Patent Publication Number: US-2007111102-A1

Title: Negative electrode for non-aqueous electrolyte secondary batteries, non-aqueous electrolyte secondary battery having the electrode, and method for producing negative electrode for non-aqueous electrolyte secondary batteries

Description:
BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to a negative electrode for non-aqueous electrolyte secondary batteries which contains a composite negative electrode active material. The invention more particularly relates to a technique for providing a high-capacity negative electrode without sacrificing battery characteristics.  
      2. Background Art  
      With the advancement of portable and cordless electronic instruments, growing expectation has been directed to non-aqueous electrolyte secondary batteries smaller in size, lighter in weight, and higher in energy density. In non-aqueous electrolyte secondary batteries, carbon materials such as graphite are used as a negative electrode active material in practical applications. However, carbon materials have a theoretical capacity density of as low as 372 mAh/g. In order to increase the energy density of non-aqueous electrolyte secondary batteries, an attempt has been made where as the negative electrode active material are used silicon (Si), tin (Sn), germanium (Ge), an oxide thereof, and an alloy thereof which can form alloys with lithium. These materials have a higher theoretical capacity density than carbon materials. In particular, particles formed of active material cores such as silicon particles and silicon oxide particles have been widely studied because they are less expensive.  
      However, when these materials are used as a negative electrode active material and are subjected to repeated charging and discharging, the particles of the negative electrode active material change their volume with the number of charge-discharge cycles. This change in volume causes the active material particles to be collapsed into fine particles, thereby lowering conductivity among the particles. As a result, satisfactory charge-discharge cycle characteristics (hereinafter, cycle characteristics) are not attained.  
      To solve this problem, it has been proposed that the active material particles containing a metal or a semimetal that can form alloys with lithium are used as the cores and are bound to carbon fibers so as to be formed into composite particles. It has been reported that this structure can ensure the conductivity even if the active material particles change in volume, thereby maintaining sufficient cycle characteristics. One such technique is disclosed in Japanese Patent Application Laid-Open No.2004-349056.  
      Electrodes for non-aqueous electrolyte secondary batteries are generally produced as follows: a paste of an active material-containing mixture is applied on a metallic foil which works as a current collector, and dried; then, the resulting layer was rolled to attain higher density and desired thickness. Negative electrodes containing as an active material a carbon material such as graphite are also produced in a similar manner. A battery having a negative electrode produced in this manner performs smooth changing and discharging operations and exhibits excellent cycle characteristics. However, a battery having a negative electrode which is made from the aforementioned composite negative electrode active material and is rolled to achieve higher density provides greatly deteriorated cycle characteristics, probably due to the following mechanism. When rolled with an excessive load, the composite negative electrode active material particles are collapsed and generate new active material cores that have no carbon fibers attached to their surfaces. When a large number of such active material cores are generated, this means that active material cores separated from the conductive network of the negative electrode are born therein in a greater amount. These active material cores increase their influence over repeated charge-discharge cycles, thereby causing a deterioration in cycle characteristics.  
     SUMMARY OF THE INVENTION  
      The present invention is directed to provide a negative electrode for non-aqueous electrolyte secondary batteries which enables a battery to exhibit excellent cycle characteristics by preventing the collapse of the conductive network and thereby avoiding an increase in the impedance of the negative electrode as a whole. The present invention is directed also to provide a non-aqueous electrolyte secondary battery having the negative electrode. The negative electrode of the present invention has a mixture layer containing a composite negative electrode active material which is composed of particles formed of active material cores capable of charging and discharging at least lithium ions, carbon nanofibers (hereinafter, CNFs), and catalyst elements. The CNFs are attached to the surfaces of the particles formed of the active material cores. The catalyst elements are at least one selected from the group consisting of copper (Cu), iron (Fe), cobalt (Co), nickel (Ni), molybdenum (Mo) and manganese (Mn), and promote the growth of the CNFs. The active material cores have CNFs therebetween. In a negative electrode with this structure, the presence of the CNFs between the particles formed of the active material cores ensures the conductive network. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1A  is a transparent plan view showing a structure of a model cell of an embodiment of the present invention.  
       FIG. 1B  is a sectional view of the model cell shown in  FIG. 1A  taken along line  1 B- 1 B.  
       FIGS. 2A  to  2 C are schematic diagrams showing changes in composite negative electrode active material particles contained in a negative electrode for non-aqueous electrolyte secondary batteries of the embodiment of the present invention when the composite negative electrode active material is rolled.  
       FIGS. 3A  to  3 C are schematic diagrams showing changes in composite negative electrode active material particles different from the particles used in the embodiment of the present invention when the composite negative electrode active material is rolled. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      An embodiment of the present invention will be described as follows with reference to drawings. Note that the present invention is not limited to the following description except for its fundamental features.  
       FIG. 1A  is a transparent plan view showing the structure of a model cell produced to evaluate a negative electrode for non-aqueous electrolyte secondary batteries of the embodiment of the present invention.  FIG. 1B  is a cross sectional view taken along line  1 B- 1 B.  
      Negative electrode  1  shown in  FIGS. 1A and 1B  has current collector  1 A and mixture layer  1 B formed thereon. As shown in  FIG. 2A , mixture layer  1 B contains a composite negative electrode active material which is composed of active material cores  11  (hereinafter, cores  11 ) which can charge and discharge at least lithium ions, and carbon nanofibers  12  (hereinafter, CNFs  12 ) attached to the surfaces of cores  11 . CNFs  12  are grown using catalyst elements  13  as nuclei which are supported on the surfaces of cores  11 . Catalyst elements  13  are at least one selected from the group consisting of Cu, Fe, Co, Ni, Mo, and Mn, and promote the growth of CNFs  12 . The volume proportion occupied by cores  11  in mixture layer  1 B is 19% or more and 44% or less, and cores  11  have CNFs  12  therebetween.  
      Counter electrode  2  made of metallic lithium is faced to negative electrode  1  via separator  3  and is bonded to current collector  6  on the side opposite to separator  3 . These components are housed in laminate bag  4  made by laminating a hot-melt resin film such as polyethylene to at least one side of a metal foil such as an aluminum foil. Laminate bag  4  is then filled with non-aqueous electrolyte  5  (hereinafter, electrolyte 5). Current collectors  1 A and  6  are connected respectively with leads  8  and  9  exposed outside. Leads  8  and  9  are fixed by heating and melting modified polypropylene films  7  placed at the opening of laminate bag  4 , so that laminate bag  4  is sealed.  
      The following is a detailed description of the composite negative electrode active material. Cores  11  are characterized by having a larger volume in a charged condition than in a discharged condition, and having a larger theoretical capacity density than carbon materials. The ratio of the volume A of cores  11  in a charged condition to the volume B in a discharged condition (A/B) is generally 1.2 or more. The theoretical capacity density of cores  11  is 833 mAh/cm 3  or more. The composite negative electrode active material which contains cores  11  having such properties can maintain the original high-capacity characteristics regardless of the repeated expansion and contraction during charge-discharge cycles. This allows a secondary battery to achieve a practical level of cycle characteristics.  
      Cores  11  can be made of Si or SiOx where 0.05&lt;x&lt;1.95, or can be an alloy, a compound, a solid solution or the like in which Si is partly replaced with one or more elements selected from B, Mg, Ni, Ti, Mo, Co, Ca, Cr, Cu, Fe, Mn, Nb, Ta, V, W, Zn, C, N, and Sn. These elements can compose cores  11  either on their own or in combination. Examples of composing cores  11  in combination include a composite of a Si—O compound and a Si—N compound, and a composite of a plurality of compounds which contain silicon and oxygen in different ratios. Thus, cores  11  contain at least one selected from the group consisting of pure silicon, a silicon-containing alloy, a silicon-containing compound, and a silicon-containing solid solution. Of these, SiOx where 0.05&lt;x&lt;1.95 is desirable because of its comparative inexpensiveness and high stability.  
      CNFs  12  attach to the surfaces of cores  11  where they start to grow. In other words, CNFs  12  attach directly to the surfaces of cores  11  without adhesive resin therebetween. In some growing conditions, CNFs  12  may be chemically bonded to the surfaces of cores  11  at least at one end thereof which is the starting point of the growth. This reduces the resistance for current collection and assures high electronic conductivity in the battery, thereby providing excellent charge-discharge characteristics. In a case where CNFs  12  attach to cores  11  with catalyst elements  13 , CNFs  12  are not easily detached from cores  11 , making negative electrode  1  more resistant to mechanical load (hereinafter, called as rolling load) which is applied to negative electrode  1  when it is rolled to attain a higher density.  
      In order to allow catalyst elements  13  to exhibit excellent catalytic activity until CNFs  12  are fully grown, catalyst elements  13  are preferably present in a metallic state in the surface parts of cores  11 . More specifically, catalyst elements  13  are preferably present in the form of metal particles having a diameter of, for example, 1 nm to 1000 nm. On the other hand, when the growth of CNFs  12  is complete, the metal particles of catalyst elements  13  are preferably oxidized.  
      CNFs  12  have a fiber length of preferably 1 nm to 1 mm, and more preferably 500 nm to 100 μm. When the fiber length is less than 1 nm, the effect to increase electrode conductivity is too small. In contrast, the fiber lengths of over 1 mm tend to reduce the active material density or capacity of the electrode. Although not limited, CNFs  12  are preferably in the form of at least one selected from the group consisting of a tube shape, an accordion shape, a plate shape, and a herringbone shape. CNFs  12  may absorb catalyst elements  13  during their growth. CNFs  12  have a fiber diameter of preferably 1 nm to 1000 nm, and more preferably 50 nm to 300 nm.  
      Catalyst elements  13  in a metallic state work as active sites to grow CNFs  12 . More specifically, CNFs  12  start to grow when cores  11  with catalyst elements  13  exposed in a metallic state on their surfaces are introduced into a high-temperature atmosphere containing the source gas of CNFs  12 . When cores  11  have no catalyst elements  13  on their surfaces, CNFs  12  do not grow.  
      Methods for providing metal particles of catalyst elements  13  on the surfaces of cores  11  are not particularly limited; however, one preferable method is to support metal particles on the surfaces of particles that can charge and discharge lithium ions.  
      When the metal particles are supported in this manner, it is possible to mix cores  11  with the metal particles in solid form; however, it is preferable to soak cores  11  in a solution of a metal compound which is the source material of the metal particles. After soaking, the solvent is removed from cores  11 , which can be heated if necessary, to obtain cores  11  which support on their surfaces catalyst elements  13  in the form of metal particles having a diameter of 1 nm to 1000 nm and preferably 10 nm to 100 nm in a highly and uniformly dispersed state.  
      It is difficult to form the metal particles of catalyst elements  13  having a diameter of less than 1 nm. On the other hand, when formed to have a diameter of over 1000 nm, the metal particles of catalyst elements  13  may be extremely uneven in size, making CNFs  12  difficult to grow, or making it difficult to form a highly conductive electrode. Therefore, the diameter of the metal particles of catalyst elements  13  is preferably 1 nm or more and 1000 nm or less.  
      Specific examples of the metal compound to obtain the aforementioned solution include nickel nitrate, cobalt nitrate, iron nitrate, copper nitrate, manganese nitrate, and hexaammonium heptamolybdate tetrahydrate. The solvent used for the solution can be selected from water, an organic solvent and a mixture of water and an organic solvent as appropriate according to the solubility of the compound and the compatibility of the compound with the electrochemical active phases contained in cores  11 . Specific examples of the organic solvent include ethanol, isopropyl alcohol, toluene, benzene, hexane, and tetrahydrofuran.  
      Alternatively, it is also possible to synthesize alloy particles containing cores  11  and catalyst elements  13 . This synthesis is performed by a common alloying method. The metallic materials of cores  11  such as silicon, elements react electrochemically with lithium to form alloys, thereby forming electrochemical active phases in cores  11 . On the other hand, the metallic phases of catalyst elements  13  are at least partly exposed in the form of particles having a diameter of 10 nm to 100 nm on the surfaces of the alloy particles.  
      The metal particles or metallic phases of catalyst elements  13  are preferably 0.01 wt % to 10 wt % of cores  11 , and more preferably 1 wt % to 3 wt %. When the content of the metal particles or the metallic phases is too low, it takes a lot of time to grow CNFs  12 , thereby decreasing production efficiency. In contrast, when the content is too high, catalyst elements  13  agglomerate, causing CNFs  12  to grow unevenly and to have large fiber diameters. This leads to a decrease in the conductivity and active material density of mixture layer  1 B. This also leads to a decrease in the proportion of the electrochemical active phases, making it difficult to use the composite negative electrode active material as a high-capacity electrode material.  
      The following is a description of a method for producing the composite negative electrode active material particles composed of cores  11 , CNFs  12 , and catalyst elements  13 . This production method includes the following four steps of (a) to (d).  
      (a) A step of loading catalyst elements  13  at least in the surface parts of cores  11  which can charge and discharge lithium ions. Catalyst elements  13  are at least one selected from the group consisting of Cu, Fe, Co, Ni, Mo, and Mn which promotes the growth of CNFs  12 .  
      (b) A step of growing CNFs  12  on the surfaces of cores  11  in an atmosphere containing carbon-containing gas and hydrogen gas.  
      (c) A step of sintering cores  11  with CNFs  12  attached thereto in an inert gas atmosphere at 400° C. or more and 1600° C. or less.  
      (d) A step of crushing cores  11  with CNFs  12  attached thereto so as to adjust the tap density of cores  11  to 0.42 g/cm 3  or more and 0.91 g/cm 3  or less.  
      After Step (c), the composite negative electrode active material particles can be subjected to heat treatment in the air at 100° C. or more and 400° C. or less so as to oxidize catalyst elements  13 . The heat treatment at this temperature range can oxidize only catalyst elements  13  without oxidizing CNFs  12 .  
      As Step (a), there may be mentioned a step of supporting the metal particles of catalyst elements  13  on the surfaces of cores  11 ; a step of reducing the surfaces of cores  11  containing catalyst elements  13 ; a step of synthesizing alloy particles of silicon and catalyst elements  13 ; and other steps.  
      The following is a description of conditions when CNFs  12  are grown on the surfaces of cores  11  at Step (b). CNFs  12  start to grow when cores  11  having catalyst elements  13  at least in the surface parts thereof are introduced into a high-temperature atmosphere containing the source gases of CNFs  12 . For example, cores  11  are placed in a ceramic reaction vessel and heated to high temperatures of 100° C. to 1000° C., and more preferably to 300° C. to 600° C. in an inert gas or a gas having a reducing power. Then, carbon-containing gas and hydrogen gas, which are the source gases of CNFs  12 , are introduced into the reaction vessel. When the temperature in the reaction vessel is less than 100° C., CNFs  12  either do not grow or grow very slowly, thereby damaging the productivity. In contrast, when the temperature in the reaction vessel exceeds 1000° C., the source gases are decomposed rapidly, making it harder to grow CNFs  12 .  
      The source gases are preferably a mixture gas of carbon-containing gas and hydrogen gas. Specific examples of the carbon-containing gas include methane, ethane, ethylene, butane, and carbon monoxide. The molar ratio (volume ratio) of the carbon-containing gas in the mixture gas is preferably 20% to 80%. When catalyst elements  13  in a metallic state are not exposed on the surfaces of cores  11 , the proportion of the hydrogen gas can be increased to perform the reduction of catalyst elements  13  and the growth of CNFs  12  in parallel. When the growth of CNFs  12  is terminated, the mixture gas of the carbon-containing gas and the hydrogen gas is replaced with an inert gas and the inside of the reaction vessel is cooled to room temperature.  
      Next, in Step (c), cores  11  having CNFs  12  attached thereto are sintered in an inert gas atmosphere at 400° C. or more and 1600° C. or less. This sintering is preferable because it can prevent the irreversible reaction between electrolyte  5  and CNFs  12  which progresses at the initial charge of the battery, thereby achieving excellent charge-discharge efficiency of the battery. When such sintering process is either not performed or performed at a temperature less than 400° C., the irreversible reaction may not be prevented, causing a decrease in the charge-discharge efficiency. In contrast, when sintering temperatures exceed 1600° C., the electrochemical active phases of cores  11  react with CNFs  12  and may be inactivated or reduced, so that the capacity may be decreased. For example, when the electrochemical active phases of cores  11  are made of silicon, the silicon reacts with CNFs  12  to generate inert silicon carbide, thereby causing a decrease in the charge-discharge capacity of the battery. When cores  11  are made of silicon, the sintering temperature is particularly preferably 1000° C. or more and 1600° C. or less. Some growth conditions could improve the crystallinity of CNFs  12 . When CNFs  12  have high crystallinity, the irreversible reaction between electrolyte  5  and CNFs  12  can be prevented. In this case, Step (c) is not necessary.  
      After being sintered in the inert gas, the composite negative electrode active material particles are preferably heat-treated in the air at 100° C. or more and 400° C. or less in order to oxidize at least parts (surfaces, for example) of the metal particles or metallic phases of catalyst elements  13 . When the heat-treatment temperature is less than 100° C., it is difficult to oxidize the metal, whereas temperatures exceeding 400° C. may burn CNFs  12  thus grown.  
      In Step (d), sintered cores  11  with CNFs  12  attached thereto are crushed. Crushing is preferred, because the particles of the composite negative electrode active material acquire good filling ability (compactability). However, when the tap density is 0.42 g/cm 3  or more and 0.91 g/cm 3  or less, crushing may not be necessary. In other words, when cores  11  with excellent compactability are used as a source material, crushing may not be necessary.  
      The following is a description of a method for producing negative electrode  1 . The composite negative electrode active material composed of cores  11  with CNFs  12  attached to their surfaces is mixed with a binder and a solvent so as to prepare a mixture slurry. Combination examples of the binder and the solvent include polyvinylidene fluoride (PVDF) and N-methyl-2-pyrrolidone (NMP); and an emulsion of polytetrafluoroethylene and water. Other examples of the binder include polyethylene, polypropylene, an aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, poly(methyl acrylate), poly(ethyl acrylate), polyhexylacrylate, poly(methacrylic acid), poly(methyl methacrylate), poly(ethyl methacrylate), poly(hexyl methacrylate), polyvinyl acetate, polyvinylpyrrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethyl cellulose. Further other examples of the binder include copolymers containing at least two selected from tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluoroalkylvinylether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethylvinylether, acrylic acid, and hexadiene.  
      The obtained mixture slurry is applied on current collector  1 A using a doctor blade and dried so as to form mixture layer  1 B on current collector  1 A. Mixture layer  1 B is rolled to adjust its thickness and also to adjust the volume proportion occupied by cores  11  in mixture layer  1 B to 19% or more and 44% or less. A long strip of negative electrodes thus obtained is either stamped or cut into a predetermined size. Lead  8  made of nickel or copper is, for example, welded to the exposed part of current collector  1 A so as to complete negative electrode  1 .  
      Current collector  1 A can be a metal foil made of stainless steel, nickel, copper, or titanium, or a thin film made of carbon or a conductive resin. Current collector  1 A can also be surface-treated with carbon, nickel, titanium, or the like.  
      It is also possible to add the following conductive agent to mixture layer  1 B when necessary. Specific examples of the conductive agent include graphites such as expanded graphite, artificial graphite, and natural graphite such as scaly graphite; carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as copper powder and nickel powder; and organic conductive materials such as a polyphenylene derivative.  
      The following is a description of changes in the. composite negative electrode active material particles during the rolling of negative electrode  1  with reference to  FIGS. 2A  to  2 C and  FIGS. 3A  to  3 C. As shown in  FIG. 2A , the composite negative electrode active material particles of the present embodiment have cores  11  and CNFs  12  attached to them. CNFs  12  are present at least between cores  11 . The presence of highly conductive CNFs  12  between poorly conductive cores  11  allows cores  11  to maintain conductive network. Maintaining such an electrode structure can improve the electronic conductivity in mixture layer  1 B, thereby improving cycle characteristics.  
      The form (properties) of the composite negative electrode active material particles affects the ease with which the aforementioned electrode structure is obtained. This ease closely relates to the compactability of the composite negative electrode active material particles. When the form of cores  11  is controlled enough to make the composite negative electrode active material particles highly compactable, as shown in  FIG. 2B , there is small need to increase rolling load. Therefore, the cracking of the composite negative electrode active material particles can be reduced to some extent when a rolling load is applied. The composite negative electrode active material particles thus cracked are relocated at random in mixture layer  1 B, and as a result, the structure where CNFs  12  are present between cores  11  is developed as shown in  FIG. 2C .  
      On the other hand, in a situation that the composite negative electrode active material particles are hard to be compacted, CNFs  12  are unlikely to form conductive network. Cores  11  are generally not spherical, but are indefinite in shape. Composite negative electrode active material particles containing active material cores  21  (hereinafter, cores  21 ) which are very far from being spherical as shown in  FIG. 3A  are poorly compactable. Applying a rolling load to a negative electrode having such composite negative electrode active material particles in a mixture layer causes a lot of cracks in cores  21  as shown in  FIG. 3B . The occurrence of cracks is affected by the shapes of cores  21  and the grain boundaries of the particles. Applying a large rolling load in an attempt to increase the packing density of the completed composite negative electrode active material particles with poor compactability would cause cracks.  
      Consequently, as shown in  FIG. 3C , the cracked negative electrode active material particles randomly present on the mixture layer generate a lot of portions  22  having no CNFs  12  between cores  21 . In other words, the proportion of CNFs  12 A that are present somewhere other than between cores  21  increases. As a result, electronic conductivity and ionic conductivity in mixture layer  1 B both decrease, thereby causing the battery to decrease both in cycle characteristics and high-load characteristics. Reducing rolling load causes a reduction in the packing density of cores  11 , which are the main body of the active material to charge and discharge lithium ions. This reduces battery energy density.  
      The volume proportion occupied by cores  11  in mixture layer  1 B should be in an appropriate range. More specifically, when there are a lot of CNFs  12  and the volume proportion occupied by cores  11  is less than 19%, conductive network is formed properly, but the capacity density becomes low because the packing density of cores  11  is low.  
      On the other hand, when the volume proportion occupied by cores  11  exceeds 44%, cores  11  are too compact to have enough space formed with CNFs  12 . This makes the ion supply from electrolyte  5  to cores  11  insufficient and therefore the high-load characteristics are lowered. Furthermore, increasing rolling load to achieve this condition causes more damage to the composite negative electrode active material particles. This results in a reduction in electronic conductivity and also in a slight reduction in cycle characteristics.  
      When the volume proportion occupied by cores  11  in mixture layer  1 B is 19% or more and 44% or less, the space formed with CNFs  12  serves as the path of electrolyte  5  so that the ion conductivity can be high. This allows electrolyte  5  to be supplied sufficiently to cores  11  so that the battery can have smooth charge-discharge reaction, thereby having excellent high-load characteristics. In addition, the conductive network formed among cores  11  is well developed enough to achieve excellent electronic conductivity, so that the battery cycle characteristics can be high. In conclusion, it is preferable that the volume proportion occupied by cores  11  in mixture layer  1 B be 19% or more and 44% or less.  
      A large proportion of CNFs  12  tends to decrease the compactability of the composite negative electrode active material particles because CNFs  12  are bulky. In contrast, when the proportion of CNFs  12  is too small, conductive network is not well developed between composite negative electrode active material particles even when the volume proportion occupied by cores  11  is in an appropriate range. Therefore, the weight proportion of CNFs  12  in the composite negative electrode active material particles should be in an appropriate range. When the weight proportion is less than 6%, the electronic conductivity becomes lower than a required level, making the cycle characteristics slightly low. On the other hand, when the weight proportion is over 35%, the composite negative electrode active material particles become too bulky and are required to have higher rolling load, thereby causing more damage to the composite negative electrode active material particles. As a result, the weight proportion of CNFs  12  in the composite negative electrode active material particles is preferably 6% or more and 35% or less. Thus, the appropriate volume proportion occupied by cores  11  in mixture layer  1 B and the appropriate weight proportion of CNFs  12  in the composite negative electrode active material particles achieve well-developed conductive network between the composite negative electrode active material particles.  
      One index to evaluate the compactability of the composite negative electrode active material particles is tap density. Tap density is determined in the following manner basically compliant with JIS-K5101. “Powder Tester” manufactured by Hosokawa Micron Corp. is used for the measurement; a sieve with an aperture of 710 μm is used, through which sample powders are passed. The sample powders are dropped in a tapping cell of 25 cc until the cell is full. Then, tapping one time per second with a stroke length of 18 mm is performed 600 times. The height and weight of the powder in the cell are measured to calculate the tap density.  
      When the tap density is less than 0.42 g/cm 3 , the poor compactability of the composite negative electrode active material particles requires large rolling load in order to ensure good battery energy density. This causes more damage to the composite negative electrode active material particles, thereby making cores  11  crack and fall apart, and decreasing the electronic conductivity. As a result, cycle characteristics decrease. In contrast, when the composite negative electrode active material particles are highly compactable, a necessary packing density can be obtained without the application of large rolling load. The damage to the composite negative electrode active material particles is reduced.  
      The tap density of the composite negative electrode active material particles increases as the particles are closer to spherical and are larger in diameter. Therefore, when the tap density is too large, the surface area of the particles is relatively small. The tap density exceeding 0.91 g/cm 3  lowers the high-load characteristics because the surface area of cores  11  is too small. In conclusion, it is preferable to use composite negative electrode active material particles having a tap density of 0.42 g/cm 3  or more and 0.91 g/cm 3  or less.  
      In order to put CNFs  12  between cores  11 , cores  11  having an appropriate tap density can be crushed after being coated with CNFs  12 . In this approach, the crushing of cores  11  and the measuring of the tap density can be repeated alternately to obtain appropriate composite negative electrode active material particles.  
      As described above, cores  11  are not perfect spheres and may have indefinite shapes, so that when the average particle diameter is less than 1 μm, the obtained composite negative electrode active material particles tend to be poorly compactable. This requires increasing the rolling load and thus causes more damage to the composite negative electrode active material particles. Consequently, the electronic conductivity decreases, and the cycle characteristics is slightly lowered. When the average particle diameter is less than 1 μm, cores  11  are likely to form strong aggregates. Such aggregates have unexposed portions where CNFs  12  do not grow. As a result, cores  11  come into direct contact with each other in many portions.  
      In contrast, when the average particle diameter exceeds 14 μm, the surface area of cores  11  is relatively small, and the high-load characteristics are slightly lowered. Furthermore, unless the amount of CNFs  12  is reduced, the volume proportion occupied by cores  11  is more likely to exceed 44%, which is the upper limit of the acceptable range. In conclusion, the average particle diameter of cores  11  is preferably 1 μm or more and 14 μm or less.  
      The following is a description of advantages of the present invention with specific experiments and the results. First, the volume proportion occupied by cores  11  in mixture layer  1 B, and the average particle diameter of cores  11  are studied as follows, using test cells  1  to  12 .  
      Preparation of Test Cells  
      Test cell  1  is prepared as follows. Firstly, silicon monoxide (SiO) particles which serve as cores  11  are pulverized and classified to have an average particle diameter of 0.5 μm. One part by weight of nickel nitrate (II) hexahydrate is dissolved in ion exchange water to prepare a solution which is used for preparing catalyst elements. To this solution are added SiO particles. The solution is stirred for one hour, and then, water is removed therefrom using an evaporator so as to load nickel nitrate on the surfaces of the SiO particles.  
      Next, the SiO particles with nickel nitrate supported thereon are placed in a ceramic reaction vessel and heated to 550° C. in the presence of helium gas. Then, the helium gas is replaced by a mixture gas consisting of hydrogen gas and methane gas in a volume ratio of 50:50, and left for ten minutes at 550° C. so as to reduce nickel nitrate (II) and to grow CNFs  12  on the SiO particles. After this, the mixture gas is replaced by helium gas, and the inside of the reaction vessel is cooled to room temperature. The temperature is then raised again to 1000° C. in argon gas, the SiO particles are sintered for one hour at 1000° C. so as to obtain a composite negative electrode active material. The weight proportion of CNFs  12  in the composite negative electrode active material particles is set to 15%. After that, the composite negative electrode active material particles are crushed. The obtained composite negative electrode active material particles have a tap density of 0.33 g/cm 3 .  
      The obtained composite negative electrode active material particles are observed with a scanning electron microscope (hereinafter, called as SEM); CNFs  12  are found to be attached to the surfaces of cores  11 . The nickel nitrate supported on cores  11  are reduced to particles having a particle diameter of about 100 nm. The nickel particles are observed with SEM for their particle diameter, fiber diameter, and fiber length. The weight of CNFs  12  is determined by changes in weight before and after CNFs  12  are grown.  
      Then, 100 parts by weight of the composite negative electrode active material are mixed with 7 parts by weight (solid content) of an N-methyl-2-pyrrolidone (hereinafter, called as NMP) solution of PVDF as a binder and an appropriate amount of NMP so as to prepare a negative electrode mixture slurry. The resulting slurry is applied on current collector  1 A made of a 15 μm—think Cu foil using a doctor blade, dried at 60° C. so as to make mixture layer  1 B supported on current collector  1 A. The volume proportion occupied by cores  11  in mixture layer  1 B is 0.18% after the drying. Mixture layer  1 B is stamped in a square having a width of 32 mm and a longitudinal length of 42 mm and is used as negative electrode  1 .  
      Negative electrode  1  thus obtained is used together with counter electrode  2  and separator  3  so as to prepare a flat test cell. Counter electrode  2  is made of metallic lithium, which is 300 μm thick, 34 mm wide, and 44 mm long. Separator  3  is made of a polyethylene microporous membrane having a thickness of 20 μm and a porosity of about 40%. The test cell is then housed in laminate bag  4 . Laminate bag  4  is sealed after being filled with electrolyte  5 ; electrolyte  5  is a solution containing a mixture solvent of ethylene carbonate and diethyl carbonate, and LiPF 6  dissolved at a concentration of 1 mol/dm 3  in the mixture solvent.  
      Test cells  2  to  9  are prepared in the same manner as test cell  1  except that the pulverizing and classifying conditions of SiO particles are changed so that the average particle diameters of the SiO particles can be 1, 2, 4, 8, 10, 12, 14, and 18 μm, respectively.  
      Test cell  10  is prepared in the same manner as test cell  6  except that the sintered composite negative electrode active material particles are used without being crushed.  
      Test cells  11  and  12  are prepared in the same manner as test cell  6  except that mixture layer  1 B is supported on current collector  1 A, and then negative electrode  1  that has been dried but not been cut yet is rolled with a load of 300 kgf/cm and 1000 kgf/cm, respectively.  
      Evaluations of Test Cell Characteristics  
      The test cells use metallic lithium as counter electrode  2 , so that negative electrode  1  has a higher charge-discharge potential than counter electrode  2 . In the following description, charging operation and discharging operation respectively indicate the absorption and desorption of lithium ions to and from negative electrode  1 . In other words, the voltages of the test cells are decreased by charging and increased by discharging.  
      Each test cell is measured for the initial charge and discharge capacities at a charge-discharge current of 0.1 CmA. The obtained discharge capacity is converted to the discharge capacity per apparent unit volume (1 cm 3 ) of mixture layer  1 B (apparent volume: simply calculated from the outside dimension of mixture layer  1 B) so as to calculate a discharge capacity density. The charging is performed until the voltage between the electrodes reaches 0V and the discharging is performed until the voltage reaches 1.5V. In this case, 0.1 CmA indicates a current value obtained by dividing the designed capacity of batteries by ten hours.  
      The test cells are then evaluated for high-load characteristics as follows. Each test cell is charged with a current of 0.1 CmA and then discharged with a current of 1 CmA so as to measure the discharge capacity at 1 CmA. The obtained discharge capacity is divided by the discharge capacity of 0.1 CmA to determine the high-load capacity retention rate, which is used as an index of high-load characteristics.  
      In the end, each test cell is evaluated for cycle characteristics. Charge-discharge operations are repeated for 50 cycles under the same conditions as the initial capacity measurement. The ratio of the discharge capacity at the 50th cycle to the initial discharge capacity is divided by the number of cycles (50) to determine the degradation rate per cycle (cycle degradation rate), which is used as an index of the cycle characteristics.  
      By referring to the case where a negative electrode containing graphite active material is used, the evaluation standards of the discharge capacity density and the high-load capacity retention rate are set to 500 mAh/cm 3  or more and 90% or more, respectively. The evaluation standard of the cycle degradation rate is set to 0.10% or less per cycle after practical usefulness is considered. The configuration of each test cell and the results of the aforementioned evaluations are shown in Table 1 below.  
                           TABLE 1                                      Negative electrode                                                                             Discharge   Highload               SiO   CNF   Composite negative   Rolling   volume   capacity   capacity   Cycle       Test   diameter   weight   electrode material   load   proportion   density   retention   degradation       cell   μm   proportion %   tap density g/cm 3     kgf/cm   of SiO Vol %   mAh/cm 3     rate %   rate %/cycle                                                         1   0.5   15   0.39   —   18   648   89   1.00       2   1   15   0.42   —   22   770   98   0.06       3   2   15   0.44   —   22   797   98   0.06       4   4   15   0.50   —   25   878   98   0.06       5   8   15   0.60   —   28   1013   98   0.06       6   10   15   0.63   —   30   1053   95   0.06       7   12   15   0.70   —   32   1148   94   0.06       8   14   15   0.73   —   33   1188   91   0.06       9   18   15   0.78   —   35   1256   88   0.06       10   10   15   0.42   —   22   770   95   0.06       11   10   15   0.63   300   44   1418   95   0.06       12   10   15   0.63   1000   46   1620   89   1.00                  
 
      Comparison of test cells  1  to  9  indicates the following. In test cells  2  to  8  where the average particle diameters of SiO particles are between 1 μm and 14 μm inclusive, the volume proportions occupied by cores  11  are in the range of 22% or more and 33% or less, and the discharge capacity density, high-load characteristics, and cycle characteristics are all excellent. In test cell  1 , on the other hand, the average particle diameter of the SiO particles is as small as 0.5 μm, so that the crushed composite negative electrode active material particles have a small tap density and are hard to be compacted. The volume proportion occupied by cores  11  is also as small as 18%, so that the conductive network is not developed sufficiently and that the high-load characteristics are slightly low. In addition, the SiO particles form strong aggregates, thereby having some portions where CNFs  12  are not present. Consequently, cycle characteristics are very low. In test cell  9 , the SiO particles in the composite negative electrode active material have a large particle diameter, so that the SiO particles have a small surface area, making high-load characteristics slightly low. In conclusion, the average particle diameter of the SiO particles, which form cores  11 , is preferably 1 μm or more and 14 μm or less. In terms of high-load characteristics, the average particle diameter is more preferably 1 μm or more and 12 μm or less, and further more preferably 1 μm or more and 10 μm or less.  
      The following is a comparison between test cells  10  to  12  and test cell  6 . In the preparation of negative electrode  1 , the sintered composite negative electrode active material particles are crushed in test cell  6 , but are not crushed in test cell  10 . The absence of crushing makes the composite negative electrode active material particles have a slightly small tap density and unlikely to be compacted. As a result, the volume proportion occupied by the SiO particles which are cores  11  in mixture layer  1 B is as small as 22%, making the discharge capacity density small. The reason for this seems to be that the packing density of the SiO particles (or the composite negative electrode active material particles) is slightly low. Even so, test cell  10  is as excellent in quality as test cell  2 . This is because the SiO particles which are a source material of the composite negative electrode active material and have an average particle diameter of 10 μm are highly compactable. Therefore, it is not essential to crush the composite negative electrode active material particles. Although the experimental results are not shown, when the SiO particles which are a source material are not so compactable and the volume proportion occupied by the SiO particles in mixture layer  1 B is less than 19%, the capacity density, high-load characteristics, and cycle characteristics are all low.  
      While in test cell  6  is not performed a rolling operation in the preparation of negative electrode  1 , in test cells  11  and  12  is performed the rolling operation at different loads from each other. The volume proportion occupied by cores  11  is 44% in cell  11 , and 46% in cell  12 . In test cell  11 , the SiO particles are not cracked because the rolling load is not large, so that both high-load characteristics and cycle characteristics are as excellent as in test cell  6 . In addition, the thickness is reduced by rolling, so that the discharge capacity density is increased as compared with test cell  6 .  
      On the other hand, in test cell  12 , the rolling operation is performed at a large load in an attempt to achieve higher capacity by increasing the volume proportion occupied by cores  11 . As a result, the spaces between the SiO particles are not enough, and the high-load characteristics are lowered. Furthermore, it seems that the large rolling load causes the SiO particles to be in direct contact with each other in some portions, and that the SiO particles are cracked. As a result, cycle characteristics are lowered.  
      The following is a description of study on the optimum range of the weight proportion of CNFs  12  in the composite negative electrode active material particles. The weight proportion of CNF 12  to SiO is changed by using as a source material SiO particles having average particle diameters of 1 μm, 8 μm, and 14 μm, respectively, and changing the reaction time.  
      First, the case of using as a source material SiO particles having an average particle diameter of 1 μm is described with reference to Table 2 below. Test cells  13  to  18  are prepared in the same manner as test cell  2  except that the weight proportions of CNFs  12  in the composite negative electrode active material particles are set to 5%, 6%, 10%, 20%, 30%, and 35%, respectively. In the following evaluation results including the evaluation results shown in Table 2, the discharge capacity density is calculated on the basis of the tap volume of the composite negative electrode active material particles. “Tap volume” stands for a volume of powder or particles in a state where the powder or particles are packed and compressed to determine the tap density.  
                           TABLE 2                                      Negative electrode                                                                             Discharge   Highload               SiO   CNF   Composite negative   Rolling   volume   capacity   capacity   Cycle       Test   diameter   weight   electrode material   load   proportion   density   retention   degradation       cell   μm   proportion %   tap density g/cm 3     kgf/cm   of SiO Vol %   mAh/cm 3     rate %   rate %/cycle                                                         13   1   5   0.56   —   30   840   93   1.00       14   1   6   0.52   —   28   764   98   0.10       15   1   10   0.47   —   25   658   98   0.06       2   1   15   0.42   —   22   567   98   0.06       16   1   20   0.35   —   18   455   98   0.06       17   1   30   0.31   —   14   372   95   0.06       18   1   35   0.28   —   12   322   94   0.06                  
 
      As shown in Table 2, it is considered that in test cell  13 , the weight proportion of CNFs  12  is too small to form a well-developed conductive network, and this makes the cycle characteristics low. In contrast, in test cells  16  to  18 , the amount of CNFs  12  is so large that the volume proportion occupied by cores  11  in mixture layer  1 B is less than 19%, making the discharge capacity density small. Thus, when SiO particles having an average particle diameter of 1 μm are used as a source material, the weight proportion of CNFs  12  in the composite negative electrode active material particles is preferably 6% or more and 15% or less.  
      The following is a description of the case of using, as a source material, SiO particles having an average particle diameter of 8 μm with reference to Table 3 blew. Test cells  19  to  24  are prepared in the same manner as test cell  5  except that the weight proportions of CNFs  12  in the composite negative electrode active material particles are set to 5%, 6%, 10%, 20%, 30%, and 35%, respectively.  
                               TABLE 3                                      Negative electrode   High-                                                                             Discharge   load               SiO   CNF   Composite negative   Rolling   volume   capacity   capacity   Cycle       Test   diameter   weight   electrode material   load   Proportion   density   retention   degradation       cell   μm   proportion %   tap density g/cm 3     kgf/cm   of SiO Vol %   mAh/cm 3     rate %   rate %/cycle                                                         19   8   5   0.80   —   40   1200   93   1.00       20   8   6   0.75   —   38   1103   98   0.10       21   8   10   0.67   —   33   938   98   0.08       5   8   15   0.60   —   28   810   98   0.06       22   8   20   0.55   —   25   715   97   0.06       23   8   30   0.45   —   19   540   97   0.06       24   8   35   0.33   —   14   380   97   0.06                  
 
      As shown in Table 3, it is considered that in test cell  19 , the weight proportion of CNFs  12  is too small to form a well-developed conductive network, and this makes the cycle characteristics low. In contrast, in test cell  24 , the amount of CNFs  12  is so large that the volume proportion occupied by cores  11  in mixture layer  1 B is less than 19%, making the discharge capacity density small. Thus, when SiO particles having an average particle diameter of 8 μm are used as a source material, the weight proportion of CNFs  12  in the composite negative electrode active material particles is preferably 6% or more and 30% or less.  
      The following is a description of the case of using, as a source material, SiO particles having an average particle diameter of 14 μm with reference to Table 4 blew. Test cells  25  to  31  are prepared in the same manner as test cell  8  except that the weight proportions of CNFs  12  in the composite negative electrode active material particles are set to 5%, 6%, 10%, 20%, 30%, 35%, and 40% respectively.  
                               TABLE 4                                      Negative electrode   High-                                                                             Discharge   load               SiO   CNF   Composite negative   Rolling   volume   capacity   capacity   Cycle       Test   diameter   weight   electrode material   load   Proportion   density   retention   degradation       cell   μm   proportion %   tap density g/cm 3     kgf/cm   of SiO Vol %   mAh/cm 3     rate %   rate %/cycle                                                         25   14   5   0.97   —   48   1455   87   1.00       26   14   6   0.91   —   44   1338   92   0.10       27   14   10   0.82   —   39   1148   92   0.08       8   14   15   0.73   —   33   986   91   0.06       28   14   20   0.67   —   29   871   91   0.06       29   14   30   0.55   —   22   660   91   0.06       30   14   35   0.49   —   19   564   91   0.06       31   14   40   0.43   —   16   470   91   0.06                  
 
      As shown in Table 4, it is considered that in test cell  25 , the weight proportion of CNFs  12  is too small to form a well-developed conductive network, and this makes the cycle characteristics low. In contrast, in test cell  31 , the amount of CNFs  12  is so large that the volume proportion occupied by cores  11  in mixture layer  1 B is less than 19%, making the discharge capacity density small. Thus, when SiO particles having an average particle diameter of 14 μm are used as a source material, the weight proportion of CNFs  12  in the composite negative electrode active material particles is preferably 6% or more and 35% or less.  
      In conclusion, the weight proportion of CNFs  12  to the composite negative electrode active material particles is preferably 6% or more and 35% or less although it is also affected by the average particle diameter of the SiO particles. In terms of cycle characteristics, the weight proportion is more preferably 10% or more. In order to achieve excellent properties regardless of the average particle diameter of the SiO particles, the weight proportion is preferably 6% or more and 15% or less, and more preferably 10% or more and 15% or less. The results of test cells  23 ,  24 ,  25  and  26  and test cells  10  to  12  indicate that the volume proportion occupied by cores  11  is preferably 19% or more and 44% or less.  
      In order to achieve an appropriate volume proportion occupied by the SiO particles without rolling the negative electrode, the tap density of the composite negative electrode active material particles is preferably 0. 42 g/cm 3  or more and 0.91 g/cm 3  or less based on the results of test cells  2 ,  16 ,  23 ,  24 ,  25 , and  26 .  
      The results of the experiments using the test cell shown in  FIGS. 1A and 1B  are described hereinbefore. Instead of counter electrode  2  made of metallic lithium, a positive electrode having a mixture layer containing the following positive electrode active material can be used to compose a laminate-type non-aqueous electrolyte secondary battery. Examples of the positive electrode active material include lithium-containing complex oxides such as LiCoO 2 , LiNiO 2 , Li 2 MnO 4 , and a mixture or composite thereof. Such a positive electrode active material reduces lithium ions at least during discharge, and contains lithium ions in an uncharged state. In a case where negative electrode  1  does not contain lithium in an uncharged state, the positive electrode need to contain lithium ions as in the present case. Negative electrode  1  having the configuration as described above can be used in a non-aqueous electrolyte secondary battery having this structure so as to achieve a battery excellent in high-load characteristics as well as cycle characteristics.  
      Besides the aforementioned solution, electrolyte  5  may be made of other various electrolyte solutions containing an organic solvent and a solute dissolved in the solvent. Electrolyte  5  may also be a so-called polymer electrolyte layer containing such an electrolyte solution immobilized in a polymer. In the case of using an electrolyte solution, it is preferable to provide a separator impregnated with the electrolyte solution between counter electrode  2  and negative electrode  1 . The separator can be nonwoven fabric or microporous membrane made of polyethylene, polypropylene, an aramid resin, amideimide, polyphenylene sulfide, or polyimide. The separator may also contain a heat resistant filler such as alumina, magnesia, silica, or titania either inside or on its surface. Besides the separator, can be used a heat resistant layer made of one of the fillers and the same binder as used in the negative electrode.  
      The material of electrolyte  5  is selected based on the oxidation-reduction potential of the active material and other conditions. As the solute for electrolyte  5 , salts commonly used in lithium batteries can be used. Examples of the salt include LiPF 6 , LiBF 4 , LiClO 4 , LiAlCl 4 , LiSbF 6 , LiSCN, LiCF 3 SO 3 , LiN(CF 3 SO 2 ), LiN(C 2 F 5 SO 2 ) 2 , LiAsF 6 , LiB 10 Cl 10 , lower aliphatic lithium carboxylate, LiF, LiCl, LiBr, LiI, chloroborane lithium; various borates such as bis(1,2-benzenediolate(2-)-O,O′)lithium borate, bis(2,3-naphthalenediolate(2-)-O,O′)lithium borate, bis(2,2′-biphenyldiolate(2-)-O,O′)lithium borate, bis(5-fluoro-2-olate-1-benzenesulfonic acid-O,O′)lithium borate; and tetraphenyl lithium borate.  
      As the organic solvent in which the aforementioned salts are dissolved, solvents commonly used in lithium batteries can be used. Examples of the organic solvent include the following which can be used either on their own or in combination: ethylene carbonate, propylene carbonate, butylene carbonate, vinylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dipropyl carbonate, methyl formate, methyl acetate, methyl propionate, ethyl propionate, dimethoxymethane, γ-butyrolactone, γ-valerolactone, 1,2-diethoxyethane, 1,2-dimethoxyethane, ethoxymethoxyethane, trimethoxymethane, tetrahydrofuran derivatives such as tetrahydrofuran and 2-methyl-tetrahydrofuran, dimethyl sulfoxide, dioxolane derivatives such as 1,3-dioxolane and 4-methyl-1,3-dioxolane, formamide, acetamide, dimethylformamide, acetonitrile, propylnitrile, nitromethane, ethylmonoglyme, trimester phosphate, acetate ester, propionate ester, sulfolane, 3-methyl-sulfolane, 1,3-dimethyl-2-imidazolidinone, 3-methyl-2-oxazolidinone, a propylene carbonate derivative, ethyl ether, diethyl ether, 1,3-propane sultone, anisole, and fluorobenzene.  
      Electrolyte  5  may further contain an additive such as vinylene carbonate, cyclohexylbenzene, biphenyl, diphenyl ether, vinylethylene carbonate, divinylethylene carbonate, phenylethylene carbonate, diallyl carbonate, fluoroethylene carbonate, catechol carbonate, vinyl acetate, ethylene sulfite, propane sultone, trifluoropropylene carbonate, dibenzofuran, 2,4-difluoroanisole, o-terphenyl, and m-terphenyl.  
      Electrolyte  5  may alternatively be used in the form of a solid electrolyte by adding the aforementioned solute to the following polymeric materials either on their own or in combination: poly(ethylene oxide), poly(propylene oxide), polyphosphazene, polyaziridine, polyethylene sulfide, polyvinyl alcohol, polyvinylidene fluoride, and polyhexafluoropropylene.  
      Electrolyte  5  having the polymeric materials may alternatively be used in the form of gel by being mixed with one of the aforementioned organic solvents. As the solid electrolyte can be used an inorganic material such as a lithium nitride, a lithium halide, lithium oxoate, Li 4 SiO 4 , Li 4 SiO 4 -LiI-LiOH, Li 3 PO 4 -Li 4 SiO 4 , Li 2 SiS 3 , Li 3 PO 4 -Li 2 S—SiS 2 , and a phosphorus sulfide compound.  
      Specific examples of the positive electrode active material other than the lithium-containing complex oxides mentioned above include olivine-type lithium phosphate expressed by a general formula: LiMPO 4 , where M=V, Fe, Ni, or Mn, and lithium fluorophosphates expressed by a general formula: Li 2 MPO 4 F, where M=V, Fe, Ni, or Mn. It is also possible to replace part of the constituent elements of these lithium-containing compounds by a different element. The surfaces of lithium-containing compounds may be treated with a metal oxide, a lithium oxide, a conductive agent or the like, or may be subjected to hydrophobic treatment.  
      Specific examples of the conductive agent to be used for the positive electrode include graphites such as natural graphite and artificial graphite; carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride powder and aluminum powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and organic conductive materials such as a phenylene derivative.  
      The binder for the positive electrode can be the same as for negative electrode  1 . Specific examples of the binder include PVDF, polytetrafluoroethylene, polyethylene, polypropylene, an aramid resin, polyamide, polyimide, polyamideimide, polyacrylonitrile, polyacrylic acid, poly(methyl acrylate), poly(ethyl acrylate), polyhexylacrylate, poly(methacrylic acid), poly(methyl methacrylate), poly(ethyl methacrylate), poly(hexyl methacrylate), polyvinyl acetate, polyvinylpyrrolidone, polyether, polyethersulfone, hexafluoropolypropylene, styrene-butadiene rubber, and carboxymethyl cellulose. Further other examples of the binder include copolymers containing at least two of the following: tetrafluoroethylene, hexafluoroethylene, hexafluoropropylene, perfluorbalkylvinylether, vinylidene fluoride, chlorotrifluoroethylene, ethylene, propylene, pentafluoropropylene, fluoromethylvinylether, acrylic acid, and hexadiene. Alternatively, two or more of these elements can be mixed.  
      The current collector and the lead to be used for the positive electrode can be made of stainless steel, aluminum, titanium, carbon, a conductive resin, or the like, and these materials may be surface-treated with carbon, nickel, titanium or the like.  
      The structure of the battery is not limited to the Aforementioned structure in which the electrode plates face each other. A coin shaped battery, or either a cylindrical or prismatic battery which is obtained by winding thin and long strips of positive and negative electrodes can also provide the same advantages as the battery having the aforementioned structure. Coin shaped batteries do not always need current collector  1 A; mixture layer  1 B may be formed directly on the inner surface of a metal case, which is made of iron, nickel-plated iron, or the like and also serves as an external terminal. Furthermore, instead of using a wet process with a mixture paste, the composite negative electrode active material may be mixed with a powdered binder and then pressed.