Patent Publication Number: US-2004048161-A1

Title: Rechargeable battery using nonaqeous electorlyte

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to a nonaqueous electrolyte rechargeable battery including a positive electrode, a negative electrode and a nonaqueous electrolyte, and more particularly to a nonaqueous electrolyte rechargeable battery using molybdenum oxide as the positive electrode material.  
       BACKGROUND ART  
       [0002] Rechargeable lithium batteries, currently under practical use, use lithium cobalt oxide (LiCoO 2 ) or lithium manganate (LiMn 2 O 4 ) as the positive electrode material and carbon material as the negative electrode material. However, portable equipments demand rechargeable batteries having further longer operating time. There accordingly is a need for rechargeable batteries which have the increased capacities and energy densities. Meanwhile, lithium cobalt oxide, the most popular positive electrode material, is a rare resource and expensive. Accordingly, its substituents have been sought extensively.  
       [0003] One possible substituent of lithium cobalt oxide may be molybdenum oxide. Although cobalt in lithium cobalt oxide changes in oxidation state from trivalent to tetravalent form, molybdenum in molybdenum oxide is changeable in oxidation state from tetravalent to hexavalent form. Hence, the increased capacity and energy density can be expected from the use of molybdenum oxide in place of lithium cobalt oxide.  
       [0004] However, conventional rechargeable lithium batteries using molybdenum oxide in place of lithium cobalt oxide remain to exhibit lower discharge capacities than theoretical capacities. Japanese Patent Laid-Open Nos. Hei 11-250907 and Hei 3-88269 propose the use of molybdenum oxide in the noncrystalline form. This however has been still insufficient in terms of capacity and energy density.  
       [0005] The present applicant has proposed the use of a silicon thin film deposited on a negative current collector as a high-capacity negative electrode material for rechargeable lithium batteries (Japanese Patent Application No. Hei 11-301646). This negative electrode material is capable of 3,000-4,000 mAh/g or higher at the negative electrode.  
       [0006] However, in the case where a rechargeable lithium battery is fabricated using such a silicon thin film as the negative electrode material and lithium cobalt oxide as the positive electrode material, a thickness of the layer of positive active material must be made considerably large to balance the positive electrode capacity against the negative electrode capacity. This makes it difficult for the electrolyte to penetrate through the layer of positive active material during the manufacturing process. This may also cause problematic depletion of the electrolyte present in the layer of positive active material during charge-discharge cycles, which results in the reduced charge-discharge cycle characteristics. There accordingly remains a need for development of a positive electrode material having a positive electrode capacity well balanced in level against a high negative electrode capacity.  
       DISCLOSURE OF THE INVENTION  
       [0007] It is a first object of the present invention to provide a nonaqueous electrolyte rechargeable battery using molybdenum oxide as a positive electrode material, which has high capacity and energy density.  
       [0008] It is a second object of the present invention to provide a nonaqueous electrolyte rechargeable battery using a thin film of silicon as negative active material, which exhibits high capacity and energy density and excellent charge-discharge cycle characteristics.  
       [0009] A nonaqueous electrolyte rechargeable battery in accordance with a first aspect of the present invention includes a positive electrode, a negative electrode, and a nonaqueous electrolyte. Characteristically, it uses molybdenum oxide in the form of a thin film deposited on an aluminum-containing substrate as the positive electrode.  
       [0010] In the first aspect of the present invention, a surface of the substrate that carries the thin film of molybdenum oxide thereon contains aluminum, specifically comprises aluminum or its alloy. Preferably, such thin film-forming techniques as CVD, sputtering, vacuum evaporation and spraying may be used to deposit the thin film on the substrate.  
       [0011] Conventionally, the provision of a thin film of molybdenum oxide on a silica glass substrate or a silicon wafer has been studied (see, for example, C. Julien, G. A. Nazri, J. P. Guesdon, A. Gorenstein, A. Khelfa, O. M. Hussain, Solid State Ionics 73(1994) 319-326). However, the molybdenum oxide thin film deposited on the aluminum-containing substrate in the first aspect of the present invention has a crystal structure which is different from that of the conventionally-used thin film of molybdenum oxide. This is probably because the crystal structure of the deposited film has been subjected to the influence of the substrate. More specifically, the surface structure, physical and chemical properties of the substrate are believed to affect the crystal structure, grain shape, physical and chemical properties of the molybdenum oxide thin film to be deposited thereon. This is believed to change the site at which insertion and deinsertion of lithium occurs to result in the increased discharge capacity and energy density.  
       [0012] A first thin film of molybdenum oxide in accordance with the first aspect of the present invention is characterized as satisfying the relationship 0≦I(020)/I(110)≦0.3, wherein I(020) is an intensity of a peak present in the range of 2θ=12.7±1.0° and I(110) is an intensity of a peak present in the range of 2θ=23.3±1.0°, when determined by X-ray diffraction analysis using Cu—Kα as an X-ray source.  
       [0013] A second thin film of molybdenum oxide in accordance with the first aspect of the present invention is characterized as satisfying the relationship 0≦I(020)/I(021)≦0.2, wherein I(020) is an intensity of a peak present in the range of 2θ=12.7±1.0° and I(021) is an intensity of a peak present in the range of 2θ=27.3±1.0°, when determined by X-ray diffraction analysis using Cu—Kκ as an X-ray source.  
       [0014] A third thin film of molybdenum oxide in accordance with the first aspect of the present invention is characterized as satisfying the relationship 0≦I(040)/I(110)≦0.6, wherein I(040) is an intensity of a peak present in the range of 2θ=25.7±1.0° and I(110) is an intensity of a peak present in the range of 2θ=23.3±1.0°, when determined by X-ray diffraction analysis using Cu—Kκ as an X-ray source.  
       [0015] A fourth thin film of molybdenum oxide in accordance with the first aspect of the present invention is characterized as satisfying the relationship 0≦I(040)/I(021)≦0.5, wherein I(040) is an intensity of a peak present in the range of 2θ=25.7±1.0° and I(021) is an intensity of a peak present in the range of 2θ=27.3±1.0°, when determined by X-ray diffraction analysis using Cu—Kκ as an X-ray source.  
       [0016] In the first aspect of the present invention, the substrate can preferably serve as a current collector for the electrode. Preferably, a surface of the substrate that carries the thin film thereon has a surface roughness Ra of 0.001-1 μm. The use of the substrate having such surface roughness Ra enables efficient current collection since it assures good adhesion between the substrate serving as the current collector and the thin film of molybdenum oxide even when the latter is subjected to expansion and shrinkage during charge and discharge. The surface roughness Ra is defined in Japanese Industrial Standards (JIS B 0601-1994) and can be measured as by a surface roughness meter.  
       [0017] In the first aspect of the present invention, the surface roughness Ra of the substrate preferably satisfies the relationship Ra≦t, where t is a thickness of the molybdenum oxide thin film.  
       [0018] Also in the first aspect of the present invention, the surface roughness Ra of the substrate and a mean spacing of local peaks of profile S preferably satisfy the relationship S≦100Ra. The mean spacing of local peaks of profile S is also defined in Japanese Industrial Standards (JIS B 0601-1994) and can be measured as by a surface roughness meter.  
       [0019] In the first aspect of the present invention, the surface roughness Ra of the substrate is preferably 0.0105 μm or larger, more preferably in the range of 0.011-0.1 μm, most preferably in the range of 0.012-0.09 μm. By depositing, in the form of a thin film, molybdenum oxide on the substrate having a surface roughness Ra within the specified range, a morphology of the deposited molybdenum oxide thin film can be varied so that an electrode having improved cycle characteristics can be obtained. That is, the molybdenum oxide thin film, if deposited on the substrate roughened at its surface to a surface roughness Ra within the specified range, has a structure which shows good adhesion to the substrate serving as a current collector.  
       [0020] In the first aspect of the present invention, an oxidation number of molybdenum in the molybdenum oxide thin film is preferably 5 or larger. Molybdenum with an oxidation number of 5 or larger undergoes a large change in valence number during charge and discharge. Also, an accompanying change of an electronic structure of the active material increases a discharge potential. As a result, the further increased energy density is obtained.  
       [0021] In the first aspect of the present invention, the molybdenum oxide thin film may further contain a dissimilar element.  
       [0022] A nonaqueous electrolyte rechargeable battery in accordance with a second aspect of the present invention has a positive electrode, a negative electrode and a nonaqueous electrolyte. Characteristically, an active material of the positive electrode is molybdenum oxide and an active material of the negative electrode is silicon in the form of a thin film deposited on a negative current collector.  
       [0023] In the second aspect of the present invention, molybdenum oxide is used as active material of the positive electrode. Lithium cobalt oxide is the most widely-used positive active material in the art and cobalt therein undergoes a change of oxidation number from 3 to 4. On the other hand, molybdenum in molybdenum oxide for use in the second aspect of the present invention is changeable in oxidation number from 4 to 5 or larger. For this reason, molybdenum oxide provides a higher capacity than lithium cobalt oxide. Accordingly, the use of molybdenum oxide in combination with the silicon thin film results in the successful increase of an energy density. It also betters a balance between positive and negative electrode capacities and thereby permits a control of respective thicknesses of the positive and negative electrode plates. This prevents shortage of the electrolyte at each electrode plate and thus improves cycle characteristics.  
       [0024] In the second aspect of the present invention, the molybdenum oxide for use as the positive active material may be in the form of a powder, or alternatively, a thin film deposited on the positive current collector. Molybdenum oxide, if used in the powder form, may be mixed with a binder and a conductive filler such as graphite powder to provide a slurry which is subsequently coated onto a positive current collector such as an aluminum foil.  
       [0025] Preferably, the thin film of molybdenum oxide may be deposited on the aluminum-containing substrate, e.g., an aluminum foil, by such thin film-forming techniques as CVD, sputtering and spraying.  
       [0026] The molybdenum oxide thin film in accordance with the second aspect of the present invention may be either noncrystalline or crystalline. If crystalline, it preferably has X-ray diffraction peaks as illustrated in the description of the first aspect of the present invention.  
       [0027] The oxidation number of molybdenum in the molybdenum oxide thin film is preferably 5 or larger.  
       [0028] Molybdenum in molybdenum oxide, if having an oxidation number of 5 or larger, undergoes a large change in valence number during charge and discharge. Also, an accompanying change of an electronic structure of the active material increases a discharge potential. As a result, the further increased energy density is obtained. The molybdenum oxide thin film may further contain a dissimilar element.  
       [0029] In the second aspect of the present invention, used as the negative active material is silicon in the form of a thin film deposited on the negative current collector.  
       [0030] Preferably, the silicon thin film has a microcrystalline or noncrystalline form. Silicon is identified as being microcrystalline when Raman spectroscopy detects substantial presence of a peak around 520 cm −1  which corresponds to a crystalline region and a peak around 480 cm −1  which corresponds to a noncrystalline region, and as being noncrystalline when Raman spectroscopy detects substantial absence of a peak around 520 cm −1  corresponding to the crystalline region and substantial presence of a peak around 480 cm −1  corresponding to the noncrystalline region.  
       [0031] In the second aspect, the silicon thin film can be deposited on a negative current collector, such as a copper foil, by such thin film-forming techniques as CVD, sputtering, spraying and vapor evaporation.  
       [0032] In the second aspect of the present invention, an aluminum foil and a copper foil are particularly preferred for use as the positive and negative current collectors, respectively. Where a copper foil is used as the negative current collector, the use of an electrolytic copper foil having a large value for surface roughness Ra is particularly preferred.  
       [0033] The positive and negative current collectors are hereinafter referred to as a “current collector”, collectively.  
       [0034] In the second aspect of the present invention, a surface of the current collector that carries the silicon thin film thereon preferably has a surface roughness Ra of 0.01-1 μm, and a surface of the current collector that carries the molybdenum oxide thin film thereon preferably has a surface roughness Ra of 0.001-1 μm. The use of the current collector having such surface roughness Ra enables efficient current collection since it assures good adhesion between the current collector and the thin film of silicon or molybdenum oxide even when the thin film is subjected to expansion and shrinkage during charge and discharge. The surface roughness Ra is defined in Japanese Industrial Standards (JIS B 0601-1994) and can be measured as by a surface roughness meter.  
       [0035] In the second aspect of the present invention, the surface roughness Ra of the current collector preferably satisfies the relationship Ra≦t, where t is a thickness of the silicon or molybdenum oxide thin film.  
       [0036] Also in the second aspect of the present invention, the surface roughness Ra of the current collector and a mean spacing of local peaks of profile S preferably satisfy the relationship S≦100Ra. The mean spacing of local peaks of profile S is also defined in Japanese Industrial Standards (JIS B 0601-1994) and can be measured as by a surface roughness meter. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0037]FIG. 1 is a chart showing an X-ray diffraction pattern of the molybdenum oxide thin film obtained in one example of the present invention;  
     [0038]FIG. 2 is a photomicrograph taken using an electron microscope, showing a surface of the molybdenum oxide thin film obtained in one example of the present invention;  
     [0039]FIG. 3 is a chart showing an X-ray diffraction pattern of the molybdenum oxide thin film obtained in another example of the present invention;  
     [0040]FIG. 4 is a chart showing an X-ray diffraction pattern of a commercially available crystal MoO 3 ;  
     [0041]FIG. 5 is a schematic sectional view showing the beaker cell constructed in examples;  
     [0042]FIG. 6 is a schematic sectional view showing the thin film-forming apparatus employed in examples of the present invention;  
     [0043]FIG. 7 is a photomicrograph taken using an electron microscope, showing a surface of the molybdenum oxide thin film of the positive electrode b 1  fabricated in an example of the present invention;  
     [0044]FIG. 8 is a photomicrograph taken using an electron microscope, showing a surface of the molybdenum oxide thin film of the positive electrode b 2  fabricated in an example of the present invention;  
     [0045]FIG. 9 is a photomicrograph taken using an electron microscope, showing a surface of the molybdenum oxide thin film of the positive electrode b 3  fabricated in an example of the present invention; and  
     [0046]FIG. 10 is a graph showing a capacity retention rate on each cycle for cells using the positive electrodes b 1 , b 2  and b 3 . 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
     [0047] The present invention is below described in more detail by way of Examples. It will be recognized that the following examples merely illustrate the practice of the present invention but are not intended to be limiting thereof. Suitable changes and modifications can be effected without departing from the scope of the present invention.  
     [0048] Experiment 1  
     [0049] A rechargeable lithium battery was constructed having a positive electrode comprised of molybdenum oxide in the form of a thin film deposited on an aluminum foil substrate by a sputtering technique. Another rechargeable lithium battery was constructed using a marketed MoO 3  crystal powder as the positive electrode material. The electrochemical properties thereof were compared.  
     [0050] (Fabrication of Electrode)  
     [0051] A thin film of molybdenum oxide was deposited by a sputtering technique on a rolled aluminum foil (20 μm thick) as a substrate, using MoO 3  as a target. The detailed thin film-forming conditions are listed in Table 1. The deposited thin film of molybdenum oxide was about 2 μm thick. A surface roughness Ra of the rolled aluminum foil was 0.001-0.010 μm.  
                       TABLE 1                                   During Film Formation                                                        Argon Gas Flow Rate   100   sccm           Oxygen Gas Flow Rate   10   sccm           Reaction Pressure   0.1   Pa           RF Power   200   W                             Substrate Temp.   Not Heated               (Without Temp. Control)           Target Material   MoO 3                        
 
     [0052] X-ray diffraction (XRD) measurement was performed for the obtained molybdenum oxide thin film. The resulting XRD chart is shown in FIG. 1. As shown in FIG. 1, the measurements revealed no clear peak other than a peak for aluminum of the substrate and confirmed this molybdenum oxide thin film as being noncrystalline (amorphous). Next, the molybdenum oxide thin film was observed using a scanning electron microscope (SEM) at a magnification of 5,000×. A photomicrograph taken using SEM is shown in FIG. 2. As shown in FIG. 2, a dense aggregate of particles having diameters of 1.5 μm and below was observed at a surface of the oxide thin film obtained.  
     [0053] Also, an oxidation number of molybdenum in this thin film was found to be 4.2. The aluminum foil substrate carrying the thin film thereon was cut into a specific size to fabricate a positive electrode a 1 .  
     [0054] Next, a thin film of molybdenum oxide was deposited by a sputtering technique on a rolled aluminum foil similar to the preceding one, while accompanied by activation of oxygen in a sputtering gas and a surface reaction, so that an oxidation number of molybdenum was increased in a resulting thin film than in the thin film of the positive electrode a 1 . The employed thin film-forming apparatus is shown in FIG. 6.  
     [0055] As shown in FIG. 6, located within a vacuum chamber  10  is a substrate holder  11  on which an aluminum foil, as a substrate  12 , is placed. A target  13  of MoO 3  is provided on an electrode  14  to lie beneath the substrate  12 . An RF power source  15  is connected to the electrode  14 .  
     [0056] A side wall of the vacuum chamber that positions sideward of the substrate  12  has an opening  10   a.  An ECR plasma generation chamber  16  is provided to cover the opening  10   a  from outside. Solenoid coils  17  are mounted circumferentially of the ECR plasma generation chamber  16  for production of an external magnetic field. The ECR plasma generation chamber  16  has at its end a microwave inlet window  19  to which a microwave waveguide  18  is connected. Also, a gas inlet tube  20  is also connected to the plasma generation chamber  16  for introduction of an argon or oxygen gas thereinto.  
     [0057] A microwave generated in a microwave supplying means, not shown in the drawing, is passed through the microwave waveguide  18  and the microwave inlet window  19  and introduced into the ECR plasma generation chamber  16 . A high density plasma is produced within the ECR plasma generation chamber 16 by interaction of a radio-frequency field produced by the microwave with a magnetic field produced within the solenoid coils  17 . This plasma is guided to pass through the opening  10   a  along a diverging magnetic field produced by the solenoid coils  17  into the vacuum chamber  10 .  
     [0058] Using the apparatus shown in FIG. 6, an argon gas at 100 sccm and an oxygen gas at 10 sccm, as sputtering gases, were introduced though the gas inlet tube  20  into the ECR plasma generation chamber  16  where an ECR plasma was produced by the interaction of the radio-frequency field and magnetic field. The ECR plasma was then directed to pass through the opening  10   a  and bombard the substrate  12 . Concurrently, an RF power was applied to the electrode  14  from the RF power source  15  so that a plasma is produced between the substrate  12  and the target  13 . The subsequent sputtering of MoO 3  of the target  13  by the plasma resulted in the formation of a molybdenum oxide thin film on the substrate  12 . An aluminum foil roughened at its surface by polishing with a #800 sand paper served as the preceding substrate  12 . The detailed thin film-forming conditions are listed in Table 2. A thickness of the deposited thin film of molybdenum oxide was about 2 μm. The surface-roughened aluminum foil was found to have a surface roughness Ra of 0.093 μm.  
                       TABLE 2                                   During Film Formation                                                        Argon Gas Flow Rate   100   sccm           Oxygen Gas Flow Rate   10   sccm           Reaction Pressure   0.1   Pa           RF Power   200   W           Microwave Power   200   W                             Substrate Temp.   Not Heated               (Without Temp. Control)           Target Material   MoO 3                        
 
     [0059]FIG. 3 is an XRD chart of the obtained molybdenum oxide thin film. FIG. 4 is an XRD chart of the commercially available powder of crystalline MoO 3  for a comparative purpose. The XRD chart of FIG. 3 and the XRD chart of FIG. 4 for the crystalline MoO 3  show a similar pattern of peaks but differ largely from each other in terms of peak intensity ratios. Particularly, the molybdenum oxide thin film exhibits the reduced (0k0) reflection peak relative to crystalline MoO 3 . That is, the XRD chart of FIG. 4 for the crystalline MoO 3  shows that a ratio of the (020) peak intensity to (110) peak intensity, I(020)/I(110), is 3.61; a ratio of the (020) peak intensity to (021) peak intensity, I(020)/I(021), is 2.68; a ratio of the (040) peak intensity to (110) peak intensity, I(040)/I(110), is 4.35; and a ratio of the (040) peak intensity to (021) peak intensity, I(040)/I(021), is 3.23. In contrast, the XRD chart of FIG. 3 shows that I(020)/I(110) is 0.01, I(020)/I(021) is 0.09, I(040)/I(110) is 0.01 and I(040)/I(021) is 0.09. These are considered to result from different conditions of crystal growth between the thin film of molybdenum oxide and the crystalline MoO 3  marketed in the trade.  
     [0060] An oxidation number of molybdenum in the deposited molybdenum oxide thin film was measured to be 5.5. This thin film, together with underlying aluminum foil substrate, were cut into a predetermined size to fabricate a positive electrode a 2 .  
     [0061] For a comparative purpose, 90 parts by weight of the marketed crystalline powder of MoO 3 , 5 parts by weight of artificial graphite powder, and an N-methyl-2-pyrrolidone (NMP) solution containing 5 parts by weight of polyvinylidene fluoride were mixed to prepare a slurry. This slurry was coated on one surface of a rolled aluminum foil as similar to the preceding one by a doctor blade technique to provide a layer of active material thereon, vacuum dried at 150° C. for 2 hours and cut into a predetermined size to fabricate a positive electrode a 3 .  
     [0062] (Preparation of Electrolyte Solution)  
     [0063] 1 mole/liter of LiPF 6  was dissolved in a mixed solvent containing ethylene carbonate and diethyl carbonate at a 1:1 ratio by volume to prepare an electrolyte solution.  
     [0064] (Construction of Beaker Cells)  
     [0065] Using each of the above-fabricated electrodes a 1 -a 3  as a working electrode, a beaker cell as shown in FIG. 5 was constructed. As shown in FIG. 5, the beaker cell includes a counter electrode  3 , a working electrode  4  and a reference electrode  5 , which are all immersed in an electrolyte solution  2  contained in a container  1 . The above-prepared electrolyte solution was used as the electrolyte solution  2 . Metallic lithium was used for both the counter electrode  3  and the reference electrode  5 .  
     [0066] (Charge-Discharge Test)  
     [0067] Each of the above-constructed beaker cells was discharged at 25° C. at a constant current of 0.2 mA to 1.0 V (vs. Li/Li + ) This was recorded as an initial discharge. It was then charged at a constant current of 0.2 mA to 3.5 V (vs. Li/Li + ) and further at a constant current of 0.067 mA to 3.5 V (vs. Li/Li + ), and then discharged under the same conditions as described above. This was recorded as a second-cycle discharge. Discharge capacity, average discharge potential and discharge energy density for each beaker cell are given in Table 3.  
                                   TABLE 3                                           Average                   Discharge   Discharge   Discharge           Positive   Capacity   Potential   Energy Density           Electrode   (mAh/g)   (V(vs.Li/Li + ))   (mWh/g)                                        Initial Discharge                                     a1   460   1.80   827           a2   336   2.26   757           a3   185   2.36   437                 2nd-cycle Discharge                                     a1   294   1.54   453           a2   280   2.17   609           a3   157   2.24   352                      
 
     [0068] As can be clearly seen from the results shown in Table 3, the discharge capacity and average discharge potential of each of the positive electrodes a 1 -a 3  drop markedly between initial discharge and 2nd-cycle discharge but remain relatively constant on the subsequent cycles.  
     [0069] On the 2nd-cycle discharge, the positive electrodes a 1  and a 2  both exhibit higher discharge capacities and discharge energy densities than the positive electrode a 3 . This is most probably due to the difference in crystal structure between the molybdenum oxide thin film and crystalline MoO 3 .  
     [0070] Also on the 2nd-cycle discharge, the positive electrode a 2  exhibits lower discharge capacity and higher average discharge potential than the electrode a 1 . Accordingly, the positive electrode a 2  exhibits higher discharge energy density than the positive electrode a 1 . In the construction of a rechargeable lithium battery, the use of a material other than a lithium metal for the negative electrode may cause a potential rise of the positive electrode at the final stage of discharge. Then, discharge is most likely terminated before the positive electrode potential drops to 1.0 V (vs. Li/Li + ). It is thus considered that the use of the electrode a 2  having the higher average discharge potential than the electrode a 1  is preferred for use as a positive electrode of a rechargeable lithium battery.  
     [0071] The preceding results demonstrate that the molybdenum oxide thin film deposited on the aluminum foil by a sputtering technique, when used as a positive active material, exhibits the increased energy density compared to the marketed crystalline MoO 3 . It has been also confirmed that the similar effects are obtained with the use of respective molybdenum oxide thin films deposited by vacuum evaporation, CVD and spraying techniques.  
     [0072] Experiment 2  
     [0073] (Fabrication of Positive Electrode b 1 )  
     [0074] A rolled aluminum foil (20 μm thick) was polished with a #4000 sand paper to provide a substrate having a rough surface. The substrate was found to have a surface roughness of 0.0128 μm. Measurement of surface roughness Ra was performed using a feeler-type surface profilimeter Detak ST (available from Nippon Shinku Co., Ltd.) at a measurement distance of 2.0 mm. The correction for a deflection gain was carried out prior to calculation of the surface roughness Ra. The correction values, a low pass=200 μm and a high pass=20 μm, were added to achieve deflection correction. The surface roughness Ra is given by an automatically calculated value. The surface roughness Ra in the preceding Experiment 1 was measured in a similar fashion.  
     [0075] Next, using the apparatus shown in FIG. 6, a thin film of molybdenum oxide was deposited by a sputtering technique on the roughened surface of the preceding aluminum foil, while accompanied by activation of oxygen in a sputtering gas and a surface reaction.  
     [0076] Specifically, an argon gas at 100 sccm and an oxygen gas at 20 sccm, both as sputtering gases, were introduced into the ECR plasma source where an ECR plasma was generated by the action of a microwave power and a magnetic field. The ECR plasma generated was then directed onto the aluminum foil substrate. Concurrently, an RF power was applied to a sputtering source where a target of molybdenum oxide was disposed, so that aplasma was produced between the substrate and the target. The subsequent sputtering of the molybdenum oxide by the produced plasma resulted in the formation of a molybdenum oxide thin film on the substrate. The detailed thin film-forming conditions are listed in Table 4. The deposited thin film of molybdenum oxide was about 2 μm thick.  
                       TABLE 4                                   During Film Formation                                                        Argon Gas Flow Rate   100   sccm           Oxygen Gas Flow Rate   20   sccm           Reaction Pressure   0.1   Pa           RF Power   350   W           Microwave Power   200   W                             Substrate Temp.   Not Heated               (Without Temp. Control)           Target Material   MoO 3                        
 
     [0077]FIG. 7 is a photomicrograph taken using a scanning electron microscope (at a magnification of 20,000×), showing a surface of the deposited thin film of molybdenum oxide.  
     [0078] The deposited thin film, together with the underlying aluminum foil substrate, were cut into a predetermined size to fabricate a positive electrode b 1 .  
     [0079] (Fabrication of Positive Electrode b 2 )  
     [0080] A rolled aluminum foil (20 μm thick) was polished with a #800 sand paper to a surface roughness Ra of 0.0930 μm.  
     [0081] This aluminum foil was used as a substrate on which a thin film of molybdenum oxide was to be deposited. Otherwise, the procedure used in the fabrication of the positive electrode b 1  was followed to fabricate a positive electrode b 2  with a predetermined size.  
     [0082]FIG. 8 is a photomicrograph taken using a scanning electron microscope (at a magnification of 20,000×), showing a surface of the deposited thin film of molybdenum oxide.  
     [0083] (Fabrication of Positive Electrode b 3 )  
     [0084] For a comparative purpose, a rolled aluminum foil (20 μm thick) was left unpolished and used as a substrate on which a thin film of molybdenum oxide was to be deposited. Otherwise, the procedure used in the fabrication of the positive electrode b 1  was followed to fabricate a positive electrode b 3  with a predetermined size. The substrate was found to have a surface roughness Ra of 0.0026 μm.  
     [0085]FIG. 9 is a photomicrograph taken using a scanning electron microscope (at a magnification of 20,000×), showing a surface of the deposited thin film of molybdenum oxide.  
     [0086] (Preparation of Electrolyte Solution)  
     [0087] 1 mole/liter of LiPF 6  was dissolved in a mixed solvent containing ethylene carbonate and diethyl carbonate at a 4:6 ratio by volume to prepare an electrolyte solution.  
     [0088] (Construction of Beaker Cells)  
     [0089] The procedure of Experiment 1 was followed, except that the positive electrodes b 1 , b 2  and b 3  were used as the working electrode, to construct beaker cells.  
     [0090] (Charge-Discharge Test)  
     [0091] Each beaker cell was charged and discharged under the same conditions as used in the charge-discharge test of Experiment 1. After 10 cycles, a ratio of a discharge capacity on each cycle to the initial discharge capacity, i.e., a capacity retention rate, was calculated. The results are shown in FIG. 10.  
     [0092] As apparent from FIG. 10, the positive electrodes b 1  and b 2  having the molybdenum oxide thin films deposited on the roughened surfaces of their respective substrates exhibit the improved cycle characteristics relative to the positive electrode b 3  having the molybdenum oxide thin film deposited on the unroughened surface of its substrate. This shows that a thin film of molybdenum oxide, if deposited on a roughened surface of a substrate, exhibits improved cycle characteristics. Superior cycle characteristics are obtained particularly when the positive electrode b 1  is used. This demonstrates that a surface roughness Ra of a substrate is preferably 0.0105 μm or larger, more preferably 0.011-0.1 μm, still more preferably 0.012-0.09 μm.  
     [0093] As apparent from comparisons of FIGS. 7, 8 and  9  showing respective surfaces of the positive electrodes b 1 , b 2  and b 3 , the texture of the deposited thin film of molybdenum oxide varies depending upon the degree of surface roughness of the substrate. As such, the roughened surface of the substrate is considered as being an important contributor to the highly adherent structure of the molybdenum oxide thin film deposited thereon.  
     [0094] Experiment 3  
     [0095] An electrode was fabricated by depositing a microcrystalline thin film of silicon active material on an electrolytic copper foil as a current collector. Electrochemical properties of this electrode when used as a negative electrode of a rechargeable lithium battery were examined.  
     [0096] (Fabrication of Negative Electrode)  
     [0097] A microcrystalline silicon thin film was deposited on an electrolytic copper foil (thickness=17 μm and surface roughness Ra=0.188 μm), as a substrate, by a CVD process using silane (SiH 4 ) as a source gas and hydrogen as a carrier gas. Specifically, the copper foil substrate was placed on a heater in a reaction chamber. An interior of the reaction chamber was evacuated by a vacuum exhausting system to a pressure of 1 Pa or below. Thereafter, silane (SiH 4 ) as a source gas and hydrogen (H 2 ) as a carrier gas were introduced from a source gas inlet port into the reaction chamber. The substrate was heated by a heater to a temperature of 180° C. A degree of vacuum was adjusted to a reaction pressure by a vacuum exhausting system. An RF power source was operated to generate a radio frequency which was subsequently introduced via the electrode to induce a glow discharge. The detailed thin film-forming conditions are listed in Table 5.  
                           TABLE 5                                   Item   During Film Formation                                                        Source Gas (SiH 4 ) Flow Rate   10   sccm           Carrier Gas (H 2 ) Flow Rate   200   sccm           Substrate Temp.   180°   C.           Reaction Pressure   40   Pa           RF Power   555   W                      
 
     [0098] The microcrystalline silicon thin film was deposited under the above-specified conditions to a thickness of about 2 μm. Observation of the resulting thin film by an electron microscope (at a magnification of 2,000,000×) revealed the presence of noncrystalline regions located in a manner to surround a crystal region consisting of minute crystal grains, and confirmed a noncrystalline nature of the thin film. Raman spectroscopic analysis revealed the presence of a peak around 480 cm −1  and a peak around 520 cm −1 . Thus, the resulting silicon thin film is identified as being of microcrystalline nature.  
     [0099] The electrolytic copper foil carrying thereon a deposit of microcrystalline silicon thin film was cut into a 2 cm×2 cm size to fabricate an electrode c 1 .  
     [0100] (Preparation of Electrolyte Solution)  
     [0101] An electrolyte solution was prepared in the same manner as in Experiment 1.  
     [0102] (Construction of Beaker Cell)  
     [0103] Using the electrode c 1  as a working electrode, a beaker cell was constructed in the same manner as in Experiment 1.  
     [0104] (Charge-Discharge Test)  
     [0105] The above beaker cell was charged at 25° C. and at a constant current of 0.5 mA until a potential based on the reference electrode reached 0 V, and then discharged to 0.5 V. The discharge capacity on this cycle was 1,550 mAh/g. The beaker cell was also confirmed to exhibit stable discharge capacity on the second and subsequent cycles.  
     [0106] After the above beaker cell was charged to a potential of 0 V (vs. Li/Li + ), the negative electrode c 1  was removed and combined with any one of the molybdenum oxide positive electrodes a 1 -a 3  to fabricate a nonaqueous electrolyte rechargeable battery. A ratio by weight of the required positive active material to the negative active material is given in Table 6. For a comparative purpose, the result of the case using conventional positive active material, lithium cobalt oxide, is also given in Table 6. In Table 6, a discharge capacity of each positive active material is also shown. A value for discharge capacity of molybdenum oxide was measured at 1.0-3.5 V (vs. Li/Li + ) and a value for discharge capacity of lithium cobalt oxide was measured at 2.75-4.3 V (vs. Li/Li + ).  
                       TABLE 6                               Required Weight of Positive           Discharge   Active Material When Negative       Positive   Capacity   Active Material Weight is       Electrode   (mAh/g)   Taken as 1                  a1   460   3.4       a2   336   4.6       a3   185   8.4       LiCoO 2     157   9.9                  
 
     [0107] As can be appreciated from the results shown in Table 6, in the case where the silicon thin film is used as negative active material, the reduced weight of positive active material is required when using molybdenum oxide as positive active material than when using lithium cobalt oxide, leading to the reduction in thickness of an electrode plate.  
     [0108] The second aspect of the present invention enables increase in energy density of a nonaqueous electrolyte rechargeable battery, improves a balance between a positive electrode capacity and a negative electrode capacity, and achieves thickness control of an electrode plate. Accordingly, it prevents shortage of electrolyte solution in the electrode plate and improves cycle characteristics.  
     Utility in Industry  
     [0109] In accordance with the present invention, a nonaqueous electrolyte rechargeable battery using molybdenum oxide as positive active material is provided which has the improved capacity and energy density.