104] 
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 peak intensity 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 ratio 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Exp. Ex. 1-10 
                 8 
                 5 
                 0.5 
                 93 
               
               
                 Exp. Ex. 1-11 
                 8 
                 5 
                 0.7 
                 90 
               
               
                 Exp. Ex. 1-4 
                 8 
                 5 
                 1.6 
                 88 
               
               
                 Exp. Ex. 1-12 
                 8 
                 5 
                 5 
                 60 
               
               
                 Exp. Ex. 1-13 
                 8 
                 5 
                 10 
                 50 
               
               
                   
               
            
           
         
       
     
     As is clear from Table 4, in Experiment Examples 1-4, 1-10, and 1-11, in which the peak intensity ratio [003]/[104] was 2 or lower, excellent cycle characteristic was attained. In contrast, in Experiment Examples 1-12 and 1-13, in which the peak intensity ratio [003]/[104] was in excess of 2, cycle characteristic was impaired. 
     When the ceramic sheet of the embodiment for a cathode active material layer has a structure in which numerous primary particles (crystal particles) have been bound to one another (confirmed by the scanning electron microscopic images (photograph  FIG. 4C ) of the cross section, although the contrast is insufficient). Thus, in order to evaluate the influence of the primary particle size, there were produced ceramic sheet samples of Experiment Examples 1-14 to 1-18 having different primary particle sizes at a constant voidage and pore size. In Experiment Examples 1-14 to 1-18, the ceramic sheet samples having different primary particle sizes were formed by appropriately modifying conditions employed in incorporation of lithium (i.e., type and amount of lithium compound and treatment temperature, see Table 5), while the voidage and pore size were maintained by controlling, to constant values, the raw material particle size and firing temperature for yielding an intermediate fired product. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 5 
               
               
                   
                   
               
               
                   
                   
                 Li/M 
                 Li incorporation 
               
               
                   
                 Li compd. 
                 mole ratio 
                 temp. (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. Ex. 1-14 
                 Li 2 O 2   
                 1 
                 775 
               
               
                   
                 Exp. Ex. 1-15 
                 LiNO 3   
                 1 
                 800 
               
               
                   
                 Exp. Ex. 1-16 
                 LiNO 3   
                 1.2 
                 840 
               
               
                   
                 Exp. Ex. 1-17 
                 LiNO 3   
                 2 
                 870 
               
               
                   
                 Exp. Ex. 1-18 
                 LiNO 3   
                 3 
                 900 
               
               
                   
                   
               
            
           
         
       
     
     The primary particle size was determined through the following procedure. Specifically, a sample for observation under an electron microscope was prepared by thinning a ceramic sheet for a cathode active material layer to a thickness of about 80 nm through FIB (focused ion beam) working. The sample had a cross section in parallel with the plate surface of the ceramic sheet. The magnification of the transmission electron microscope was adjusted so that 10 or more primary particles were included in a vision field, and a bright image of the sample was taken. An inscribed circle was drawn in each of the 10 primary particles observed in the bright image, and the diameter of the inscribed circle was determined. The thus-obtained diameters were averaged, to thereby obtain a primary particle size. The evaluation results of Experiment Examples 1-14 to 1-18 are shown in Table 6. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 6 
               
               
                   
                   
               
               
                   
                   
                   
                 Primary 
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 particle 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 size (μm) 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Exp. Ex. 1-14 
                 8 
                 5 
                 0.2 
                 92 
               
               
                 Exp. Ex. 1-15 
                 8 
                 5 
                 1.2 
                 90 
               
               
                 Exp. Ex. 1-16 
                 8 
                 5 
                 3 
                 88 
               
               
                 Exp. Ex. 1-17 
                 8 
                 5 
                 5 
                 80 
               
               
                 Exp. Ex. 1-18 
                 8 
                 5 
                 8 
                 50 
               
               
                   
               
            
           
         
       
     
     As is clear from Table 6, the ceramic sheet samples of Experiment Examples 1-14 to 1-17, having a primary particle size of 5 μm or less, exhibited a favorable cycle characteristic. In contrast, the ceramic sheet sample of Experiment Example 1-18, having a primary particle size in excess of 5 μm, exhibited an impaired cycle characteristic. 
     Furthermore, ceramic sheet samples for a cathode active material layer having a ceramic sheet thickness more than 30 μm (Experiment Examples 1-19 to 1-21, see Table 7) were evaluated. As is clear from Table 7, when the thickness of the ceramic sheet for a cathode active material layer was in excess of 30 μm (for example, 100 μm), a favorable cycle characteristic was found to be attained by virtue of a controlled voidage and pore size, similar to the case of a thickness of 30 μm. This feature was the same in the case of the below-mentioned Li(Ni,Co,Al)O 2  ceramic sheets. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 7 
               
               
                   
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 Thickness 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 (μm) 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Exp. Ex. 1-19 
                 10 
                 1 
                 100 
                 90 
               
               
                 Exp. Ex. 1-20 
                 5 
                 1 
                 100 
                 81 
               
               
                 Exp. Ex. 1-21 
                 3 
                 1 
                 100 
                 70 
               
               
                   
               
            
           
         
       
     
     Example 2 
     Specific Example 2 
     Ni-Based Composition 
     In Experiment Examples 2-1 to 2-10, Li(Ni,Co,Al)O 2  ceramic sheets were produced under the conditions shown in Table 8. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 8 
               
               
                   
                   
               
               
                   
                 Material 
                 Intermediate 
                 Pore-forming material 
               
               
                   
                 particle size 
                 firing temp. 
                 shape/amount 
               
               
                   
                 (μm) 
                 (° C.) 
                 (wt. parts) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Exp. Ex. 2-1 
                 0.2 
                 1,000 
                 0 
               
               
                 Exp. Ex. 2-2 
                 0.7 
                 1,000 
                 0 
               
               
                 Exp. Ex. 2-3 
                 0.3 
                 900 
                  fibrous/5 
               
               
                 Exp. Ex. 2-4 
                 0.7 
                 900 
                 spherical/5  
               
               
                 Exp. Ex. 2-5 
                 0.2 
                 700 
                     fibrous/10 
               
               
                 Exp. Ex. 2-6 
                 0.7 
                 800 
                 spherical/10 
               
               
                 Exp. Ex. 2-7 
                 0.2 
                 1,000 
                 0 
               
               
                 Exp. Ex. 2-8 
                 0.1 
                 1,100 
                 0 
               
               
                 Exp. Ex. 2-9 
                 2.0 
                 1,000 
                 0 
               
               
                 Exp. Ex. 2-10 
               
               
                   
               
            
           
         
       
     
     The detailed production conditions employed in Experiment Examples 2-1, 2-2, and 2-7 to 2-9 shown in Table 8 are as follows. 
     (1) Preparation of Slurry 
     NiO powder (particle size: 1 to 10 μm; product of Seido Kagaku Kogyo Co., Ltd.) (75.1 parts by weight), CO 3 O 4  powder (particle size: 1 to 5 μm; product of Seido Kagaku Kogyo Co., Ltd.) (21.5 parts by weight), and Al 2 O 3  powder (particle size: 1 to 10 μm; product of Showa Denko K.K.) (3.4 parts by weight) were mixed together and pulverized, and the resultant mixture was thermally treated in air at 1,000° C. for five hours, to thereby synthesize (Ni 0.75 , CO 0.2 , Al 0.05 )O powder. 
     The thus-synthesized powder was pulverized by means of a pot mill, and the resultant (Ni 0.75 , Co 0.2 , Al 0.05 )O raw material particles (100 parts by weight) were mixed with a dispersion medium (toluene:isopropanol=1:1) (100 parts by weight), a binder (polyvinyl butyral: product No. “BM-2,” product of Sekisui Chemical Co. Ltd.) (10 parts by weight), a plasticizer (DOP: di(2-ethylhexyl)phthalate, product of Kurogane Kasei Co., Ltd.) (4 parts by weight), and a dispersant (trade name “RHEODOL SP-O30,” product of Kao Corporation) (2 parts by weight). The resultant mixture was stirred under reduced pressure for defoaming, and the viscosity thereof was adjusted to 3,000 to 4,000 cP. 
     (2) Sheet Forming 
     The thus-prepared slurry was formed into a sheet-like compact on a PET film through the doctor blade method so that the thickness of the compact as measured after drying was adjusted to 30 μm. 
     (3) Calcination 
     The sheet-like compact was removed from the PET film and was cut into square pieces (50 mm×50 mm) by means of a cutter. Each piece was placed at the center of the aforementioned zirconia-made setter. The piece was heated in air at a specific temperature (firing temperature for yielding an intermediate fired product) for 10 hours. Thereafter, the atmosphere in the firing furnace was replaced with an oxygen atmosphere, followed by cooling to room temperature at 200 degrees (° C.)/h. Then, a portion of the piece which was not fused to the setter was removed from the furnace. 
     (4) Lithium Incorporation 
     LiOH powder (product of Kanto Chemical Co., Inc.) was applied to the thus-obtained (Ni 0.75 , CO 0.2 , Al 0.05 )O ceramic sheet so that the ratio by mole of Li/(NiCoAl) was 3.0, and the mixture was thermally treated in an oxygen atmosphere (0.1 MPa) at 775° C. for 48 hours, to thereby produce an “independent” Li 1.0 (Ni 0.75 , CO 0.2 , Al 0.05 )O 2  ceramic sheet for a cathode active material layer. 
     The detailed production conditions employed in Experiment Examples 2-3 to 2-6 shown in Table 8 were the same as those employed in other Experiment Examples (e.g., 2-1), except that the aforementioned pore-forming material was added. The detailed production conditions employed in Experiment Example 2-10 shown in Table 8 are as follows. 
     NiO powder (particle size: 1 to 10 μm; product of Seido Kagaku Kogyo Co., Ltd.), CO 3 O 4  powder (particle size: 1 to 5 μm; product of Seido Kagaku Kogyo Co., Ltd.), Al 2 O 3 —H 2 O (particle size: 1 to 3 μm; product of SASOL), and Li 2 CO 3  powder (particle size: 10 to 50 μm; product of Kanto Chemical Co., Inc.) were mixed at predetermined proportions so as to attain a composition of Li 1.50  (Ni 0.75 Co 0.2 Al 0.05 )O 2 . The mixture was heated under oxygen (0.1 MPa) at 775° C. for 48 hours, to thereby yield a powder of Li 1.0 (Ni 0.75 CO 0.2 Al 0.05 )O 2  having a mean particle size of 3.0 μm. 
     The same procedure employed in, for example, Experiment Example 2-1, was repeated, except that the above-yielded powder was used, to thereby form a ceramic sheet having a thickness of 30 μm. The sheet was removed from the PET film and was cut into square pieces (70 mm×70 mm) by means of a cutter. Each piece was placed at the center of the aforementioned zirconia-made setter. The piece was heated in air at 900° C. for 10 hours. Thereafter, the piece was cooled at 200 degree (° C.)/h, and a portion of the piece which was not fused to the setter was removed from the furnace, to thereby produce an “independent” Li 1.0 (Ni 0.75 CO 0.2 Al 0.05 )O 2  ceramic sheet having a thickness of 30 μm for a cathode active material layer. 
     The ceramic sheets of the Experiment Examples shown in Table 8 were evaluated, and the results are shown in Table 9. As is clear from Table 9, the ceramic sheets having a nickel-based composition were found to have the same characteristics as those of the ceramic sheets having a cobalt-based composition. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 9 
               
               
                   
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. Ex. 2-1 
                 3.2 
                 0.1 
                 88 
               
               
                   
                 Exp. Ex. 2-2 
                 3 
                 4.8 
                 84 
               
               
                   
                 Exp. Ex. 2-3 
                 7.5 
                 0.3 
                 91 
               
               
                   
                 Exp. Ex. 2-4 
                 8 
                 5.1 
                 87 
               
               
                   
                 Exp. Ex. 2-5 
                 14.8 
                 0.1 
                 94 
               
               
                   
                 Exp. Ex. 2-6 
                 14.9 
                 4.9 
                 91 
               
               
                   
                 Exp. Ex. 2-7 
                 2 
                 3.1 
                 36 
               
               
                   
                 Exp. Ex. 2-8 
                 2.9 
                 0.05 
                 58 
               
               
                   
                 Exp. Ex. 2-9 
                 3 
                 7.2 
                 49 
               
               
                   
                 Exp. Ex. 2-10 
                 21 
                 6.5 
                 92 
               
               
                   
                   
               
            
           
         
       
     
     Similar to the aforementioned LiCoO 2  sheets, lithium oxide ceramic sheets of a nickel-based composition were evaluated in terms of peak intensity ratio [003]/[104] and effect of primary particle size. The results are shown in Tables 10 to 13. As is clear from Tables 10 to 13, the ceramic sheets having a nickel-based composition were found to have the same characteristics as those of the aforementioned LiCoO 2  sheets. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 10 
               
               
                   
                   
               
               
                   
                   
                 Li/M 
                 Li incorporation 
               
               
                   
                 Li compd. 
                 mole ratio 
                 temp. (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. Ex. 2-11 
                 Li 2 O 2   
                 2 
                 750 
               
               
                   
                 Exp. Ex. 2-12 
                 Li 2 O 2   
                 3 
                 750 
               
               
                   
                 Exp. Ex. 2-13 
                 LiOH 
                 3 
                 775 
               
               
                   
                 Exp. Ex. 2-14 
                 LiOH 
                 5 
                 800 
               
               
                   
                 Exp. Ex. 2-15 
                 LiOH 
                 10 
                 800 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 11 
               
               
                   
                   
               
               
                   
                   
                   
                 [003]/[104] 
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 peak intensity 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 ratio 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Exp. Ex. 2-11 
                 8 
                 4.9 
                 0.3 
                 95 
               
               
                 Exp. Ex. 2-12 
                 8 
                 4.9 
                 0.7 
                 92 
               
               
                 Exp. Ex. 2-13 
                 8 
                 4.9 
                 1.1 
                 81 
               
               
                 Exp. Ex. 2-14 
                 8 
                 4.9 
                 6 
                 65 
               
               
                 Exp. Ex. 2-15 
                 8 
                 4.9 
                 9 
                 57 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 12 
               
               
                   
                   
               
               
                   
                   
                 Li/M 
                 Li incorporation 
               
               
                   
                 Li compd. 
                 mole ratio 
                 temp. (° C.) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 Exp. Ex. 2-16 
                 Li 2 O 2   
                 2 
                 750 
               
               
                   
                 Exp. Ex. 2-17 
                 Li 2 O 2   
                 3 
                 750 
               
               
                   
                 Exp. Ex. 2-18 
                 LiOH 
                 3 
                 775 
               
               
                   
                 Exp. Ex. 2-19 
                 LiOH 
                 5 
                 800 
               
               
                   
                 Exp. Ex. 2-20 
                 LiOH 
                 10 
                 800 
               
               
                   
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 13 
               
               
                   
                   
               
               
                   
                   
                   
                 Primary 
                   
               
               
                   
                 Voidage 
                 Mean pore 
                 particle 
                 Percent capacity 
               
               
                   
                 (%) 
                 size (μm) 
                 size (μm) 
                 maintenance (%) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Exp. Ex. 2-16 
                 8 
                 4.9 
                 0.1 
                 93 
               
               
                 Exp. Ex. 2-17 
                 8 
                 4.9 
                 0.5 
                 90 
               
               
                 Exp. Ex. 2-18 
                 8 
                 4.9 
                 1 
                 87 
               
               
                 Exp. Ex. 2-19 
                 8 
                 4.9 
                 3 
                 81 
               
               
                 Exp. Ex. 2-20 
                 8 
                 4.9 
                 6 
                 57 
               
               
                   
               
            
           
         
       
     
     Examples of Modifications 
     The above-described embodiment and specific examples are, as mentioned above, mere examples of the best mode of the present invention which the applicant of the present invention contemplated at the time of filing the present application. The above-described embodiment and specific examples should not be construed as limiting the invention. Various modifications to the above-described embodiment and specific examples are possible, so long as the invention is not modified in essence. 
     Several modifications will next be exemplified. In the following description of the modifications, component members similar in structure and function to those of the above-described embodiment are denoted by names and reference numerals similar to those of the above-described embodiment. The description of the component members appearing in the above description of the embodiment can be applied as appropriate, so long as no inconsistencies are involved. 
     Needless to say, even modifications are not limited to those described below. Limitingly construing the present invention based on the above-described embodiment and the following modifications impairs the interests of an applicant (particularly, an applicant who is motivated to file as quickly as possible under the first-to-file system) while unfairly benefiting imitators, and is thus impermissible. 
     The structure of the above-described embodiment and the structures of the modifications to be described below are entirely or partially applicable in appropriate combination, so long as no technical inconsistencies are involved. 
     The present invention is not limited to the constitution which is specifically disclosed in the description of the above embodiments. For example, the cathode collector  15   a  of the cathode  15  may be omitted. That is, the sintered lithium composite oxide sheet as is may be employed as the cathode  15 . 
       FIG. 6  is a side sectional view of the configuration of a modification of the cathode  15  shown in  FIG. 1B . As shown in  FIG. 6 , in one exemplary mode, a plurality of sintered lithium composite oxide sheets  15   b   1  are bonded to the cathode collector  15   a , while the sheets are one-dimensionally or two dimensionally arranged, to thereby form the cathode  15 . In this case, the cathode active material layer  15   b  is formed of a plurality of sintered lithium composite oxide sheets  15   b   1  which are arranged one-dimensionally or two-dimensionally. 
     In the configuration shown in  FIG. 6 , each sintered lithium composite oxide sheet  15   b   1  may be obtained through cutting an edge of the aforementioned lithium-incorporated sintered sheet to predetermined dimensions as viewed from the top plane. Alternatively, each sintered lithium composite oxide sheet  15   b   1  may be obtained through one-dimensionally or two-dimensionally dividing the aforementioned lithium-incorporated sintered sheet into pieces as viewed from the top plane. In either case, preferably, the cathode  15  is formed such that one sintered lithium composite oxide sheet  15   b   1  is closely bonded to another adjacent sheet so as to reduce the space between two sintered lithium composite oxide sheets  15   b   1  adjacent to each other, and such that the sintered sheets are bonded to the cathode collector  15   a  so as to prevent intrusion of the conductive bonding layer  15   c  into the spaces, whereby the electrolyte  13  enters the very narrow spaces between two adjacent sintered lithium composite oxide sheets  15   b   1 . 
       FIG. 7  is a sectional view of the schematic configuration of an embodiment of the lithium secondary battery  20  of the present invention. As shown in  FIG. 7 , the lithium secondary battery  20  is of a so-called complete solid type and has a cathode collector  21 , a cathode active material layer  22 , a solid electrolyte layer  23 , an anode active material layer  24 , and an anode collector  25 . The lithium secondary battery  20  is formed by stacking, on the cathode collector  21 , the cathode active material layer  22 , the solid electrolyte layer  23 , the anode active material layer  24 , and the anode collector  25 , in this order. The sintered lithium composite oxide sheet of the present invention preferably formed the cathode active material layer  22  of the aforementioned secondary battery. In this case, preferably, the open pores of the cathode active material layer  22  are continuously filled with the solid electrolyte  23 , from the viewpoint of activation of intercalation/deintercalation of lithium ions to/from the cathode active material. 
       FIG. 8  is a sectional view of the schematic configuration of an embodiment of the lithium secondary battery  30  of the present invention. As shown in  FIG. 8 , the lithium secondary battery  30  is of a so-called polymer type and has a cathode collector  31 , a cathode active material layer  32 , a polymer electrolyte layer  33 , an anode active material layer  34 , and an anode collector  35 . The lithium secondary battery  30  is formed by stacking, on the cathode collector  31 , the cathode active material layer  32 , the polymer electrolyte layer  33 , the anode active material layer  34 , and the anode collector  35 , in this order. The sintered lithium composite oxide sheet of the present invention preferably formed the cathode active material layer  32  of the aforementioned secondary battery. 
     As shown in  FIG. 1B , the cathode collector  15   a  may be disposed on at least one of the two plate surfaces of the cathode active material layer  15   b . Alternatively, as shown in  FIG. 9 , the cathode collector  15   a  may be disposed on each of the two plate surfaces of the cathode active material layer  15   b.    
     As shown in  FIG. 9 , in the case where the cathode collector  15   a  is disposed on each plate surface of the cathode active material layer  15   b , one cathode collector  15   a   1  may have a thickness greater than the other cathode collector  15   a   2  so as to sustain the cathode active material layer  15   b . In this case, the counter cathode collector disposed  15   a   2  may have such a structure as not inhibit intercalation/deintercalation of lithium ions into/from the cathode active material layer  15   b  (e.g., a mesh-like structure or a porous structure). The cathode collector  15   a   2  may also be applicable to the cathode  15  shown in  FIG. 1B . 
     Moreover, a conductive bonding layer  15   c   1  disposed on the side of the cathode collector  15   a   1  and a conductive bonding layer  15   c   2  disposed on the side of the cathode collector  15   a   2  may have thicknesses identical to or different from each other. The two conductive bonding layers may be formed of the same material or different materials. Furthermore, one of the two conductive layers may be omitted. In other words, the cathode collector  15   a   1  or the cathode collector  15   a   2  may be formed directly on a plate surface of the cathode active material layer  15   b  through a process such as coating or sputtering. 
     As shown in  FIG. 1B , in the case where the cathode collector  15   a  is disposed on only one plate surface of the cathode active material layer  15   b , lithium ions and electrons move in the directions opposite to each other in the cell reaction occurring at the cathode  15  during the charge-discharge process, whereby potential gradient is generated in the cathode active material layer  15   b . When a large potential gradient is generated, diffusion of lithium ions is impeded. 
     In contrast, as shown in  FIG. 9 , the aforementioned generation of potential gradient is suppressed through formation of a cathode collector  15   a   2  which does not inhibit intercalation/deintercalation of lithium ions, on the side of the cathode active material layer  15   b  in contact with the electrolyte  13 , leading to enhancement of battery performance. 
     In the present invention, the method of producing the sintered lithium composite oxide sheet is not limited to the aforementioned procedures. For example, the sintered lithium composite oxide sheet of the present invention is formed readily and reliably through the following production method. 
     1. Preparation of Raw Material Particles 
     As raw material particles, there is used an appropriate particle mixture of compounds of Li, Co, Ni, Mn, etc. at such proportions that the formed cathode active material LiMO 2  has a layered rock salt structure. Alternatively, raw material particles having a LiMO 2  composition (synthesized) may also be used. To such raw material particles, the aforementioned pore-forming material is added. 
     In the below-mentioned thermal treatment step, a lithium compound may be added in an excessive amount of about 0.5 to about 30 mol % in order to promote grain growth or compensate for volatile matter. Also, for promoting grain growth, a low-melting-point oxide such as bismuth oxide or a low-melting-point glass such as borosilicate glass may be added in an amount of 0.001 to 30 wt. %. 
     Particularly when a thick sintered sheet having a thickness of 50 μm or more is formed, sheet-like LiMO 2  oriented particles whose crystal planes other than the (003) plane are oriented to a plate surface and which have a thickness of about 0.5 to about 20 μm may be added in an amount of 2 to 30 wt. % in order to enhance the orientation degree. Such sheet-like oriented particles can enhance the orientation degree when particles are dispersed in such a state that a plate surface is in parallel to a sheet surface during sheet forming in which shear force is applied (e.g., the doctor blade method) and when grain growth occurs together with co-present raw material particles in the firing step. The sheet-like oriented particles may be produced through, for example, producing sintered sheets having a thickness of 20 μm or less according to the production method disclosed in the specification and appropriately crushing the sheets after firing. 
     2. Raw Material Particle Sheet Forming Step 
     Through subjecting a mixture of raw material particles and a pore-forming material to sheet forming, an “independent” sheet-like compact is produced. The sheet forming step is the same as the aforementioned two-step process. 
     3. Compact Thermal Treatment (Firing) Step The sheet-like compact formed through the aforementioned forming step is thermally treated (fired) at a relatively high temperature (e.g., 1,200° C. to 1,500° C.) in order to proceed crystal growth with formation of pores, to thereby yield an “independent” sintered lithium composite oxide sheet for a cathode active material layer. This thermal treatment may be carried out in an oxygen atmosphere where, for example, the above-formed sheet-like compact is placed on a zirconia-made embossed setter. 
     Differing from the above embodiment employing a two-step thermal treatment step including formation of an intermediate fired product (calcination) and lithium incorporation, the variation of the production method is performed as a one-step thermal treatment process. Therefore, it is called a “one-step process.” 
     The ratio by mole of lithium to M; i.e., LiIM (Li/Co or Li/(Co,Ni,Mn)) of a sintered sheet represented by formula Li x MO 2  is not limited to 1.0. However, the ratio is preferably 0.9 to 1.2, more preferably 1.0 to 1.1. Through controlling the ratio, excellent charge-discharge capacity is attained. 
     Needless to say, those modifications which are not particularly referred to are also encompassed in the technical scope of the present invention, so long as the invention is not modified in essence. 
     Those components which partially constitute means for solving the problems to be solved by the present invention and are illustrated with respect to operations and functions encompass not only the specific structures disclosed above in the description of the above embodiment and modifications but also any other structures that can implement the operations and functions. Further, the contents (including specifications and drawings) of the prior application and publications cited herein can be incorporated herein as appropriate by reference.