Patent Publication Number: US-2011070492-A1

Title: Electrode for a lithium secondary battery and lithium secondary battery equipped with same

Description:
TECHNICAL FIELD 
     The present invention relates to an electrode for a lithium secondary battery and a lithium secondary battery including the same. 
     BACKGROUND ART 
     Recently, along with the development of portable devices such as personal computers, mobile phones and the like, there is an increasing demand for cells as power supplies of these devices. Cells for such uses are required to have a high energy density. In order to fulfill such a requirement, lithium secondary batteries are paid attention to. For both of a positive electrode and a negative electrode of a lithium secondary battery, active materials having a larger capacity than conventional active materials have been developed. Among these materials, single elements of silicon (Si) and tin (Sn), an oxide and an alloy thereof are considered to be promising as an active material for providing a very large capacity. 
     However, where an electrode for a lithium secondary battery is formed using such an active material, there is a problem that the electrode is deformed as charge/discharge is repeated. Any of the above-described active materials causes a significant volume change when reacting with lithium ions, and therefore significantly expands or contracts by the reaction occurring when lithium ions are inserted into, or released from, the active material during charge/discharge. Therefore, when the charge/discharge is repeated, the electrode receives a significant stress and so is distorted, which may undesirably cause wrinkles, breaks or the like. When the electrode is distorted and thus deformed, a gap is made between the electrode and the separator, which may undesirably make the charge/discharge reaction non-uniform and thus locally reduce the cell characteristics. For these reasons, it has been difficult to obtain a lithium secondary battery having a sufficiently high charge/discharge cycle characteristic using any of the above-mentioned active materials. 
     In order to solve these problems, it has been proposed to form a space in the active material layer for allowing the active material to expand. 
     For example, Patent Document No. 1 filed by the present applicant proposes an electrode in which an active material layer including a plurality of column-like active material bodies is formed on a surface of a current collector. The plurality of active material bodies are arranged with an interval held therebetween on the surface of the current collector. Such an electrode is produced by depositing a vaporized vapor deposition material (for example, silicon) on a current collector, having a plurality of bumps at a surface thereof, in a direction inclined with respect to the normal direction to the surface of the current collector (oblique vapor deposition). When being deposited by oblique vapor deposition, silicon is easily incident and deposited on a top part of each bump of the surface of the current collector, and is difficult to be deposited on an area of the surface which is in the shadow of the bump (and of the material deposited on the bump). Therefore, a column-like active material body containing silicon can be formed on each bump, and also a space for allowing the volume of the active material body to expand can be obtained with certainty between adjacent active material bodies. Owing to such a structure, the stress applied on the current collector by contact between the active material bodies expanded at the time of charge can be reduced, and thus the deformation of the electrode due to the contraction and expansion of the active material bodies can be suppressed. 
     Patent Document No. 2 proposes applying a tensile load on the current collector after forming an active material layer on the current collector, thus to form breaking lines in the active material layer. Patent Document No. 2 describes that the breaking lines formed in the active material layer act as a space for alleviating the stress generated by the expansion of the active material, and so the charge/discharge cycle characteristic can be improved. 
     CITATION LIST 
     Patent Literature 
     Patent Document No. 1: International Publication WO2009/019869 pamphlet 
     Patent Document No. 2: Japanese Laid-Open Patent Publication No. 2006-260928 
     SUMMARY OF INVENTION 
     Technical Problem 
     With the conventional method for producing an electrode, there are cases where a space for sufficiently alleviating the stress generated by the expansion of the active material is not formed in the active material layer. 
     As described in Patent Document No. 1, with the method of producing an electrode using oblique vapor deposition, a plurality of active material bodies are formed with an interval held therebetween using a shadowing effect. Therefore, it is difficult to make the interval between the active material bodies larger than an area of the surface of the current collector which is in the shadow of the bump or the like. As a result, as described below in more detail, it is occasionally difficult to increase the ratio of the space with respect to the active material layer, although the ratio depends on the height of the bumps of the current collector or the vapor deposition angle. 
     With the method described in Patent Document No. 2, there is a possibility that a sufficiently large space for alleviating the stress generated by charge/discharge is not formed merely by the breaking lines made in the active material layer by the tensile load. In addition, the electrode is further elongated by charge/discharge and as a result, the electrode may be undesirably wrinkled. Therefore, there is an undesirable possibility that the deformation of the electrode or the reduction of the cell reliability caused by the stress generated by charge/discharge of the cell is not effectively suppressed. 
     Solution to Problem 
     A method for producing an electrode for a lithium secondary battery according to the present invention includes (A) a step of causing a vaporized vapor deposition material to be incident on a surface of a current collector, having a plurality of bumps at the surface thereof, in a direction inclined with respect to the normal direction to the surface of the current collector, thus to form an active material body on each of the plurality of bumps of the current collector; and (B) a step of stretching the current collector having the active material bodies formed thereon in at least one axial direction parallel to the surface of the current collector. 
     According to the present invention, in the step (A), the active material body is formed on each bump of the current collector, and so a space can be formed between adjacent active material bodies. Then, by stretching the current collector having the active material bodies formed thereon in step (B), the space between the active material bodies can be further enlarged. Therefore, a sufficiently large space for alleviating the expansion of the active material caused by charge/discharge can be formed. 
     Therefore, the deformation of the electrode due to the stress generated by the expansion of the active material can be suppressed, and so a lithium secondary battery having a splendid charge/discharge cycle characteristic and a high reliability can be provided. 
     ADVANTAGEOUS EFFECTS OF INVENTION 
     According to the present invention, in an electrode for a lithium secondary battery including a current collector and a plurality of active material bodies formed on a surface of the current collector, a larger space can be formed between the active material bodies. As a result, the ratio of the space with respect to the active material layer including the plurality of active material bodies can be increased than in the conventional electrodes. Therefore, the stress applied on the current collector by contact between adjacent active material bodies when the active material bodies are expanded can be significantly suppressed. Thus, the deformation of the current collector due to the contraction and expansion of the active material bodies can be suppressed, and so the charge/discharge cycle characteristic and the reliability can be improved. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view showing an example of a conventional method for producing an electrode using oblique vapor deposition. 
         FIGS. 2(   a ) and ( b ) are schematic cross-sectional views for illustrating a vapor deposition step and a stretching step in an embodiment according to the present invention, respectively. 
         FIGS. 3(   a ) through ( d ) are schematic cross-sectional views for illustrating stages of the vapor deposition step in the method for producing an electrode in an embodiment according to the present invention. 
         FIGS. 4(   a ) and ( b ) are respectively a plan view and a cross-sectional view showing an example of a current collector usable in an embodiment according to the present invention. 
         FIG. 5  is a cross-sectional view for illustrating a vapor deposition device usable in the vapor deposition step in an embodiment according to the present invention. 
         FIGS. 6(   a ) through ( c ) schematically show a current collector after an active material layer is formed by the method for producing an electrode in an embodiment according to the present invention, and  FIGS. 6(   a ) and ( b ) are cross-sectional views respectively taken along line I-I′ and line II-II′ in  FIG. 6(   c ), and  FIG. 6(   c ) is a plan view. 
         FIGS. 7(   a ) through ( c ) schematically show an electrode  200  obtained by the method for producing an electrode in an embodiment according to the present invention, and  FIGS. 7(   a ) and ( b ) are cross-sectional views respectively taken along line I-I′ and line II-II′ in  FIG. 7(   c ), and  FIG. 7(   c ) is a plan view. 
         FIG. 8  is a schematic view showing a cylindrical cell using an electrode according to the present invention. 
         FIGS. 9(   a ) through ( c ) are schematic cross-sectional views for illustrating steps of a method for producing a current collector in an example and a comparative example. 
         FIG. 10  is a plan view showing the shape of a surface of the current collector in the example and the comparative example. 
         FIG. 11  shows a top surface of the active material layer in the comparative example. 
         FIG. 12(   a ) shows a top surface of an active material layer in Example 1, and  FIG. 12(   b ) is an enlarged view of  12 ( a ). 
         FIG. 13  illustrates a stretching step in Example 2. 
         FIG. 14  illustrates another stretching step performed by rolling. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an embodiment of a method for producing an electrode for a lithium secondary battery according to the present invention will be described. The method in the embodiment is applicable to produce both a negative electrode and a positive electrode of a lithium secondary battery, but is preferably usable to produce a negative electrode. 
     A method for producing an electrode according to the present invention includes a vapor deposition step of forming active material bodies on a surface of a current collector by oblique vapor deposition; and a stretching step of stretching the current collector having the active material bodies formed thereon in at least one axial direction which is parallel to the surface of the current collector. Owing to this, the space between the active material bodies can be enlarged as compared with in the conventional electrodes produced using oblique vapor deposition. The reason for this will be described below. 
       FIG. 1  is a schematic cross-sectional view showing an example of a conventional method for producing an electrode using oblique vapor deposition.  FIGS. 2(   a ) and ( b ) are schematic cross-sectional views respectively for illustrating an overview of the vapor deposition step and an overview of the stretching step in this embodiment. For easier understanding, in the stretching step shown in  FIG. 2(   b ), the current collector is stretched in a direction parallel to the vapor deposition direction on a plane parallel to the surface of the current collector, and the cross-section will be compared with the cross-section including the vapor deposition direction of the conventional electrode shown in  FIG. 1 . 
     First, a reason why it is difficult to make the interval between the active material bodies larger than a prescribed interval in the conventional electrode produced by oblique vapor deposition will be described in detail. 
     In the example shown in  FIG. 1 , a vapor deposition material is incident on a current collector  11  having a plurality of bumps  12  at a surface thereof in a direction  52  inclined by angle ω with respect to the normal D to the surface of the current collector  11 . As a result, an active material body  14 ′ is formed on each bump  12 . Between adjacent active material bodies  14 ′, a space  16 ′ is formed. The space  16 ′ is formed in an area which is in the shadow of the bump  12  having height H and on which the vapor deposition material is not incident. Therefore, where the bypassing of, or the collision between, the vapor deposition particles are ignored, a width of the space  16 ′ on the surface of the current collector  11  is determined by the height H of the bump  12  and the vapor deposition angle ω, and is H·tan ω. 
     With this method, even if the interval between the bumps  12  of the current collector  11  is made larger than H·tan ω, the width of the space  16 ′ on the surface of the current collector  11  is not made larger than H·tan ω. When the interval between the bumps  12  is increased, the amount of the active material deposited in an area (dent) of the current collector  11  on which the bumps  12  are not formed is increased, and the width of the active material bodies  14 ′ is increased. As a result, the ratio of the space with respect to a layer (active material layer)  15 ′ which includes the plurality of active material bodies  14 ′ and the space  16 ′ is rather decreased. 
     Therefore, in order to increase the width of the space  16 ′, the height H of each bump  12  needs to be increased or the vapor deposition angle ω needs to be made closer to 90°. However, if the height H is increased, the thickness of the current collector  11  is increased. As a result, the volume of the current collector  11  with respect to the electrode is increased, which decreases the capacity. Therefore, it may be undesirably impossible to obtain a high energy density with certainty. It is not practical to make the vapor deposition angle ω closer to 90° because the productivity is drastically reduced. Therefore, it may be occasionally difficult to increase the ratio of the space  16 ′ with respect to the active material layer without decreasing the energy density or the adherence of the active material bodies. 
     By contrast, in this embodiment, as shown in  FIG. 2(   a ), first, an active material layer  15   a  including a plurality of active material bodies  14  is formed by oblique vapor deposition. In the active material layer  15   a , each active material body  14  is formed on the bump  12  and a space  16   a  is formed between adjacent active material bodies  14 . Width Ea of the space  16   a  on the surface of the current collector  11  is H·tan ω as in the conventional electrode shown in  FIG. 1 . 
     Next, as shown in  FIG. 2(   b ), the current collector  11  having the active material layer  15   a  formed thereon is stretched. Thus, an electrode  200  is obtained. 
     By the stretching step, an area of the current collector  11  on which the active material bodies  14  are not formed is elongated, and so the interval between the active material bodies  14  on the surface of the current collector  11  is increased. Therefore, in the post-stretching active material layer  15   b , width Eb of a space  16   b  on the surface of the current collector  11  is larger than the width of the space  16   a  of the pre-stretching active material layer  15   a . The width Eb of the space  16   b  can be made larger than the length determined by the height H of the bump  12  and the vapor deposition angle ω, namely, can be larger than H·tan ω. 
     Moreover, areas of the current collector at which the bumps  12  are formed, namely, areas on which the active material bodies  14  are formed, are thicker than the area of the current collector  11  which is a dent and on which the active material bodies  14  are not formed, and so are unlikely to be elongated by the stretching step. Therefore, with the method in this embodiment, the space  16   b  between the active material bodies  14  can be enlarged without increasing the width of the active material bodies  14 . As a result, the ratio of the space  16   b  with respect to the active material layer  15   b  can be increased as compared with in the conventional electrode. 
     Now, with reference to the figures, the method for producing an electrode in this embodiment will be described more specifically. 
       FIGS. 3(   a ) through ( d ) are schematic views for illustrating stages of the vapor deposition step in this embodiment. Herein, the vapor deposition is performed a plurality of times while the vapor deposition direction is alternately inverted with respect to the normal to the surface of the current collector. When the vapor deposition step is performed for first through n&#39;th stages (n≧2) while the vapor deposition direction is changed, the obtained active material bodies each include n pieces of portions in accordance with the growth direction thereof. In this specification, the n pieces of portions will be referred to as the “first portion, second portion, . . . n&#39;th portion” from the surface side of the current collector. 
     First, as shown in  FIG. 3(   a ), the current collector  11  having a plurality of bumps  12  at the surface thereof is produced. The current collector  11  is obtained by, for example, transferring a concaved and convexed pattern on a copper foil using a roller used for a rolling method and having the concaved and convexed pattern formed at a surface thereof. The plurality of bumps  12  are regularly arranged with an interval held therebetween at the surface of the current collector  11 . The expression “regularly arranged” indicates that the interval between adjacent bumps  12  is adjusted to be equal to, or greater than, a prescribed distance, and this concept does not encompass concaved and bumps formed at the surface by roughening. The plurality of bumps  12  do not need to be arranged at an equal interval. The bumps  12  do not need to have generally the same shape, and may be different in the width or the height. 
       FIG. 4(   a ) is a schematic plan view showing an example of the bumps  12  of the current collector  11  in this embodiment, and  FIG. 4(   b ) is a cross-sectional view taken along line I-I′ of  FIG. 4(   a ). In the example shown here, the bumps  12  are each like a column having a rhombic top surface, and are arranged in a lattice (including houndstooth check) at the surface of the current collector  11 . Preferable ranges of the height H and arrangement pitches Pa, Pb and Pc of the bumps  12 , etc. will be described later. 
     Next, as shown in  FIG. 3(   b ), a vaporized vapor deposition material (for example, silicon) is incident on the surface of the current collector  11  in the direction  52  inclined at a prescribed angle (hereinafter, referred to as the “vapor deposition angle”) ω with respect to the normal D to the surface of the current collector  11 . As a result, a first portion  14   a  of the active material body containing silicon is formed on each bump  12  (first-stage vapor deposition step). 
       FIG. 5  is a schematic view showing an example of a structure of a vapor deposition device usable for forming the active material bodies. A vapor deposition device  40  includes a vacuum chamber  41  and an exhaust pump  47  for exhausting the vacuum chamber  41 . Inside the vacuum chamber  41 , a fixing table  43  for fixing the current collector  11 , a gas introduction pipe  42  for introducing oxygen gas into the vacuum chamber  41 , and a crucible  46  loaded with a vapor deposition source for supplying silicon to the surface of the current collector  11  are installed. As the vapor deposition source, silicon is usable, for example. Although not shown, electron beam heating means for vaporizing a material of the vapor deposition source is provided. The gas introduction pipe  42  includes an oxygen nozzle  45 , and is located such that oxygen gas ejected from the oxygen nozzle  45  is supplied to the vicinity of the surface of the current collector  11 . The fixing table  43  and the crucible  46  are located such that vapor deposition particles (herein, silicon atoms)  49  from the crucible  46  are incident on the surface of the current collector  11  in a direction having the angle (vapor deposition angle) ω with respect to the normal direction D to the current collector  11 . In this example, the fixing table  43  has a rotation shaft. By rotating the fixing table  43  around the rotation shaft, angle θ of the normal to the fixing table  43  with respect to a horizontal face  50  is adjusted to be equal to the prescribed vapor deposition angle ω (for example, θ=65′). Herein, the term “horizontal face” refers to a face perpendicular to a direction in which the material of the vapor deposition source placed in the crucible  46  and directed toward the fixing table  43  after being gasified. 
     In this embodiment, vapor deposition (EB vapor deposition) is performed as follows. While oxygen gas is blown to the vicinity of the surface of the current collector  11  from the oxygen nozzle  45 , silicon in the crucible  46  is dissolved by being irradiated with an electron beam (EB) provided by an electron gun (not shown) and thus is incident on the current collector  11 . In this case, on the surface of the current collector  11 , the silicon atoms  49  react with the oxygen gas and thus a silicon oxide is grown. On this stage, the silicon atoms  49  are incident on the surface of the current collector  11  in a direction inclined with respect to the normal D to the current collector  11 , and so is easily vapor-deposited on the bumps  12  at the surface of the current collector  11 . Thus, the silicon oxide is grown like columns only on the bumps  12 . By contrast, on an area of the surface of the current collector  11  which is in the shadow of the silicon oxide growing like columns, the silicon atoms are not incident and so the silicon oxide is not vapor-deposited (shadowing effect). As a result, the active material (herein, a silicon oxide) is deposited like a column on each bump  12  of the current collector  11 . Thus, the first portion  14   a  is obtained. 
     Growing direction S 1  of the first portion  14   a  is inclined at angle α 1  with respect to the normal direction D to the current collector  11 . The inclining angle α 1  is determined by the vapor deposition angle (incidence angle of silicon) ω. Specifically, it is empirically known that the inclining angle α 1  of the growth direction and the vapor deposition angle ω of silicon fulfill the relationship of 2 tan α 1 =tan ω. It is also known that by changing the amount of oxygen to be introduced and thus controlling the inner pressure of the vacuum tank, the inclining angle is decreased as compared with the inclining angle calculated based on the above-described relationship. Thus, the inclining angle α 1  can be controlled by changing the vapor deposition angle and the inner pressure of the vacuum tank. 
     The obtained first portion  14   a  has a chemical composition of SiO x . An average value of the molar ratio x of the amount of oxygen with respect to the amount of silicon in the first portion  14   a , and the thickness of the first portion  14   a , are controlled by adjusting the output at the time of vapor deposition, the time, the amount of oxygen gas to be introduced into the vacuum chamber  41  (namely, the concentration of oxygen in the atmosphere), and the like. 
     Next, as shown in  FIG. 3(   c ), the vaporized vapor deposition material (herein, silicon) is incident on the surface of the current collector  11  in a direction  62  inclined oppositely to the vapor deposition direction  52  in the first-stage vapor deposition step. As a result, a second portion  14   b  is formed on each first portion  14   a  (second-stage vapor deposition step). 
     The second-stage vapor deposition step is also performed using the vapor deposition device shown in  FIG. 5 . Specifically, after the first-stage vapor deposition step, the fixing table  43  is rotated clockwise around the rotation shaft to be inclined in the opposite direction to the inclining direction of the fixing table  43  in the first-stage vapor deposition step with respect to the horizontal face  50  (for example, θ=−65′). After this, like the first-stage vapor deposition step, silicon in the crucible  46  is vaporized and incident on the first portion  14   a  on the current collector  11 . In the cross-section shown here, the direction  62  in which the silicon atoms  49  are incident is inclined at, for example, 65° in the opposite direction to the direction  52  with respect to the normal D to the current collector  11  (ω=−65′). Like in the first-stage vapor deposition step, at the same time as the incidence of the silicon atoms  49 , oxygen gas is supplied from the oxygen nozzle  45  toward the current collector  11 . As a result, a silicon oxide (SiO x ) is selectively deposited on the first portion  14   a  on the current collector  11 , and thus the second portion  14   b  is obtained. In the cross-section shown here, growth direction S 2  of the second portion  14   b  is inclined at angle α 2  in the opposite direction to the growth direction of the first portion  14   a  with respect to the normal direction D to the current collector  11  (α 2 =−α 1 ). 
     Then, as shown in  FIG. 3(   d ), the angle θ of the fixing table  43  may be returned to the same angle as in the first-stage vapor deposition step (herein, 65°) to grow a silicon oxide in the same conditions as those of the first-stage vapor deposition step (third-stage vapor deposition step). As a result, a third portion  14   c  is further formed on the second portion  14   b . Inclining angle α 3  of the third portion  14   c  in growth direction S 3  is the same as the inclining angle α 1  of the first portion  14   a.    
     In this manner, vapor deposition is performed up to the n&#39;th stage (n≧2) while the vapor deposition angle ω is alternately changed between, for example, 65° and −65°. As a result, an active material body including n pieces portions can be formed. In this embodiment, the vapor deposition is performed up to the 35th stage to obtain a plurality of active material bodies. 
     Then, the current collector having the active material bodies formed thereon is stretched in a direction parallel to the surface of the current collector. Herein, the current collector is stretched on a plane parallel to the surface of the current collector in a direction perpendicular to the vapor deposition direction. 
       FIGS. 6(   a ), ( b ) and ( c ) schematically show the current collector after having the active material bodies formed thereon but before being stretched.  FIG. 6(   c ) is a plan view, and  FIGS. 6(   a ) and ( b ) are cross-sectional views respectively taken along line I-I′ and line II-II′ in  FIG. 6(   c ). 
     As shown in  FIG. 6(   c ), the plurality of active material bodies  14  obtained by the above-described vapor deposition step are arranged regularly in correspondence with the positions of the bumps  12  shown in  FIG. 4 . These active material bodies  14  do not contact each other, and a space  16   a  is existent between adjacent active material bodies  14 . In this specification, a layer including the plurality of active material bodies  14  and the space  16  between adjacent active material bodies  14  will be referred to as the “active material layer  15   a”.    
     Each active material body  14  may occasionally have a zigzag shape in correspondence with growth direction S, but herein, are like columns upright in the normal direction D to the current collector  11 . In the case where, as in this embodiment, the vapor deposition step is performed in many stages, for example, 30 or more stages (n≧30) or the portion formed by each stage of the vapor deposition step has an especially small thickness (for example, 0.5 μm or less), the obtained active material bodies  14  are like upright columns. Even in this case, the growth direction S of the active material bodies  14  can be confirmed to be zigzag from a bottom surface to a top surface thereof by observing the cross-section of the active material bodies  14 . 
     The active material bodies  14  may be formed of a one-stage vapor deposition step and have a shape inclined in one direction. However, it is preferable that the active material bodies  14  are formed of a multiple-stage vapor deposition step and each have a plurality of layers which are different in the growth direction S. Owing to such a structure, the stress applied on the current collector  11  by the expansion of the volume of the active material bodies  14  at the time when the active material bodies  14  occlude lithium ions can be more effectively alleviated. 
     As is seen from  FIGS. 6(   a ) and ( c ), in the example shown here, the active material bodies  14  are arranged along a direction  19  parallel to the vapor deposition direction on a plane parallel to the surface of the current collector  11  while the space  16   a , which is sufficiently large, is provided between adjacent active material bodies  14 . By contrast, along a direction  18 , as can be seen from  FIGS. 6(   b ) and ( c ), the plurality of active material bodies  14  are closest to each other and the width of the space  16   a  between adjacent active material bodies  14  is smallest. In this manner, the direction  18  defines a distance La 2  between each two active material bodies  14  which are closest to each other (minimum distance La 2 ). The term “minimum distance” refers to the distance between adjacent active material bodies  14  on a plane parallel to the surface of the current collector  11  when the active material bodies  14  do not occlude lithium ions, namely, the minimum value of the width of the space between adjacent active material bodies  14 . 
     Accordingly, in this embodiment, a linear void ratio in the direction  18  has the minimum value of the linear void ratio in an arbitrary direction (hereinafter, this minimum value will be referred to as the “minimum linear void ratio”). Herein, the “linear void ratio” is the ratio of the space  16  with respect to the active material layer  15   a  in an arbitrary direction (for example, directions  18 ,  19 ,  21 , etc.) on a plane parallel to the surface of the current collector  11 . For example, where the arrangement pitch of the active material bodies  14  in the direction  18  is La 1 , the linear void ratio in the direction  18  is represented by (La 2 /La 1 )×100(%). The minimum linear void ratio of the pre-stretching current collector  11  is preferably greater than 0% (namely, the active material bodies  14  do not contact each other), and is, for example, 0.5% or greater. 
     Next, the current collector  11  shown in  FIG. 6  is supplied with a tensile load and is stretched in one axial direction. Herein, the current collector  11  is stretched in a direction  21  perpendicular to the vapor deposition direction on a plane parallel to the surface of the current collector  11 . As a result, the length of the current collector  11  along the direction  21  is made larger than the length of the pre-stretching current collector  11  along the direction  21  (for example, 100.5% or greater) by plastic deformation. Thus, the electrode  200  is obtained. 
       FIGS. 7(   a ), ( b ) and ( c ) schematically show the electrode  200 .  FIG. 7(   c ) is a plan view, and  FIGS. 7(   a ) and ( b ) are cross-sectional views respectively taken along lines I-I′ and II-II′ in  FIG. 7(   c ). 
     As is understood from  FIGS. 7(   b ) and ( c ), the current collector  11  is plastically deformed in the direction  21  by the stretching step, and the space  16   b  between the active material bodies  14  is larger than the pre-stretching space  16   a  ( FIG. 6) . In this step, an area of the current collector  11  on which the active material bodies  14  are not formed, namely, the dent, is mainly elongated, whereas the areas on which the active material bodies  14  are formed are not elongated almost at all. In other words, the interval between the active material bodies  14  can be increased without increasing the width of the active material bodies  14  or making cracks in the active material bodies  14 . Therefore, the linear void ratio of the electrode  200  in the direction  18  (minimum linear void ratio), i.e., (Lb 2 /Lb 1 )×100(%), is larger than the pre-stretching minimum linear void ratio, i.e., (La 2 /La 1 )×100(%). 
     As understood from this, in this embodiment, the ratio of the space  16   b  between the active material bodies  14  (linear void ratio) can be made larger than that provided by the conventional art, by stretching the current collector  11  after the active material bodies  14  are formed. For example, according to the conventional method described above with reference to  FIG. 1 , it is difficult to obtain a minimum linear void ratio of 10% or greater while keeping the productivity. According to the method in this embodiment, the minimum linear void ratio of 10% or greater can be more easily obtained without spoiling the productivity, for example, without increasing the height of the bumps  12  of the current collector or increasing the vapor deposition angle ω. 
     Accordingly, a space sufficiently large for alleviating the expansion and contraction of the active material bodies  14  can be obtained with more certainty. Also in this embodiment, a cell is produced using an electrode already stretched. Therefore, the current collector can be suppressed from being elongated by charge/discharge of the cell, and so the deformation of the electrode such as generation of wrinkles can be suppressed. Thus, the deformation of the cell due to the expansion and contraction of the active material bodies  14  can be suppressed, and the charge/discharge cycle characteristic can be improved. 
     In the stretching step in this embodiment, it is sufficient to stretch the current collector  11  having the active material layer  15   a  at least in one axial direction on a plane parallel to the surface of the current collector  11 . There is no specific limitation on the method of stretching, but it is preferable to apply a load uniformly in the stretching direction. In the case where a sheet-like current collector is used, for example, while the current collector having the active material layer is fed out from one of the rollers and wound around the other roller, a load is applied between the two rollers. In this manner, the current collector can be stretched in a longitudinal direction (MD direction) of the current collector. Alternatively, the current collector having the active material layer may be fed out and wound around the roller in the state where a tensile load is applied on the current collector in a direction perpendicular to the MD direction (width direction of the current collector; hereinafter, referred to as the “TD direction”). In this case, the current collector is stretched in the TD direction. 
     The current collector  11  may be stretched in two axial directions on a plane parallel to the surface of the current collector  11 . For example, a tensile load may be applied in two axial directions perpendicular to each other (for example, the MD direction and the TD direction) at the same time or sequentially to stretch the current collector  11 . Alternatively, the current collector  11  having the active material layer  15   a  may be rolled to be stretched. A device usable for this method will be described later. 
     There is no specific limitation on the direction of stretching the current collector  11 , but it is preferable that the current collector  11  is stretched so as to enlarge the space  16   a  between the active material bodies  14 . Especially where the current collector  11  is stretched so as to enlarge the minimum distance between adjacent active material bodies  14 , the stress applied on the current collector  11  by contact between the active material bodies  14  can be effectively reduced. 
     It is preferable to stretch the current collector such that the length of the post-stretching current collector  11  in the stretching direction is 100.5% or greater of the length of the pre-stretching current collector  11  in the stretching direction owing to plastic deformation. A reason for this is that by plastically deforming the current collector  11  so as to have a length which is 100.5% or greater of the pre-stretching length, a space which is sufficiently large to alleviate the stress generated by the expansion and contraction can be formed between adjacent active material bodies  14 . Herein, the term “plastic deformation” means deformation which is caused by a load exceeding the elastic limit of the material and is kept without returning to the original shape even after the load is removed. The “plastic deformation” does not encompass elastic deformation. Accordingly, the expression “stretching the current collector  11  by plastic deformation” means that the current collector  11  is deformed by applying a tensile load thereto and is kept stretched even after the tensile load is removed. 
     An elongation ratio (rupture elongation ratio) of the pre-stretching current collector  11  is preferably 1.0% or greater. The “elongation ratio (rupture elongation ratio)” refers to the elongation ratio obtained at the time when the current collector  11  is ruptured by a tensile test. When the current collector  11  has a rupture elongation ratio of 1.0% or greater, the current collector  11  can be easily stretched by 0.5% or greater without being broken. 
     Before being stretched, the current collector  11  may be annealed. Owing to the annealing, the rupture elongation ratio of the current collector  11  can be increased, and so the current collector  11  is easier to be elongated. Annealing is not indispensable regardless of the type of the current collector  11 , but annealing provides the effect of the present invention with more certainty. For example, in the case where the current collector  11  is formed using a rolled copper foil, the rolled copper foil has a low rupture elongation ratio but can be sufficiently elongated by being annealed before being stretched. Therefore, a sufficiently large space can be formed in the active material layer with more certainty. There is no specific limitation on the conditions of annealing, and conditions of annealing are appropriately selectable in accordance with the material of the current collector  11  or the like. 
     The minimum linear void ratio (post-stretching minimum linear void ratio) of the active material layer  15   b  of the electrode  200  in this embodiment can be controlled by appropriately selecting the location and the size of the bumps  12  formed at the surface of the current collector  11 , the vapor deposition conditions for forming the active material bodies  14 , and the stretching conditions (the stretching direction, the magnitude of the tensile load, etc.). When the minimum linear void ratio of the electrode  200  obtained after the stretching is 5% or greater, an effect of suppressing the deformation of the electrode plate is provided. More preferably, the minimum linear void ratio is 8% or greater. With such values, the contact between the active material bodies  14  is suppressed with more certainty. From the viewpoint of obtaining a certain level of charge capacity, the minimum linear void ratio is preferably 30% or less. More preferably, the average value of the linear void ratio in an arbitrary direction is 20% or less. With such values, a large charge capacity can be realized with more certainty. 
     In this specification, the terms “linear void ratio” and “minimum linear void ratio” respectively mean the average linear void ratio and the average minimum linear void ratio of the active material layer  15   b  after the electrode  200  is produced but before lithium is occluded. The linear void ratio and the minimum linear void ratio before lithium is occluded or after charge/discharge are found by, for example, observing a top surface of the active material layer  15   b  using a scanning electron microscope (SEM). 
     The electrode  200  is stretched in advance, and so is unlikely to be deformed, for example, wrinkled, by charge/discharge of the cell. Even in a cell using a conventional electrode which is not stretched in advance, the electrode is stretched on a plane parallel to the current collector by charge/discharge of the cell. However, a portion of the current collector on which the active material layer is not formed (a portion to which the lead is connected, etc.) is not elongated almost at all. By contrast, in a cell using the electrode  200  in this embodiment, even a portion of the current collector on which the active material layer is not formed is stretched. Accordingly, in order to determine whether the electrode has been stretched or not during the production thereof, the elongation of the portion to which the lead is connected, for example, may be checked in addition to whether or not the electrode is wrinkled or not after the charge/discharge. 
     Now, with reference to the figures, preferable locations and sizes of the bumps  12  of the pre-current collector  11  in this embodiment will be described. 
       FIGS. 4(   a ) and ( b ) will be referred to again. In the example shown here, the bumps  12  are each like a column having a rhombic top surface. However, the shape of the bump  12  is not limited to this. An orthogonal projection image of the bump  12  as seen in the normal direction D to the current collector  11  may be square, rectangular, trapezoidal, rhombic, parallelogramic, pentagonal, polygonal such as home plate-shaped, circular, elliptical or the like. The shape of the cross-section of the bump  12  parallel to the normal direction D to the current collector  11  may be square, rectangular, polygonal, semicircular, or a combination thereof. The shape of the bump  12  along a cross-section perpendicular to the surface of the current collector  11  may be, for example, polygonal, semicircular, bow-shaped or the like. In the case where the border between the bump  12  and a portion other than the bump  12  (also referred to as the “groove”, “dent” or the like) is not clear, for example, in the case where the cross-section of the concaved and convexed pattern formed at the current collector  11  has a curved shape, among the entire surface having the concaved and convexed pattern, a portion above the line of the average height will be referred to as the “bump  12 ” and a portion below the line of the average height will be referred to as the “groove” or the “dent”. The “dent” may be a single continuous area as in the example shown here or a plurality of areas separated from each other by the bumps  12 . In this specification, the “interval between adjacent bumps  12 ” means a distance between adjacent bumps  12  on a plane parallel to the surface of the current collector  11  and is the “width of the groove” or the “width of the dent”. 
     The height H of the bump  12  is preferably 3 μm or greater, more preferably 4 μm or greater, and still more preferably 5 μm or greater. When the height H is 3 μm or greater, the active material body  14  can be located only on the bump  12  by oblique vapor deposition using the shadowing effect. Therefore, the space  16  can be formed between the active material bodies  14  with certainty. The height H of the bump  12  is preferably 15 μm or less, and more preferably 12 μm. When the bump  12  is 15 μm or less, the volume ratio of the current collector  11  with respect to the electrode can be suppressed low. Therefore, a high energy density can be obtained. 
     The bumps  12  are preferably arranged regularly at a prescribed arrangement pitch. For example, the bumps  12  may be arranged in a houndstooth check-like or grid-like pattern. The arrangement pitch of the bumps  12  (distance between the centers of adjacent bumps  12 ) is, for example, 10 μm or greater and 100 μm or less. Herein, the “center of the bump  12 ” refers to the central point of the maximum width of the top surface of the bump  12 . When the arrangement pitch is 10 μm or greater, a space for allowing the active material body  14  to expand can be formed between adjacent bumps  12  with certainty. The arrangement pitch is preferably 20 μm or greater, and more preferably 30 μm or greater. When the arrangement pitch P is 100 μm or less, a large capacity can be obtained certainly without increasing the height of the active material body  14 . The arrangement pitch P is preferably 80 μm or less, more preferably 60 μm or less, and still more preferably 50 μm or less. In the example shown here, the bumps  12  are arranged along three directions, and all of arrangement pitches Pa, Pb and Pc in the respective directions are preferably in the above ranges. 
     The ratio of interval d between the bumps  12  with respect to the arrangement pitch Pa of the bumps  12  is preferably ⅓ or greater and ⅔ or less. Similarly, the ratio of interval e between the bumps  12  with respect to the arrangement pitch Pb of the bumps  12 , and the ratio of interval f between the bumps  12  with respect to the arrangement pitch Pc of the bumps  12 , are each preferably ⅓ or greater and ⅔ or less. When the ratio of each of these intervals d, e and f is ⅓ or greater, the width of the space between the active material bodies  14  formed on the bumps  12  in each arrangement direction can be obtained with more certainty. Therefore, a sufficiently high linear void ratio is obtained. When the ratio of each of these intervals d, e and f is ⅔ or greater, the active material is vapor-deposited also on the groove between the bumps  12 , and so the stress applied on the current collector  11  by the expansion of the active material may be undesirably increased. 
     The width of the top surface of the bump  12  is preferably 200 μm or less, and more preferably 50 μm or less. With such values, a sufficiently large space can be obtained with certainty between the active material bodies  14  using the shadowing effect. Therefore, the deformation of the electrode  200  due to the stress generated by the expansion of the active material can be effectively suppressed. By contrast, when the width of the top surface of the bump  12  is too small, a sufficiently large size of contact area between the active material body  14  and the current collector  11  may not be obtained. Therefore, the width of the top surface of the bump  12  is preferably 1 μm or greater. Especially where the bump  12  is column-like, when the width of the top surface thereof is too small (for example, less than 2 μm), the bump  12  is too thin and so is easy to be deformed by the stress generated by charge/discharge. Therefore, the width of the top surface of the bump  12  is more preferably 2 μm or greater. With such values, the deformation of the bump  12  due to charge/discharge can be suppressed with more certainty. In the example shown here, all of widths a, b and c of the bump  12  in the respective arrangement directions are preferably in the above ranges. 
     In the case where the bump  12  is like a column having a side surface perpendicular to the surface of the current collector  11 , the intervals d, e and f between adjacent active material bodies  14  are preferably 30%, and more preferably 50%, of the widths a, b and c of the bump  12 , respectively. With such values, a sufficiently large space can be obtained with certainty between the active material bodies  14  to significantly alleviate the stress generated by the expansion of the active material. By contrast, when the distance between adjacent bumps  12  is too large, the thickness of the active material layer  15   a  needs to be increased in order to obtain a certain level of capacity. Therefore, the intervals d, e and f are preferably 250% or less, and more preferably 200% or less, of the widths a, b and c of the bump  12 , respectively. 
     The top surface of the bump  12  may be flat, but preferably is concaved and convexed. Surface roughness Ra thereof is preferably 0.1 μm or greater. Herein, the term “surface roughness Ra” refers to the “arithmetic average roughness Ra” defined by the Japanese Industrial Standards (JISB 0601-1994) and can be measured using, for example, a surface roughness meter or the like. When the surface roughness Ra of the top surface of the bump  12  is less than 0.1 μm, in the case where, for example, the plurality of active material bodies  14  are formed on the top surface of one bump  12 , each active material body  14  has an excessively small width (column diameter) and so is easy to be destroyed at the time of charge/discharge. The surface roughness Ra is more preferably 0.3 μm or greater. With such values, the active material body  14  is more easily grown on the bump  12 , and as a result, a sufficiently large space can be formed between the active material bodies  14  with certainty. By contrast, when the surface roughness Ra is too large (for example, exceeding 100 μm), the thickness of the current collector  11  is too thick and a high energy density is not obtained. Therefore, the surface roughness Ra is preferably, for example, 30 μm or less. The surface roughness Ra is more preferably 10 μm or less, and still more preferably 5.0 μm or less. Especially when the surface roughness Ra of the current collector  11  is in the range of 0.3 μm or greater and 5.0 μm or less, a sufficiently strong adherence between the current collector  11  and the active material bodies  14  is obtained with certainty. Therefore, the active material bodies  14  can be prevented from being peeled off. 
     The material of the current collector  11  is preferably, for example, copper or a copper alloy obtained by rolling or electrolysis, and more preferably a copper alloy having a relatively high strength. In this embodiment, there is no specific limitation on the material of the current collector  11 , but the current collector  11  is obtained by, for example, forming a regular concaved and convexed pattern including a plurality of bumps  12  at a surface of a metal foil of copper, a copper alloy, titanium, nickel, stainless steel of the like. As the metal foil, for example, a rolled copper foil, a rolled copper alloy foil, an electrolyzed copper foil, an electrolyzed copper alloy foil and the like are preferably usable. 
     There is no specific limitation on the thickness of the metal foil before the concaved and convexed pattern is formed. For example, the thickness is preferably 1 μm or greater and 50 μm or less. When the thickness is 50 μm or less, the current collector is sufficiently thin. Therefore, the ratio of the active material with respect to the electrode is sufficiently high and the capacity per unit volume can be increased. When the thickness is 1 μm or greater, the current collector  11  is easy to be handled. The thickness of the metal foil is more preferably 6 μm or greater and 40 μm or less, and still more preferably 8 μm or greater and 33 μm or less. 
     There is no specific limitation on the method for forming the bumps  12 . For example, the metal foil may be etched using a resist resin or the like to form a groove having a prescribed pattern. Portions in which the groove is not formed may be set as the bumps  12 . Alternatively, a resist pattern may be formed on the metal foil, and the bumps  12  may be formed on a groove of the resist pattern by electrodeposition or plating. Still alternatively, a groove formed by pattern carving in a roller usable used a rolling method may be mechanically transferred onto the surface of a metal foil. 
     As described above, the active material body  14  in this embodiment grows along the direction S inclined with respect to the normal direction D to the current collector  11 . An angle (inclining angle) α made by the growth direction S of the active material body  14  and the normal direction D is preferably 5° or greater and more preferably 10° or greater. In order to obtain a good adherence, it is preferable that the contact area size between the active material body  14  and the current collector  11  is larger, namely, the inclining angle is 0°. However, in this case, the shadowing effect is not provided, and so no space can be formed between adjacent active material bodies  14 . When the angle is 5° or greater, a space is formed between the active material bodies  14  and also a sufficiently large size of contact area is obtained. The inclining angle α is preferably less than 90°, but as the angle is closer to 90°, the vapor deposition ratio is lower. In consideration of the productivity, the inclining angle is preferably 80° or less. In the case where the active material body  14  is formed by oblique vapor deposition, the inclining angle α of the active material body  14  is determined by the vapor deposition angle at which the active material body  14  is formed. The inclining angle α can be found by, for example, measuring inclining angles of any two to 10 active material bodies  14  and calculating an average value thereof. 
     The inclining angle α of the active material body  14  may change in accordance with the height of the active material body  14 . In the case where, as in this embodiment, the active material body  14  includes a plurality of portions which are different in the growth direction S, it is preferable that all the growth directions S of the active material body  14  are inclined with respect to the normal direction D, and that all the inclining angles α are 10° or greater and less than 90°. 
     In this embodiment, the area size ratio of the space  16   b  with respect to the active material layer  15   b  (hereinafter, referred to as the “planar void ratio”) is preferably 5% or greater and 50% or less. When the planar void ratio is 5% or greater, the expansion and contraction of the active material bodies  14  can be effectively accommodated in the space  16   b . Therefore, the deformation of the electrode  200  can be reduced. From the viewpoint of obtaining a large capacity with certainty, the planar void ratio is preferably 50% or less. In the case where each active material body  14  is like a column upright along the normal D to the surface of the current collector  11 , the planar void ratio is calculated by finding an area size of the active material layer  15   b  and an area size of the space  16   b  as seen from the normal D to the surface of the current collector  11 . In the case where each active material body  14  is like a column inclining in one direction or a zigzag column, the planar void ratio is calculated by finding an area size of the active material layer  15   b  and an area size of the space  16   b  in a cross-section parallel to the surface of the current collector  11 . 
     Thickness t of the active material layer  15   b  is the distance t, along the normal direction to the current collector  11 , from the top surface of the bump  12  of the current collector  11  to the apex of the active material body  14 . The thickness t is equal to the height of the active material body  14 . The thickness t is, for example, 0.01 μm or greater and preferably 0.1 μm or greater. In this case, a sufficiently high energy density can be obtained with certainty, and so the characteristic having a large capacity of the active material containing silicon can be well utilized. When the thickness t is 3 μm or greater, the volume ratio of the active material with respect to the entire electrode is larger, and so a still higher energy density is obtained. The thickness t is more preferably 5 μm or greater, and still more preferably 8 μm or greater. The thickness t of the active material layer  15   b  is, for example, 100 μm or less, preferably 50 μm or less, and more preferably 40 μm or less. With such values, the stress generated by the expansion of the active material layer  15   b  can be suppressed. The above ranges of thickness t are advantageous for high-rate charge/discharge because the resistance against the current collection can be made low. When the thickness t is, for example, 30 μm or less and more preferably 25 μm or less, the deformation of the current collector  11  due to the stress generated by the expansion of the active material can be suppressed more effectively. 
     The thickness t of the active material layer  15   b  can be measured by, for example, the following method. First, the entire thickness of the electrode  200  after the active material layer  15   b  is formed is measured. In the case where the bumps  12  and the active material layer  15   b  are formed only on one surface of the current collector  11 , the thickness t of the active material layer  15  is found by subtracting the thickness of the current collector  11  including the bumps  12  (sum of the thickness of the metal foil and the height of the bumps  12 ) from the entire thickness of the electrode  200 . In the case where the bumps  12  and the active material layers  15   b  are formed on both surfaces of the current collector  11 , the total thickness of the active material layers  15   b  formed on both surfaces of the current collector  11  is found by subtracting the thickness of the current collector  11  including the bumps  12  (sum of the thickness of the metal foil and the total thickness of the bumps  12  formed on both surfaces thereof) from the entire thickness of the electrode  200 . 
     There is no specific limitation on the width of the active material body  14 . In order to prevent the active material body  14  from cracking due to the expansion thereof at the time of charge, the width of the active material body  14  is preferably 100 μm or less and more preferably 50 μm or less. In order to prevent the active material body  14  from being peeled off from the current collector  11 , the width of the active material body  14  is preferably 1 μm or greater. The width of the active material body  14  is obtained as an average value of the widths, of any two to 10 active material bodies  14 , along a cross-section which is parallel to the surface of the current collector  11  and which is at a level corresponding to ½ of the height t of the active material bodies  14 . In the case where the cross-section is circular, the width of the active material body  14  is an average value of the diameters of such active material bodies  14 . 
     In this embodiment, the capacity per unit area size of the active material layer  15   b  is preferably 2 mAh/cm 2  or greater. With such values, a high cell energy can be obtained. When the capacity per unit area size is increased while the linear void ratio is kept at 5% or greater, the thickness of the active material layer  15   b  (height of the active material body  14 ) t is increased and the amount of expansion of the active material at the time of charge is increased. As a result, there is an undesirable possibility that the deformation of the current collector  11  due to the stress generated by the expansion of the active material cannot be sufficiently suppressed. Therefore, the capacity per unit area size is preferably 10 mAh/cm 2  or less and more preferably 8 mAh/cm 2  or less. 
     The active material layer  15   b  in this embodiment preferably contains silicon element or tin element. With these elements, a large capacity can be obtained with certainty. More preferably, the active material layer  15   b  contains silicon element. The active material layer  15   b  may contain at least one selected from the group consisting of, for example, single element of silicon, a silicon alloy, a compound containing silicon and oxygen, and a compound containing silicon and nitrogen. The active material layer  15   b  may contain only one of, or two or more of, the above-listed substances. 
     The compound containing silicon and nitrogen may further contain oxygen. For example, the active material layer  15   b  may be formed of a plurality of compounds containing silicon, oxygen and nitrogen at different molar ratios. Alternatively, the active material layer  15   b  may be formed of a composite of a plurality of silicon oxides having different molar ratios of silicon and oxygen. 
     More preferably, the active material layer  15   b  contains a silicon oxide (SiO x , where 0&lt;x&lt;2). Generally in an active material containing a silicon oxide, as the molar ratio x of the amount of oxygen with respect to the amount of silicon (hereinafter, also referred to simply as the “oxygen ratio”) is lower, a larger charge/discharge capacity is obtained but the volume expansion ratio due to charge is higher. By contrast, as the oxygen ratio x is higher, the volume expansion ratio is suppressed but the charge/discharge capacity is decreased. When the average value of the oxygen ratio x is greater than 0, the expansion and contraction due to charge/discharge is suppressed and so the stress applied on the current collector  11  by the expansion of the active material can be suppressed. When the average value of the oxygen ratio x is less than 1.5, a sufficient charge/discharge capacity can be obtained with certainty, and the high-rate charge/discharge characteristic can be maintained. Thus, a good charge/discharge cycle characteristic and a high reliability can be realized. 
     Among portions having different growth directions, the oxygen ratios may be different. Even in such a case, the average value of the oxygen ratio x of the entire active material layer  15   b  is acceptable at 0&lt;x&lt;2 and is preferable at 0&lt;x&lt;1.5. 
     In this specification, the “average value of the molar ratio x of the amount of oxygen with respect to the amount of silicon” of the active material layer  15   b  indicates a value of a composition excluding lithium supplemented to, or occluded by, the active material layer  15   b . It is sufficient that the active material layer  15   b  contains a silicon oxide having the above range of oxygen ratio, and may contain impurities such as Fe, Al, Ca, Mn, Ti or the like. 
     Now, with reference to the figures, an example of a lithium ion secondary battery including the electrode  200  in this embodiment as a negative electrode will be described. 
       FIG. 8  is a schematic cross-sectional view of a cylindrical cell using the electrode  200  in this embodiment. The cylindrical cell  80  includes a cylindrical electrode assembly  84  and a cell can  88  for accommodating the electrode assembly  84 . The electrode assembly  84  is obtained by winding a strip-like positive electrode plate  81  and a strip-like negative electrode plate  82  together with a wide separator  83  located therebetween. Although not shown, the positive electrode plate  81  includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the negative electrode plate  82  includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector. The negative electrode plate  82  has, for example, a structure substantially the same as that of the electrode  200  described above with reference to  FIGS. 7(   a ) and ( b ). The negative electrode plate  82  and the positive electrode plate  81  are located such that the negative electrode active material layer and the positive electrode active material layer face each other while having the separator  83  therebetween. 
     The electrode assembly  84  is impregnated with an electrolyte (not shown) for conducting lithium ions. An opening of the cell can  84  is closed by a sealing plate  89  having a positive electrode terminal  85 . An end of a positive electrode lead  81   a  formed of aluminum is connected to the positive electrode plate  81 , and the other end of the positive electrode lead  81   a  is connected to a rear surface of the sealing plate  89 . Along a perimeter of the sealing plate  89 , an insulating packing  86  formed of polypropylene is located. An end of a negative electrode lead (not shown) formed of copper is connected to the negative electrode plate  82 , and the other end of the negative electrode lead is connected to the cell can  88 . A top insulating ring (not shown) and a bottom insulating ring  87  are respectively located above and below the electrode assembly  84 . 
     In the lithium ion secondary battery  80 , the positive electrode active material layer releases lithium ions at the time of charge, and occludes lithium ions, released by the negative electrode active material layer, at the time of discharge. The negative electrode active material layer occludes the lithium ions, released by the positive electrode active material layer, at the time of charge, and releases the lithium ions at the time of charge. 
     In this embodiment, there is no specific limitation on the structural elements of the lithium ion secondary battery  80  other than the negative electrode plate  82 . For example, the positive electrode active material layer may be formed of a lithium-containing transfer metal oxide such as lithium cobalt oxide (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ), lithium manganese oxide (LiMn 2 O 4 ) or the like, but is not limited to this. The positive electrode active material layer may be formed only of a positive electrode active material, or may be formed of a compound containing a positive electrode active material, a binder and a conductor. The positive electrode active material layer may include a plurality of active material bodies like the negative electrode active material layer. For the positive electrode current collector, it is preferable to use metal such as Al, an Al alloy, Ti or the like. 
     For the lithium ion-conductive electrolyte, any of various lithium ion-conductive solid electrolytes and non-aqueous electrolytic solutions is usable. A preferable non-aqueous electrolytic solution is a solution of a lithium salt dissolved in a non-aqueous solvent. There is no specific limitation on the composition of the non-aqueous electrolytic solution. There is no specific limitation on the separator or the external case, and any material usable for various forms of lithium secondary batteries is usable with no specific limitation. Instead of the separator, a lithium ion-conductive solid electrolyte, or a gel electrolyte containing such a solid electrolyte, is usable. 
       FIG. 8  shows one example of a cylindrical cell including a wound-type electrode plate assembly, but the present invention is also applicable to a wound-type polygonal cell or a laminate-type cell such as a coin-shaped cell or the like. The laminate-type cell may have a structure in which the positive electrode and the negative electrode are laminated in three or more layers. It is preferable that a positive electrode having a positive electrode active material layer on both or one of surfaces thereof and a negative electrode having a negative electrode active material layer on both or one of surfaces thereof are used, such that all the positive electrode active material layers face the negative electrode active material layers and all the negative electrode active material layers face the positive electrode active material layers. In the case where a plurality of negative electrode active material layers are included, the inclining state of the active material bodies (growth direction, number n of stages of vapor deposition, growth direction of a portion obtained by each vapor deposition stage, or the like) may be the same in, or different among, all the negative electrode active material layers. In one negative electrode active material layer, active material bodies having different inclining states may be formed. In the case where the negative electrode active material layers are formed on both surfaces of the negative electrode current collector, the inclining states of the active material bodies in the negative electrode active material layers may be the same as, or different from, each other. 
     As described above, there is no specific limitation on the structural elements of the lithium secondary battery according to the present invention except that the electrode according to the present invention is used as a negative electrode or a positive electrode. Any of various materials generally used as materials for lithium ion cells is selectable to be used. 
     Example 1 and Comparative Example 
     Hereinafter, Example 1 according to the present invention and a comparative example will be described. Herein, an electrode  1  was produced in Example 1 and an electrode A was produced in the comparative example. The void ratio of the active material layer in each electrode was measured. 
     (i) Method for Producing the Electrode 
     (i-1) Electrode  1   
     &lt;Production of the Current Collector&gt; 
     First, a method for producing the current collector used in the electrode  1  will be described. 
     Both surfaces of a copper foil having a thickness of 27 μm (HCL-02Z, produced by Hitachi Cable, Ltd.) were roughened by electrolytic plating to form copper particles having a particle diameter of 1 μm. Thus, as shown in  FIG. 9(   a ), a roughened copper foil  93  having a surface roughness Rz of 1.5 μm was obtained. The surface roughness Rz refers to the ten-point average roughness Rz defined by the Japanese Industrial Standards (JISB 0601-1994). Alternatively, a roughened copper foil commercially available for printed circuit boards may be used. 
     Next, as shown in  FIG. 9(   b ), a plurality of grooves (dents)  94  were formed in a ceramic roller  90  using laser carving. The plurality of grooves  94  were formed to be rhombic as seen in the normal direction to the ceramic roller  90 . The lengths of the diagonal lines of the rhombic shape were 10 μm and 20 μm, the interval between adjacent dents  94  along a diagonal line a was 18 μm, and the interval between adjacent dents  94  along a diagonal line b was 20 μm. The depth of each dent  94  was 10 μm. The copper foil  93  was rolled by passing the copper foil  93  between this ceramic roller  90  and another roller (not shown), located so as to face the ceramic roller  90 , at a linear pressure of 1 t/mm. 
     In this manner, as shown in  FIG. 9(   c ), a current collector  91  having a plurality of bumps  92  at a surface thereof was obtained. In this step, areas of the copper foil  93 , passed between the rollers, which were pressed by a portion of the ceramic roller  90  other than the dents  94  were flattened as shown in the figure. By contrast, areas of the copper foil  93  corresponding to the dents  94  were not flattened and entered the dents  94 , and as a result, the bumps  92  were formed. The height of the bumps  92  was about 6 μm, which was smaller than the depth of the dents  94  of the ceramic roller  90 . 
       FIG. 10  is a plan view of the current collector  91 . As shown in the figure, the shape and the arrangement of the bumps  92  of the current collector  91  correspond to those of the dents  94  formed in the ceramic roller  90 . Each bump  92  had a rhombic top surface, and the lengths a and b of the diagonal lines thereof were about 10 μm and about 20 μm. Interval e between adjacent bumps  92  along the diagonal line a was 18 μm, and interval d between adjacent bumps  92  along the diagonal line b was 20 μm. 
     &lt;Vapor Deposition Step&gt; 
     The current collector  91  obtained in the above-described method was set on the fixing table  43  located inside the vacuum chamber  41  described above with reference to  FIG. 5 . While oxygen gas having a purity of 99.7% was supplied to the vacuum chamber  41 , EB vapor deposition was performed with silicon as the vapor deposition source using a vapor deposition unit (the vapor deposition source, the crucible and the electron beam generation device which are structured into a unit). In this step, the inside of the vacuum chamber  41  had an oxygen atmosphere having a pressure of 3.5 Pa. In order to vaporize silicon as the vapor deposition source, an electron beam generated by the electron beam generation device was deflected by a deflection yoke to radiate to the vapor deposition source. As the vapor deposition source, chips of the material generated when a semiconductor wafer is formed (scrap silicon; purity: 99.999%) were used. 
     For performing vapor deposition, the fixing table  43  was inclined such that the vapor deposition angle ω would be 65°. The first-stage vapor deposition step was performed in this state to form a first-stage portion (first portion) of the active material bodies was formed. The film formation rate of the first portion was set to about 8 nm/s., and the oxygen flow rate was set to 30 sccm. The height of the first portion was set to 0.4 μm. The vapor deposition was performed on a plane parallel to the surface of the current collector  91  in a direction parallel to the shorter diagonal line of the bumps  12 . 
     Next, the fixing table  43  was rotated clockwise around the central shaft thereof to incline in the opposite direction to the inclining direction of the fixing table  43  in the first-stage vapor deposition step, namely, to incline at a vapor deposition angle ω of −65°. Vapor deposition was performed in this state at an oxygen flow rate of 25 sccm to form a second portion (second-stage vapor deposition step). Then, the inclining direction of the fixing table  43  was changed to the same direction as that in the first-stage vapor deposition step. Vapor deposition was performed in substantially the same manner at a vapor deposition angle ω of 65° and an oxygen flow rate of 20 sccm (third-stage vapor deposition step). The film formation was performed in this manner while the vapor deposition angle ω was alternately switched between 65° and −65° and the oxygen flow rate was decreased stepwise to 15 sccm to 10 sccm, 5 sccm and 10 sccm until the seventh stage and then oxygen was not introduced from the eighth to 35th stages. Thus, the active material bodies having a height of 14 μm were formed to obtain an active material layer (thickness t: 14 μm). The average value of the molar ratio x of the amount of oxygen with respect to the amount of silicon in the active material layer was 0.4. 
     Then, the current collector  91  was detached from the fixing table  43  and then set on the fixing table  43  again such that a surface opposite to the surface having the active material bodies formed thereon, namely, the rear surface, would be directed upward. Vapor deposition was performed on the rear surface of the current collector  91  by 35 stages in substantially the same manner to form an active material layer (thickness t: 14 μm) (not shown). In this manner, the active material layers were formed on both surfaces of the current collector  91 . 
     &lt;Stretching Step&gt; 
     For performing the stretching step, the rupture strength and the rupture elongation ratio of the pre-stretching current collector  91  were first found. 
     The current collector  91  having the active material layers on both surfaces thereof was cut into a piece having a width of 15 mm and a length of 70 mm, and the piece of the current collector  91  was stretched in one axial direction by a tensile test until being ruptured. The stretching direction was a direction along the longer diagonal line b of the bumps  92 , and the tensile rate was the minimum possible rate. As a result, the rupture strength was 11.2 N/mm, and the rupture elongation ratio (maximum elongation ratio) was 0.2%. 
     In order to make the current collector  91  more easily stretchable, the current collector  91  was annealed at 500° C. for 1 hour. The post-annealing current collector  91  was cut into a piece having a width of 15 mm and a length of 70 mm, and the piece was stretched in one axial direction by a tensile test until being ruptured. As a result, the rupture strength was 6.1 N/mm, and the rupture elongation ratio (maximum elongation ratio) was 8%. 
     From the above results, it was found that in order to prevent the current collector  91  from being ruptured during the stretching step, the post-annealing current collector  91  should be stretched at a ratio smaller than 8%, which is the rupture elongation ratio. 
     Therefore, in this example, the post-annealing current collector  91  was cut into a piece having a width of 15 mm and a length of 70 mm, and the piece was stretched in a direction along the longer diagonal line b of the bumps  92  using a tensile tester to stretch the length along the stretching direction by 5% by plastic deformation. Thus, the electrode  1  was obtained. 
     Comparative Example 
     On both surfaces of a current collector substantially the same as in Example 1, active material layers were formed in substantially the same method as in Example 1, except that the current collector was not annealed or stretched. Thus, the electrode A in the comparative example was obtained. 
     &lt;Results&gt; 
       FIGS. 11 and 12(   a ) are schematic views respectively showing the results of the electrode A and the electrode  1  observed in the normal direction to the current collector using a scanning electron microscope.  FIG. 12(   b ) is an enlarged view of  FIG. 12(   a ). In  FIGS. 11 and 12(   a ), X represents a direction parallel to the vapor deposition direction, and Y represents a direction perpendicular to the X direction. The X and Y directions are respectively parallel to the diagonal lines a and b ( FIG. 10)  of the current collectors  91 . 
     From the top surfaces shown in  FIGS. 11 and 12(   a ), the following sizes and ratios as seen in the normal direction to the current collector were found regarding the electrode A and the electrode  1 : width WX of each active material body in the X direction, width WY of each active material body in the Y direction, arrangement pitch PX in the X direction, arrangement pitch PY in the Y direction, the linear void ratio (minimum linear void ratio) in a Z direction connecting the closest active material bodies, and the planar void ratio. The results are shown in Table 1. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Electrode A 
                 Electrode 1 
               
               
                   
                 (annealing: no; 
                 (annealing: yes; 
               
               
                   
                 stretching: no) 
                 stretching: yes) 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 Width WX of active 
                 25.4 μm 
                 25.3 μm 
               
               
                   
                 material body 
               
               
                   
                 Width WY of active 
                 33.5 μm 
                 33.7 μm 
               
               
                   
                 material body 
               
               
                   
                 Arrangement pitch PX 
                 32.5 μm 
                 32.6 μm 
               
               
                   
                 Arrangement pitch PY 
                 49.5 μm 
                 59.2 μm 
               
               
                   
                 Linear void ratio in 
                 6.1% 
                 19.4% 
               
               
                   
                 Z direction 
               
               
                   
                 Planar void ratio 
                 21.3% 
                 31.9% 
               
               
                   
                   
               
            
           
         
       
     
     From the results shown in Table 1, it is understood that by the stretching step, the widths WX and WY of each active material body were not elongated almost at all, but the arrangement pitch PY of the active material bodies was elongated by about 20% and thus the space between the active material bodies was enlarged in the Y direction. It is also understood that the minimum linear void ratio and the planar void ratio can be enlarged up to 19.4% and 31.9%, respectively. 
     Thus, it is understood that by performing the stretching step, the ratio of the space between the active material bodies can be increased and the stress generated by the expansion of the active material can be alleviated without reducing the productivity. The ratio of the space can be appropriately adjusted by changing a condition such as the tensile load or the like. 
     Referring to  FIG. 12(   b ), on an area  98  of the current collector  91  which is not in contact with the active material bodies, the active material (a silicon oxide) is deposited in a small thickness. It is understood from  FIG. 12(   b ) that there are breaking lines generated in this deposited layer by the stretching step. Many of the breaking lines are generated along the X direction perpendicular to the stretching direction. Therefore, even when the active material of the deposited layer is expanded, these breaking lines act as a space to reduce the stress applied on the current collector  91  by the expansion. 
     Example 2 
     In Example 1 described above, the current collector was stretched in the Y direction. In Example 2, the current collector having active material layers formed thereon was stretched by rolling, and thus an electrode  2  was produced. 
     First, on both surfaces of a current collector substantially the same as in Example 1, active material layers were formed in substantially the same manner as in Example 1. Then, in substantially the same manner as in Example 1, the current collector was annealed at a temperature of 500° C. for 1 hour in an argon atmosphere. The post-annealing current collector was cut into a piece having a width of 15 mm and a length of 70 mm. 
     Then, this current collector having the active material layers formed thereon was stretched on a plane parallel to the current collector. The stretching of the current collector was performed by rolling using stretchable rubber. 
       FIG. 13  is a schematic cross-sectional view for illustrating the stretching step (rolling) performed in Example 2. As shown in the figure, a current collector  100  (15 mm×70 mm) having the active material layers formed thereon was held between two rubber plates  63  having a thickness of 1.0 mm and pressurized in a thickness direction via the plates  63 . In this example, as the stretchable rubber for the plates  63 , silicone rubber SR50 produced by Tigers Polymer Corporation was used. Thus, the current collector  100  was stretched in all the directions in a plane parallel to the surface of the current collector  100 . In this manner, the electrode  2  was obtained. 
     Next, the electrode  2  was observed using a scanning electron microscope in the normal direction to the current collector. As a result, it was confirmed that in the electrode  2 , the current collector was elongated in the X direction in addition to the Y direction. The planar void ratio of the active material layers of the electrode  2  was 28%. In this example also, like in Example 1, it was found that the width of the active material bodies was not changed almost at all but the interval between adjacent active material bodies was mainly elongated. 
     In Example 2, the current collector  100  was fixed when being rolled. In the case where, for example, a sheet-like current collector is used, the rolling may be performed using a device as shown in  FIG. 14 . Specifically, the current collector  100  is held between two rubber plates  63 . Next, the current collector  100  is pulled in a direction of the arrow while being compressed using a roller  61  provided on a surface of each plate  63  opposite to the surface in contact with the current collector  100 . Thus, the sheet-like current collector  100  can be rolled continuously and efficiently. 
     For rolling performed as shown in  FIGS. 12 and 13 , there is no specific limitation on the rubber as long as the rubber is stretchable. In the case where the rubber has anisotropy in the stretchable direction, the current collector  100  can be elongated in one axial direction. In the case where the rubber has isotropy in the stretchable direction, the current collector  100  can be elongated in two axial directions. 
     INDUSTRIAL APPLICABILITY 
     A negative electrode for a lithium secondary battery according to the present invention is applicable to various lithium secondary batteries including, for example, coin-shaped, cylindrical, flat, and polygonal lithium secondary batteries. These lithium secondary batteries have a superior charge/discharge cycle characteristic than the conventional lithium secondary batteries while having a large charge/discharge capacity. Therefore, these lithium secondary batteries are widely usable for mobile information terminals such as personal computers, mobile phones, PDAs and the like; audio visual devices including video recorders, memory audio players and the like, etc. 
     REFERENCE SIGNS LIST 
     
         
         
           
               200  Electrode 
               11 ,  91  Current collector 
               12 ,  92  Bump 
               14  Active material body 
               15   a ,  15   b  Active material layer 
               16   a ,  16   b  Space 
             D Normal direction with respect to the surface of the current collector 
           
         
       
    
     S Growth direction of the active material body
           41  Vacuum chamber     42  Gas introduction pipe     43  Fixing table     46  Crucible     45  Oxygen nozzle     49  Silicon atom     50  Horizontal face