Abstract:
Disclosed is a method for manufacturing a positive electrode including a positive electrode substrate made of aluminum foil and a positive electrode active material layer containing a positive electrode active material on the positive electrode substrate. This method includes the steps of stretching a first exposed region of the positive electrode substrate with a first stretching roller disposed upstream; stretching a second exposed region of the positive electrode substrate with a second stretching roller disposed downstream; and compressing the positive electrode active material layer with a pair of compression rollers.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present invention application claims priority to Japanese Patent Application No. 2015-213992 filed in the Japan Patent Office on Oct. 30, 2015, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    Field of the Invention 
         [0003]    The present invention relates to methods for manufacturing electrodes for applications such as nonaqueous electrolyte secondary batteries and methods for manufacturing secondary batteries. 
         [0004]    Description of Related Art 
         [0005]    Secondary batteries, such as nonaqueous electrolyte secondary batteries, have been used as power supplies for driving electric vehicles (EVs), hybrid electric vehicles (HEVs and PHEVs), and other systems. 
         [0006]    Secondary batteries include positive and negative electrodes, each composed of a substrate made of metal foil and an active material layer containing an active material on the substrate. There is a need for a secondary battery with a higher volume energy density for electric vehicles (EVs), hybrid electric vehicles (HEVs and PHEVs), and other systems. One approach to increase the volume energy density of secondary batteries is to increase the packing density of the active material layer. This increases the amount of active material present in the casing and thus increases the volume energy density. The packing density of the active material layer can be increased, for example, by forming the active material layer on the substrate and then compressing the active material layer at a higher pressure using a machine such as a roller press. 
         [0007]    However, if the active material layer formed on the substrate is compressed at a higher pressure, both the active material layer and the region of the substrate where the active material layer is formed are compressed at a higher pressure; therefore, the substrate is rolled. If the substrate has exposed regions where no active material layer is formed along the lateral edges of the electrode in the longitudinal direction, the exposed regions of the substrate are not pressed during compression since they are thinner than the region where the active material layer is formed. Thus, whereas the region of the substrate where the active material layer is formed is rolled during compression, the exposed regions of the substrate are not rolled. This results in a difference in length in the longitudinal direction between the region of the substrate where the active material layer is formed and the exposed regions of the substrate. The difference in length in the longitudinal direction over the substrate causes creases in the substrate and warpage in the electrode. 
         [0008]    To solve this problem, Japanese Patent No. 5390721 (Patent Document 1) proposes a technique involving stretching exposed regions of a substrate of an electrode in advance before roller-pressing the electrode. 
       BRIEF SUMMARY OF THE INVENTION 
       [0009]    The inventors have newly found the following problem during research on methods for manufacturing electrodes. An electrode includes a strip-shaped substrate and an active material layer extending across the substrate in the longitudinal direction. The substrate has first and second exposed regions extending in the longitudinal direction on opposing sides of the region of the substrate where the active material layer is formed. If the first and second exposed regions of the substrate are simultaneously stretched in advance before the step of compressing the active material layer, they might not be stretched as intended. If the first and second exposed regions of the substrate are not stretched as intended, creases may occur in the substrate during the compression of the active material layer despite the stretching of the exposed regions of the substrate in advance before the step of compressing the active material layer. 
         [0010]    An object of the present invention is to solve the foregoing problems and provide an electrode with high packing density and high reliability and a secondary battery with high volume energy density and high reliability. 
         [0011]    According to one aspect of the present invention, there is provided a method for manufacturing an electrode including a strip-shaped substrate and an active material layer containing an active material on the substrate. This method includes the steps of forming the active material layer on the substrate in a longitudinal direction of the substrate such that the substrate has first and second exposed regions extending in the longitudinal direction of the substrate on opposing sides of the active material layer in a lateral direction of the substrate; stretching the first exposed region of the substrate after the step of forming the active material layer; stretching the second exposed region of the substrate after the first stretching step; and compressing the active material layer after the second stretching step. 
         [0012]    The separate stretching of the first and second exposed regions of the substrate as described above allows them to be more reliably stretched as intended and to be more uniformly stretched in the longitudinal direction. 
         [0013]    The first stretching step preferably includes stretching the first exposed region of the substrate with a first stretching roller in abutment with the first exposed region of the substrate. The second stretching step preferably includes stretching the second exposed region of the substrate with a second stretching roller, different from the first stretching roller, in abutment with the second exposed region of the substrate. 
         [0014]    A guide roller is preferably disposed between the first and second stretching rollers, and the substrate is preferably tensioned by the guide roller. 
         [0015]    A path through which the substrate passes preferably includes, in order from upstream, the first stretching roller, the guide roller, and the second stretching roller. 
         [0016]    The first stretching roller preferably includes a first body and a first large-diameter portion having a larger diameter than the first body. The second stretching roller preferably includes a second body and a second large-diameter portion having a larger diameter than the second body. The first large-diameter portion preferably stretches the first exposed region of the substrate. The second large-diameter portion preferably stretches the second exposed region of the substrate. 
         [0017]    According to another aspect of the present invention, there is provided a method for manufacturing a secondary battery. This method includes the steps of fabricating an electrode assembly including a positive electrode, a negative electrode, and a separator; and placing the electrode assembly and a nonaqueous electrolyte in a casing. The positive electrode is manufactured by the method described above. 
         [0018]    The present invention provides an electrode with high packing density and high reliability. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0019]      FIG. 1  is a plan view of a positive electrode according to an embodiment before compression; 
           [0020]      FIG. 2  is a sectional view taken along line II-II in  FIG. 1 ; 
           [0021]      FIG. 3  illustrates a stretching unit and a compression unit; 
           [0022]      FIGS. 4A to 4C  illustrate stretching rollers and guide rollers; 
           [0023]      FIG. 5  illustrates a positive electrode in contact with a stretching roller; 
           [0024]      FIG. 6  illustrates a positive electrode in contact with a stretching roller according to a first modification; 
           [0025]      FIG. 7  illustrates a positive electrode in contact with a stretching roller according to a second modification; 
           [0026]      FIG. 8  is a plan view of a positive electrode according to a third modification before compression; 
           [0027]      FIG. 9  is a sectional view taken along line IX-IX in  FIG. 8 ; 
           [0028]      FIG. 10  illustrates a stretching unit and a compression unit according to the third modification; 
           [0029]      FIGS. 11A to 11C  illustrate stretching rollers and guide rollers according to the third modification; and 
           [0030]      FIG. 12  is a sectional view of a prismatic secondary battery. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0031]    A positive electrode for nonaqueous electrolyte secondary batteries according to an embodiment of the present invention will now be described by way of example. It should be understood that the following embodiments are not intended to limit the present invention. Positive Electrode Active Material Layer Slurry 
         [0032]    A positive electrode active material layer slurry is prepared by mixing lithium nickel cobalt manganese oxide, serving as a positive electrode active material, poly(vinylidene fluoride) (PVdF), serving as a binder, a carbonaceous material, serving as a conductor, and N-methylpyrrolidone (NMP), serving as a solvent. Lithium nickel cobalt manganese oxide, PVdF, and the carbonaceous material are mixed in a mass ratio of 97.5:1:1.5. The mass percentage of the positive electrode active material in the positive electrode active material layer is preferably 95% by mass or more and is preferably 99% by mass or less. The mass percentage of the binder in the positive electrode active material layer is preferably less than 5% by mass, more preferably 3% by mass or less. The mass percentage of the binder in the positive electrode active material layer is preferably 0.5% by mass or more. 
         [0033]    Protective Layer Slurry 
         [0034]    A protective layer slurry is prepared by mixing alumina powder, graphite, serving as a conductor, poly(vinylidene fluoride) (PVdF), serving as a binder, and N-methylpyrrolidone (NMP), serving as a solvent. Alumina powder, graphite, and PVdF are mixed in a mass ratio of 83:3:14. The mass percentage of the binder in the protective layers is preferably 5% by mass or more, more preferably 8% by mass or more, even more preferably 10% by mass or more. Although the protective layers may be made of the binder alone, they preferably contain inorganic oxides such as alumina, zirconia, titania, and silica. The protective layers preferably contain no positive electrode active material. If the protective layers contain a positive electrode active material, the mass percentage of the positive electrode active material in the protective layers is preferably 10% by mass or less, more preferably 5% by mass or less, even more preferably 1% by mass or less. 
         [0035]    Active-Material-Layer Forming Step and Protective-Layer Forming Step 
         [0036]    The positive electrode active material layer slurry and the protective layer slurry are applied to each surface of a strip of aluminum foil with a thickness of 15 μm, serving as a positive electrode substrate, by a die coater. The positive electrode active material layer slurry is applied to the lateral center of the positive electrode substrate, whereas the protective layer slurry is applied along the lateral edges of the region where the positive electrode active material layer slurry is applied. The positive electrode active material layer slurry and the protective layer slurry may be simultaneously applied to the positive electrode substrate using a single die coater, where the positive electrode active material layer slurry and the protective layer slurry may be brought together inside a die head near a slot. The positive electrode active material layer slurry and the protective layer slurry, however, need not be simultaneously applied to the positive electrode substrate. 
         [0037]    The positive electrode active material layer slurry and the protective layer slurry applied to the positive electrode substrate are dried to remove NMP therefrom. In this way, a positive electrode active material layer and protective layers are formed. 
         [0038]      FIG. 1  is a plan view of a positive electrode  1  fabricated as described above before compression.  FIG. 2  is a sectional view taken along line II-II in  FIG. 1 . As shown in  FIG. 1 , the positive electrode  1  includes a positive electrode substrate la and a positive electrode active material layer  1   b  extending in the lateral center of the positive electrode  1  in the longitudinal direction of the positive electrode  1 . The positive electrode  1  further includes protective layers (first protective layer  1   c   1  and second Protective layer  1   c   2 ) extending along the lateral edges of the region of the positive electrode substrate  1   a  where the positive electrode active material layer  1   b  is formed. The positive electrode substrate  1   a  has exposed regions (first exposed region  1   d   1  and second exposed region  1   d   2 ) extending along the lateral edges of the positive electrode  1  in the longitudinal direction of the positive electrode  1 . 
         [0039]      FIG. 3  illustrates a stretching unit  20  for stretching the exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate la of the positive electrode  1  and a compression unit  30  for compressing the positive electrode  1 . The positive electrode  1 , which is strip-shaped, passes through the stretching unit  20 , which stretches the exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a,  and then through the compression unit  30 , which compresses the positive electrode active material layer  1   b.  The compression of the positive electrode active material layer  1   b  increases its packing density. 
         [0040]    As shown in  FIG. 3 , the stretching unit  20  includes a first stretching roller  21 , a second stretching roller  22 , and three guide rollers  23 .  FIG. 4A  is a plan view of the first stretching roller  21 .  FIG. 4B  is a plan view of the second stretching roller  22 .  FIG. 4C  is a plan view of the guide rollers  23 . 
         [0041]    The first stretching roller  21  includes a body  21   a  and a large-diameter portion  21   b  having a larger diameter than the body  21   a  on one side of the body  21   a  in the lateral direction. The second stretching roller  22  includes a body  22   a  and a large-diameter portion  22   b  having a larger diameter than the body  22   a  on the other side of the body  22   a  in the lateral direction. The large-diameter portion  21   b  of the first stretching roller  21  is located on one side (closer to the viewer in  FIG. 3 ) in the lateral direction (from front to rear in  FIG. 3  and from left to right in  FIGS. 4A to 4C ), whereas the large-diameter portion  22   b  of the second stretching roller  22  is located on the other side (farther away from the viewer in  FIG. 3 ) in the lateral direction. 
         [0042]    The positive electrode  1  passes through the stretching unit  20  while being tensioned by the rollers  21  to  23 . The large-diameter portion  21   b  of the first stretching roller  21  pushes the first exposed region  1   d   1  of the positive electrode substrate  1   a  of the positive electrode  1  outward, thereby stretching the first exposed region  1   d   1  of the positive electrode substrate  1   a.  The large-diameter portion  22   b  of the second stretching roller  22  pushes the second exposed region  1   d   2  of the positive electrode substrate  1   a  of the positive electrode  1  outward, thereby stretching the second exposed region  1   d   2  of the positive electrode substrate  1   a.  The exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a  are thus stretched before compression. 
         [0043]      FIG. 5  illustrates how the large-diameter portion  21   b  of the first stretching roller  21  pushes the first exposed region  1   d   1  of the positive electrode substrate  1   a  of the positive electrode  1  outward. The large-diameter portion  21   b  of the first stretching roller  21  pushes the first exposed region  1   d   1  of the positive electrode substrate  1   a  of the positive electrode  1  outward, thereby stretching the first exposed region  1   d   1  of the positive electrode substrate  1   a.  Similarly, the large-diameter portion  22   b  of the second stretching roller  22  pushes the second exposed region  1   d   2  of the positive electrode substrate  1   a  of the positive electrode  1  outward. 
         [0044]    Although not illustrated in  FIG. 5 , the body  21   a  of the first stretching roller  21  is in contact with the positive electrode active material layer  1   b  of the positive electrode  1  in the lateral center of the positive electrode  1 . Similarly, the body  22   a  of the second stretching roller  22  is in contact with the positive electrode active material layer  1   b  of the positive electrode  1  in the lateral center of the positive electrode  1 . 
         [0045]    After the exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a  are stretched, the positive electrode  1  passes through the compression unit  30 , where a pair of compression rollers  31  compress the positive electrode  100  to increase the packing density of the positive electrode active material layer  1   b.  During this process, the region of the positive electrode substrate  1   a  where the positive electrode active material layer  1   b  is formed is rolled and stretched in the longitudinal direction of the positive electrode  1 . The stretching of the exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a  of the positive electrode  1  in advance prevents defects such as creases in the positive electrode substrate  1   a  and warpage in the positive electrode  1  when the region of the positive electrode substrate  1   a  where the positive electrode active material layer  1   b  is formed is rolled during compression. 
         [0046]    The compressed positive electrode  1  is cut along line C-C in  FIG. 2 , followed by coiling and cutting to a predetermined length for the fabrication of electrode assemblies. 
         [0047]    The positive electrode  1  has the protective layers  1   c   1  and  1   c   2  extending along the lateral edges of the positive electrode active material layer  1   b  on the positive electrode substrate  1   a.  This prevents problems such as damage to the positive electrode active material layer  1   b  and its separation from the positive electrode substrate  1   a  when the exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a  are stretched before the compression of the positive electrode  1 . The protective layers  1   c   1  and  1   c   2 , however, are optional and may be omitted. 
         [0048]    As described above, it is preferred to separately stretch the exposed regions of the positive electrode substrate with different stretching rollers. The separate stretching of the exposed regions of the positive electrode substrate allows them to be more reliably and uniformly stretched and, if necessary, allows them to be stretched to their respective intended degrees. Alternatively, the exposed regions of the positive electrode substrate may be simultaneously stretched with a single stretching roller. 
         [0049]    As shown in  FIG. 5 , the height H of the large-diameter portion  21   b  from the body  21   a  of the first stretching roller  21  is preferably sufficiently larger than the thickness T of the region of the positive electrode  1  where the positive electrode active material layer  1   b  is formed before compression. Specifically, H (mm)/T (mm) is preferably 3 or more, more preferably 5 or more, even more preferably 7 or more. This allows the first exposed region  1   d   1  of the positive electrode substrate  1   a  to be more reliably and uniformly stretched. In this case, the body  21   a  of the first stretching roller  21  is preferably in contact with the positive electrode active material layer  1   b  of the positive electrode  1 . The large-diameter portion  22   b  of the second stretching roller  22  is preferably similar to the large-diameter portion  21   b  of the first stretching roller  21 . 
         [0050]    The stretching unit  20  preferably applies a tension of 100 to 600 N, more preferably 200 to 500 N, to the positive electrode  1 . The positive electrode  1  preferably passes through the stretching unit  20  at a speed of 10 to 110 m/min, more preferably 30 to 100 m/min. The first stretching roller  21 , the second stretching roller  22 , and the guide rollers  23  are each preferably made of a metal or resin. For example, the central shaft of each roller may be a cylindrical member made of a metal such as stainless steel, and the surface layer may be a tubular resin member. In this case, the central shaft of the first stretching roller  21  is a cylindrical metal member, and the surface layer is a resin member. The surface layer of the body  21   a  and the large-diameter portion  21   b  are made of a resin. 
         [0051]    The protective layer slurry preferably has a lower viscosity than the positive electrode active material layer slurry when they are applied to the positive electrode substrate  1   a.  If no protective layer is provided, the positive electrode active material layer slurry may form wavy, non-straight lateral edges when applied. This may result in variations in the width of the exposed regions of the positive electrode substrate and may therefore result in variations in the degree of stretching of the exposed regions of the positive electrode substrate during stretching. In contrast, if a protective layer slurry having a lower viscosity than the positive electrode active material layer slurry is applied to the positive electrode substrate, it forms protective layers with more straight edges and thus allows for less variation in the width of the exposed regions of the positive electrode substrate. 
         [0052]    The positive electrode active material layer slurry preferably has a viscosity of 1.50 Pa—s or more, more preferably 1.50 to 3.50 Pa·s, even more preferably 1.80 to 3.00 Pa·s. The protective layer slurry preferably has a viscosity of 0.50 to 1.8 Pa·s, more preferably 0.60 to 1.50 Pa·s. The viscosity can be adjusted by changing the type and amount of binder in the slurry and the amount of solvent. The viscosity of the positive electrode active material layer slurry and the viscosity of the protective layer slurry can be measured with a spiral viscometer (PC-1TL, Malcom Co., Ltd.) at 40 rpm and 25° C. 
         [0053]    First Modification 
         [0054]    In the step of stretching the exposed regions of the positive electrode substrate, the large-diameter portions of the stretching rollers may be in contact with the exposed regions of the positive electrode substrate, the protective layers, or both during the stretching of the exposed regions of the positive electrode substrate. For example, as shown in  FIG. 6 , the large-diameter portion  21   b  of the first stretching roller  21  may be in contact with the first protective layer  1   c   1 . Preferably, the large-diameter portion  21   b  abuts the first protective layer  1   c   1  of the positive electrode  1  to stretch both the first exposed region  1   d   1  of the positive electrode substrate  1   a  and the region of the positive electrode substrate  1   a  where the first protective layer  1   c   1  is formed. This more reliably prevents creases in the first exposed region  1   d   1  of the positive electrode substrate  1   a  and warpage in the positive electrode  1  if the region of the positive electrode substrate  1   a  where the first protective layer  1   c   1  is formed is not rolled or is rolled to a lower extent than the region where the positive electrode active material layer  1   b  is formed during the compression of the positive electrode active material layer  1   b.    
         [0055]    The first modification is particularly effective if the mass fraction of the binder in the protective layers is larger than the mass fraction of the binder in the active material layer. The first modification is more effective if the first protective layer  1   c   1  is thinner than the positive electrode active material layer  1   b.  If the protective layers, such as the first protective layer  1   c   1 , contain more binder than the active material layer, such as the positive electrode active material layer  1   b,  the protective layers are more flexible than the active material layer. Such protective layers are less likely to be damaged when pushed by the large-diameter portion  21   b.    
         [0056]    The large-diameter portion  21   b  of the first stretching roller  21  is preferably not in contact with the positive electrode active material layer  1   b.  This more reliably prevents damage to the edges of the positive electrode active material layer  1   b  due to contact with the large-diameter portion  21   b  of the first stretching roller  21 . As shown in  FIG. 6 , a clearance is preferably provided between the large-diameter portion  21   b  of the first stretching roller  21  and the positive electrode active material layer  1   b.  Specifically, the shortest distance between the large-diameter portion  21   b  of the first stretching roller  21  and the positive electrode active material layer  1   b  is preferably larger than the thickness of the positive electrode substrate  1   a  before compression. 
         [0057]    As in the first modification, the first and second exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a  were stretched by 0.3% in the longitudinal direction of the positive electrode  1  before the positive electrode  1  was compressed to increase the packing density of the positive electrode active material layer  1   b  to 2.9 g/cm 3 . The resulting positive electrode  1  had no defects such as creases or warpage. 
         [0058]    Second Modification 
         [0059]      FIG. 7 , which corresponds to  FIG. 5 , illustrates a positive electrode substrate according to a second modification. As shown in  FIG. 7 , the end surfaces lx of the lateral edges of the positive electrode active material layer  1   b  may be inclined (at an angle of less than 90°) from the positive electrode substrate  1   a,  and part of the first protective layer  1   c   1  may be located on the end surfaces  1   x  of the positive electrode active material layer  1   b.  This more effectively prevents problems such as damage to the positive electrode active material layer  1   b  and its separation from the positive electrode substrate  1   a  when the exposed regions  1   d   1  and  1   d   2  of the positive electrode substrate  1   a  are stretched before the compression of the positive electrode  1 . 
         [0060]    Third Modification 
         [0061]      FIG. 8  is a plan view of a positive electrode  100  according to a third modification before compression.  FIG. 9  is a sectional view taken along line IX-IX in  FIG. 8 . The positive electrode  100  includes a positive electrode substrate  100   a  and two positive electrode active material layers (first positive electrode active material layer  100   b   1  and second positive electrode active material layer  100   b   2 ) extending across the positive electrode substrate  100   a  in the longitudinal direction. The positive electrode  100  further includes first and second protective layers  100   c   1  and  100   c   2  extending along the lateral edges of the first positive electrode active material layer  100   b   1  in the longitudinal direction of the positive electrode  100 . The positive electrode  100  further includes third and fourth protective layers  100   c   3  and  100   c   4  extending along the lateral edges of the second positive electrode active material layer  100   b   2  in the longitudinal direction of the positive electrode  100 . The positive electrode substrate  100   a  has first, second, and third exposed regions  100   d   1 ,  100   d   2 , and  100   d   3  separated from each other in the lateral direction and extending in the longitudinal direction of the positive electrode  100 . 
         [0062]      FIG. 10  illustrates a stretching unit  200  and a compression unit  30  according to the third modification.  FIGS. 11A to 11C  illustrate a first stretching roller  210 , a second stretching roller  220 , and a third stretching roller  230 , respectively, of the stretching unit  200  according to the third modification. 
         [0063]    The first stretching roller  210  includes a body  210   a  and a large-diameter portion  210   b  having a larger diameter than the body  210   a  on one side of the body  210   a  in the lateral direction. The second stretching roller  220  includes a body  220   a  and a large-diameter portion  220   b  having a larger diameter than the body  220   a  on the other side of the body  220   a  in the lateral direction. The third stretching roller  230  includes a body  230   a  and a large-diameter portion  230   b  having a larger diameter than the body  230   a  in the center of the body  230   a  in the lateral direction. 
         [0064]    In  FIG. 10 , the large-diameter portion  210   b  of the first stretching roller  210  is located closer to the viewer, the large-diameter portion  220   b  of the second stretching roller  220  is located farther away from the viewer, and the large-diameter portion  230   b  of the third stretching roller  230  is located in the center. Thus, before compression, the positive electrode  100  is stretched in the following order: the first exposed region  100   d   1 , the third exposed region  100   d   3 , and the second exposed region  100   d   2 . The positive electrode  100  is then compressed by the compression unit  30 , which compresses the positive electrode active material layer  100   b  to increase its packing density. The compressed positive electrode  100  is cut along lines C-C in  FIG. 9 , followed by coiling and cutting to a predetermined length for the fabrication of electrode assemblies. 
         [0065]    Prismatic Secondary Battery 
         [0066]    A secondary battery including a positive electrode fabricated as described above is illustrated by a prismatic secondary battery  50  shown in  FIG. 12 . 
         [0067]    Fabrication of Negative Electrode 
         [0068]    A negative electrode active material layer slurry is prepared by mixing graphite, serving as a negative electrode active material, styrene-butadiene rubber (SBR), serving as a binder, carboxymethylcellulose (CMC), serving as a thickener, and water. The negative electrode active material layer slurry is applied to each surface of a strip of copper foil with a thickness of 8 μm, serving as a negative electrode substrate. The negative electrode active material layer slurry is dried to remove water therefrom, thus forming a negative electrode active material layer on the negative electrode substrate. The negative electrode active material layer is then compressed to a predetermined thickness. The resulting negative electrode is cut into a strip such that the negative electrode substrate has an exposed region  5  extending along one lateral edge of the negative electrode in the longitudinal direction. 
         [0069]    Fabrication of Electrode Assembly 
         [0070]    The strip-shaped positive electrode  1  and negative electrode fabricated as described above are wound together with a polyolefin separator therebetween and are pressed into a flat shape. The resulting flat wound electrode assembly  2  has the exposed region ld of the positive electrode substrate wound at one end in the winding axis direction and the exposed region  5  of the negative electrode substrate wound at the other end in the winding axis direction. 
         [0071]    As shown in  FIG. 12 , the wound electrode assembly  2  and an electrolyte are placed in a prismatic metal casing  3  with an opening. The opening of the casing  3  is sealed with a metal sealing plate  4 . A resin insulating sheet  14  is disposed between the wound electrode assembly  2  and the casing  3 . 
         [0072]    A metal positive electrode current collector  6  connected to the exposed region  1   d  of the positive electrode substrate of the wound electrode assembly  2  is electrically connected to a positive terminal  7  attached to the sealing plate  4 . The positive electrode current collector  6  and the sealing plate  4  are insulated from each other by a resin insulating member  10 . The positive terminal  7  and the sealing plate  4  are insulated from each other by a resin insulating member  11 . A metal negative electrode current collector  8  connected to the exposed region  5  of the negative electrode substrate of the wound electrode assembly  2  is electrically connected to a negative terminal  9  attached to the sealing plate  4 . The negative electrode current collector  8  and the sealing plate  4  are insulated from each other by a resin insulating member  12 . The negative terminal  9  and the sealing plate  4  are insulated from each other by a resin insulating member  13 . 
         [0073]    The sealing plate  4  has an inlet  15  through which a liquid electrolyte containing an electrolyte salt such as a lithium salt and a nonaqueous solvent such as a carbonate is injected. After injection, the inlet  15  is sealed with a sealing plug  16 . The sealing plate  4  has a vent  17  that opens and releases gas from the battery when the internal pressure of the battery is at or above a predetermined level. A current interruption mechanism may be provided in the conduction path between the positive electrode  1  and the positive terminal  7  or in the conduction path between the negative electrode and the negative terminal  9 . The current interruption mechanism is preferably activated to break the conduction path when the internal pressure of the battery is at or above a predetermined level. The activation pressure of the current interruption mechanism is set to be lower than the activation pressure of the vent. 
         [0074]    Other Embodiments 
         [0075]    Although the present invention may be applied to both positive electrodes and negative electrodes, it is particularly effective for positive electrodes. Specifically, the present invention is effective for a positive electrode including a positive electrode active material layer having a packing density after compression of 2.7 g/cm 3  or more, particularly 2.85 g/cm 3  or more. 
         [0076]    Although electrodes fabricated according to the present invention may be used for both wound electrode assemblies and laminated electrode assemblies, they are particularly effective for wound electrode assemblies. 
         [0077]    Preferred positive electrode active materials include lithium transition metal oxides, particularly those containing at least one of nickel, cobalt, and manganese. 
         [0078]    The negative electrode active material may be a carbonaceous material capable of absorbing and releasing lithium ions. Examples of carbonaceous materials capable of absorbing and releasing lithium ions include graphite, nongraphitizable carbon, graphitizable carbon, filamentous carbon, coke, and carbon black. Examples of noncarbonaceous materials include silicon, tin, and alloys and oxides thereof. 
         [0079]    Examples of binders that may be present in the active material layers and protective layers of the electrodes include poly(vinylidene fluoride) (PVdF), polytetrafluoroethylene, polyethylene, polypropylene, aramid resins, polyamides, polyimides, polyamide-imides, polyacrylonitrile, poly(acrylic acid), poly(methyl acrylate), poly(ethyl acrylate), poly(hexyl acrylate), poly(methacrylic acid), poly(methyl methacrylate), poly(ethyl methacrylate), poly(hexyl methacrylate), poly(vinyl acetate), polyvinylpyrrolidone, polyethers, polyethersulfones, polyhexafluoropropylene, styrene-butadiene rubber, carboxymethylcellulose, acrylic rubbers, and acrylate binders (acrylate esters and salts). These may be used alone or in combination. The active material layers and the protective layers may contain the same or different binders. Resin binders are preferred. 
         [0080]    While detailed embodiments have been used to illustrate the present invention, to those skilled in the art, however, it will be apparent from the foregoing disclosure that various changes and modifications can be made therein without departing from the spirit and scope of the invention. Furthermore, the foregoing description of the embodiments according to the present invention is provided for illustration only, and is not intended to limit the invention.