Patent Publication Number: US-2013249498-A1

Title: Non-aqueous secondary battery and secondary battery system

Description:
TECHNICAL FIELD 
     The present invention relates to a non-aqueous secondary battery. The invention more particularly relates to a high energy density lithium ion secondary battery and a power source module thereof suitable to use, for example, in portable equipment, electric cars, power storage, etc. 
     BACKGROUND ART 
     A lithium ion secondary battery uses a carbon material as a negative electrode active material. It is known that in such a secondary battery a solid electrolyte interphase is formed on the surface of a negative electrode due to side reaction accompanying the negative electrode charge reaction during initial charging after the manufacture of the battery. It is also known that the solid electrolyte interphase grows during storage in a circumstance at a relatively high temperature or along with progress of the side reaction on the surface of the negative electrode occurring by subjecting to charge/discharge cycles. The side reaction involves lithium ion intercalation within the negative electrode, which causes degradation of battery characteristics, such as capacitance deterioration due to shift of the potential of the positive and negative electrodes to a higher potential, increase in the resistance attributable to increase in solid electrolyte interphase thickness at the surface of the negative electrode, and the like. 
     As a prior art for solving the subject, Patent Literature 1 discloses, for example, attachment of lithium to a negative carbon electrode. In the disclosed technique, since lithium attached to the negative carbon electrode dissolves by itself to release ions to the negative carbon electrode, ions deintercalated from the inside of the negative electrode due to the side reaction are compensated. This can return the negative electrode to a low potential to suppress deterioration of capacitance. 
     Further, Patent Literature 2 describes that lithium is disposed as a third electrode inside a battery, an electrode terminal connected with the third electrode is disposed on the cell surface, the amount of lithium ions deintercalated from the negative electrode is judged based on the potential difference between the third electrode and the negative electrode and those corresponding to lithium ions consumed are supplied. Also this can return the negative electrode to the low potential to suppress deterioration of capacitance. 
     Further, Patent Literature 3 describes that potential measuring means is disposed between a third electrode and a positive electrode and those corresponding to lithium ions consumed are supplied automatically when the potential difference is at a predetermined level or higher. 
     On the other hand, Patent Literature 4, Patent Literature 5, and Patent Literature 6 describe configurations in which a plurality of wound electrode bodies are disposed in the inside of a battery with an aim of increasing the capacitance density of lithium ion secondary batteries and improving the safety of the batteries. 
     Prior Art Literature 
     Patent Literature 
     Patent Literature 1: JP-05-234622-A 
     Patent Literature 2: JP-08-190934-A 
     Patent Literature 3: JP-2007-305475-A 
     Patent Literature 4: JP-09-266013-A 
     Patent Literature 5: JP-2000-311701-A 
     Patent Literature 6: JP-2003-31202-A 
     SUMMARY OF THE INVENTION 
     Problem to be Solved by the Invention 
     However, the approach to the supply of lithium ions for the solid electrolyte interphase formation described above is on the premise that the state of occurrence of the side reaction on the surface of the negative electrode and the shift of each of the electrode to higher potential are uniform in the inside of the cell. 
     It is known that the side reaction on the negative electrode proceeds acceleratingly upon elevation of temperature, increase in the number of charge/discharge cycles, and charge/discharge at high current. One example of situations in which such factors are combined with one another is repetitive charge/discharge of a large-size lithium ion battery cell with a high current applied. 
     When a lithium ion battery is charged/discharged repetitively under high current, heat is generated by joule heating in the cell due to direct current resistance of the battery. While the generated heat is dissipated from the outer periphery of the cell into air, since heat resistance is present in a central portion and an outer peripheral portion of the cell, the temperature at the central portion of the cell is higher than that of the outer peripheral portion of the cell, particularly, in a large sized cell. In addition, since the direct current resistance is generally lowered along with elevation of temperature in the lithium ion secondary battery, current is concentrated more in the central portion of the cell than in the outer peripheral portion of the cell. As described above, since it is considered that the temperature is higher and the current is larger in the central portion of the cell than those at the outer peripheral portion of the cell, it can be presumed that the side reaction on the surface of the negative electrode of the cell is accelerated more in the central portion of the cell than that in the outer peripheral portion of the cell. 
     The result of actually charging and discharging a battery cell under a high current is to be described with reference to  FIG. 17  to  FIG. 22 . The battery cell used in the experiment has 40 mm of diameter, 108 mm of length, and 5.5 Ah of electric capacitance. After charge/discharge of the cell is repeated 3000 cycles at a current of 90 A for a charge/discharge time of 90 sec, the cell was disassembled, and a positive electrode and a negative electrode were cutout from a central portion, an intermediate portion, and an outer peripheral portion of the cell as shown in  FIG. 19  and charge/discharge characteristics of partial electrodes were investigated. 
       FIG. 20  illustrates charge/discharge characteristics of an electrode at a central portion,  FIG. 21  illustrates charge/discharge characteristics of an electrode at an intermediate portion, and  FIG. 22  illustrates charge/discharge characteristics of an electrode at an outer peripheral portion. The abscissa represents a charge/discharge capacity and the ordinate represents a voltage or potential. In the graphs, a curve represented by blank circles (◯) shows a voltage between a positive electrode and a negative electrode, blank trigonals (Δ) show a potential of the positive electrode to lithium inserted as a reference electrode, and blank squares (□) also show the potential of the negative electrode to lithium. 
     Black rhombuses (♦) show characteristics upon charge/discharge measurement only by the partial electrode of the positive electrode and lithium, and black squares (▪) show characteristics upon charge/discharge measurement only by the partial electrode of the negative electrode and lithium. 
     According to the graphs, the charge/discharge capacitance is smaller in the electrode at the central portion of the cell than in the electrode at the outer peripheral portion of the cell and both of the positive electrode and the negative electrode are at higher potential. This seems that the side reaction on the surface of the negative electrode is accelerated since the temperature is high and current is concentrated in the central portion of the cell. 
       FIG. 17  illustrates a presumed charge/discharge state in the initial stage in the outer peripheral portion of the cell and the central portion of the cell. Since the side reaction on the surface of the negative electrode is accelerated due to high temperature and current concentration and lithium ions are deintercalated from the inside of the negative electrode, the negative electrode potential at the central portion of the cell shifts to a higher potential. Since the voltage defined from the outside during charge/discharge is a potential difference between the positive electrode and negative electrode, when the negative electrode is at a high potential, the positive electrode also shifts to a high potential. It can be considered that the charge/discharge state in the central portion of the electrode is as shown in  FIG. 18 . 
     Increase in the potential of the electrode at the central portion of the cell, particularly, increase in the potential of the positive electrode is not desired since this cause deterioration, for example, decay of crystals of LiCoO 2  as the positive electrode active material and oxygen deintercalation. Further, when a portion of the electrode material on one identical electrode foil (central portion in this case) is at a high potential, it is necessary that other portion (outer peripheral portion) be at a low potential in order to compensate the potential. Actually, the potential of the partial electrode of the negative electrode at the outer peripheral portion illustrated in  FIG. 22  shows an extremely low potential, and metal lithium may possibly deposit to the surface of the negative electrode during charging. 
     Such a local potential distribution in the cell cannot be detected at all based on the voltage between the positive electrode and negative electrode observed from the outside of the cell and it is impossible also by the voltage detection in a case of using lithium as the third electrode as in Patent Literature 2 and Patent Literature 3. 
     Further, also in the case of batteries where a plurality of wound electrode bodies are disposed in one identical container as shown in Patent Literature 4 to Patent Literature 6, deterioration sometimes occurs in a manner that the potential is different on each of the wound electrode bodies due to elevation of temperature and current concentration in the central portion. Also in such a case, it cannot be detected from the outside which wound electrode body generates potential difference and what level of the potential difference occurs therein. 
     The present invention intends to solve such a problem or a subject. That is, the present invention intends to provide a non-aqueous secondary battery such as a lithium ion secondary battery that can eliminate local potential distribution in the inside of a cell due to the side reaction during charge/discharge, and less undergoes deterioration of capacitance, deterioration of a positive electrode material, and deposition of metallic lithium. 
     Means for Solving the Problem 
     According to the present invention, a non-aqueous secondary battery has an electrode group and an electrolyte disposed in one container, the electrode group including a positive electrode, a negative electrode, and a separator, wherein the electrode group is divided into a plurality of electrode groups separated electrically, the electrode groups are in contact with an identical electrolyte, terminals are led out from the positive electrode and the negative electrode to the outside of the container on every electrode groups, the terminals are connected on every positive electrode and negative electrode at the outside of the container, and the terminals at the outside of the container are connectable and disconnectable easily. 
     Further, each of the terminals of the positive electrodes/negative electrodes may be connected on every electrode groups not at the outside of the container but in the inside of the container in which the terminal are also be connectable and disconnectable easily by operation from the outside. 
     Effects of the Invention 
     The present invention can realize a battery less undergoing deterioration of capacitance and deterioration of positive electrode material, and deposition of metallic lithium, and can provide a secondary battery with long life and high in safety. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic circuit diagram of a lithium ion secondary battery system in a first embodiment. 
         FIG. 2  is an upper plan view of the lithium ion secondary battery in the first embodiment. 
         FIG. 3  is an A-A′ cross sectional view of the lithium ion secondary battery cell in the first embodiment. 
         FIG. 4  is an A″-A′″ cross sectional view of the lithium ion secondary battery cell in the first embodiment. 
         FIG. 5  is a schematic circuit diagram of a lithium ion secondary battery system in a second embodiment. 
         FIG. 6  is an upper plan view of the lithium ion secondary battery in the second embodiment. 
         FIG. 7  is a B-B′ cross sectional view of the lithium ion secondary battery cell in the second embodiment. 
         FIG. 8  is a B″-B′″ cross sectional view of the lithium ion secondary battery cell in the second embodiment. 
         FIG. 9  is a schematic circuit diagram of a lithium ion second battery system in a third embodiment. 
         FIG. 10  is an upper plan view of the lithium ion secondary battery in the third embodiment. 
         FIG. 11  is a C-C′ cross sectional view of the lithium ion secondary battery cell in the third embodiment. 
         FIG. 12  is a C″-C′″ cross sectional view of the lithium ion secondary battery cell in the third embodiment. 
         FIG. 13  is a schematic circuit diagram of a lithium ion secondary battery system in a fourth embodiment. 
         FIG. 14  is an upper plan view of the lithium ion secondary battery in the fourth embodiment. 
         FIG. 15  is a C-C′ cross sectional view of the lithium ion secondary battery cell in the fourth embodiment. 
         FIG. 16  is a C″-C′″ cross sectional view of the lithium ion secondary battery cell in the fourth embodiment. 
         FIG. 17  illustrates an initial charge/discharge state of a lithium ion secondary battery cell in an embodiment having a subject. 
         FIG. 18  is a graph showing a charge/discharge state in an outer peripheral portion after a test for the lithium ion secondary battery cell in the embodiment having the subject. 
         FIG. 19  is a graph illustrating positions of partial electrodes to be investigated after the disassembly of the lithium ion secondary battery cell in the embodiment having the subject. 
         FIG. 20  is a graph illustrating charge/discharge characteristics of an electrode in a central portion of the lithium ion secondary battery cell in the embodiment having the subject. 
         FIG. 21  is a graph illustrating charge/discharge characteristics of an electrode in an intermediate portion of the lithium ion secondary battery cell in the embodiment having the subject. 
         FIG. 22  is a graph illustrating charge/discharge characteristics of an electrode in the outer peripheral portion of the lithium ion secondary battery cell in the embodiment having the subject. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     The best mode for practicing the invention will be described below. In the embodiments, description is to be made with reference to an example of a secondary battery in which a sheet-like separator retaining an electrolyte is disposed between a positive electrode and a negative electrode and in which the positive electrode, the separator, the negative electrode and the separator are alternately stacked and wound into a cylindrical form to constitute an electrode group. However, the invention can be practiced also for an electrode group which is stacked without winding. 
     FIRST EMBODIMENT 
       FIG. 1  illustrates a schematic circuit diagram of a secondary battery system having a lithium ion secondary battery in this embodiment,  FIG. 2  is an upper plan view of a lithium ion secondary battery in this embodiment,  FIG. 3  is an A-A′ cross sectional view in  FIG. 2 , and  FIG. 4  is an A″-A′″ cross sectional view in  FIG. 3 . 
     An electrolyte  102  and a plurality of electrode groups  103  are disposed inside a battery container  101 . Each of the electrode groups  103  is in contact with an identical electrolyte  102  (dipped therein). The electrode group  103  is formed by alternately stacking a positive electrode  251 , a negative electrode  252 , and a separator  253  between the positive electrode and the negative electrode and winding them into a flat elliptic shape. Positive electrode terminals  221  and negative electrodes terminals  241  are led out from the positive electrodes  251  and negative electrodes  252  of the respective electrode groups  103  to the outside of the battery container  101 . The positive electrode terminals  221  and negative electrodes terminals  241  are connected by way of positive connection opening contacts  220  to a negative electrode bus bar  201  on the positive side and by way of negative electrode connection opening contacts  240  to a negative electrode bus bar  202  on the negative side at the outside of the battery container  101 . 
     The positive electrode connection opening contact  220  has a configuration in which the positive electrode terminal  221  is fastened to the positive electrode bus bar  201  with a bolt  222  and a nut  223  for attachment. In this embodiment, five electrode groups  103  are arranged in the battery container  101  and, for connecting the positive electrode terminals  201  and the negative electrode terminal  241  from each of the electrode groups  103  with the positive electrode bus bar  221  and negative electrode bus bar  202 , they are fastened at each of the five points with the bolt  222  and the nut  223 . 
     The positive electrode connection opening contacts  220  and the negative electrode connection opening contacts  240  can be optionally disconnected with the positive electrode terminals  221  and the negative electrode terminal  241  respectively. 
     In this embodiment, a slurry of a positive electrode mix was prepared by adding LiCoO 2  as a battery positive electrode active material, 7 wt % of acetylene black as an electroconductive agent, and 5 wt % of polyvinylidene fluoride (PVDF) as a binder and admixing N-methyl-2-pyrrolidone to them. After the slurry is coated and dried on both surfaces of a positive electrode foil, i.e., a 25 μm-thick aluminum foil, it was pressed and cut to prepare a positive electrode  251  having a positive electrode material bonded to both surfaces of the positive electrode foil. 
     Likewise, a slurry of a negative electrode mix was prepared by using less graphitizable carbon as a negative electrode active material, adding 8 wt % of PVDF as a binder, and admixing N-methyl-2-pyrrolidone to them. The negative electrode mix slurry was coated on both surfaces of a negative electrode foil, i.e., a 10 μm-thick copper foil and pressed and cut to prepare a negative electrode  252  having a negative electrode material bonded on both surfaces of the negative electrode foil. 
     More specifically, Li x CoO 2 , Li x NiO 2 , Li x Mn 2 O 4 , Li x FeO 2  (x ranging from 0 to 1), etc. are preferred as the positive electrode material while carbonaceous materials such as graphite and coke having an interlayer graphite spacing of 0.344 nm or less are preferred as the negative electrode active material since they are satisfactory in charge/discharge reversibility. For the electrolyte, it is preferred to use a mixed solvent formed by adding at least one of dimethoxyethane, dienthyl carbonate, dimethyl carbonate, methyl ethyl carbonate, γ-butyrolactone, methyl propionate, and ethyl propionate to ethylene carbonate and an electrolyte of at least one of lithium-containing salts, for example, LiClO 4 , LiPF 6 , LiBF 4 , and LiCF 3 SO 3 , with a lithium concentration ranging from 0.5 to 2 mol/L. 
     During normal use of the battery, the lithium ion secondary battery of this embodiment serves to charge/discharge by connecting the positive electrode bus bar  201  and the negative electrode bus bar  202  as each of the terminals of the positive electrodes and the negative electrodes to an external circuit. At every predetermined interval or upon the battery reaching a predetermined electric amount in charge/discharge after starting of the use for charge/discharge, the state of the battery is verified. Specifically, the positive electrode bus bar  201  and the negative electrode bus bar  202  are disconnected from the circuit and the positive electrode terminals  221  are each disconnected from the positive electrode bus bar  201  and the negative electrode terminals  241  are each disconnected from the negative electrode bus bar  202 . After that, the potential difference between the positive electrode terminals  221  and that between the negative electrode terminals  241  are measured respectively. 
     When the potential difference between the electrode terminals  221  and between the negative electrode terminals  241  in each of the electrode groups  103  is not present or present slightly if any, it is regarded that less deterioration due to the formation of the solid electrolyte interphase has proceeded in each of the electrode groups  103 . Thus, after the positive electrode terminals  221  and the negative electrode terminals  241  are connected again to the bus bars, the battery is connected to the external circuit for serving to charge/discharge. 
     By contrast, the potential difference is generated between the positive electrode terminals  221  and between the negative electrode terminals  241 , and the potential of positive or negative electrode terminals of the electrode group  103  particularly near the central portion is clearly higher than the potential of the electrode group near the outer periphery. In such cases, it is estimated that deterioration due to the solid electrolyte interphase formation attributable to the side reaction during charge/discharge has proceeded in the electrode group  103  near the central portion. 
     It is not desired that the positive electrode potential of the electrode group in the central portion increases along with solid electrolyte interphase formation and the potential of the negative electrode group in the outer peripheral portion decreases in order to compensate for the potential since the safety is degraded as described above. To overcome the problem, a current is applied from the external circuit between a positive electrode at a higher potential and a positive electrode at a lower potential and the current is supplied continuously until the potential difference is eliminated substantially. Further, also for the negative electrode, a current is supplied continuously from the external circuit between the negative electrode at a higher potential and the negative electrode at a lower potential until the potential difference is eliminated substantially. Alternatively, the potential difference is eliminated by supplying a current between a positive electrode at a higher potential and a negative electrode at a lower potential until the potential of the positive electrode or the negative electrode reaches a potential identical with that of other electrodes. 
     The method as described above makes it possible to overcome a state where the potential of the positive electrode is excessively high or the potential of the negative electrode is excessively low in the electrode group  103  in the battery and to recover the safety of the battery. Accordingly, the battery can be connected again to the external circuit and served for charge/discharge. 
     As has been described above, the present invention makes it possible to open connection of each of the terminals for the positive electrodes and the negative electrodes on every electrode groups. Thus it can be confirmed whether the battery is safe or not by providing a maintenance period in usual use and inspecting the potential difference between each of the terminals for the maintenance period. Further, even if a potential difference is generated inside the battery and the safety is deteriorated, the potential difference can be eliminated by applying the current across the terminals. Thus, local potential difference in the battery can be eliminated and the safety can be recovered; therefore, a battery less undergoing deterioration of capacitance, deterioration of a positive electrode material, and deposition of metallic lithium can be provided. 
     In this embodiment, five flat wound electrode bodies are used as the electrode group  103 , and they are arranged linearly in the battery. However, the electrode group  103  may also be a cylindrical wound type or a stacked type, and the number of the electrode groups may be more than five. The wound electrode bodies may be arranged in the cell not linearly but, for example, cylindrical wound electrode bodies may also be arranged in a closed pack state. Further, in this embodiment, the terminal and the bus bar are fastened by the bolt and the nut for the connection open contacts of each of the terminals, but simpler means such as a threaded hole and a screw may also be used. 
     A configuration in which the secondary battery includes a measuring means  301  and a current application means  302  is referred to as a secondary battery system. The measuring means  301  is capable of measuring the potential differences between the positive electrode terminals  221  and between the negative electrode terminals  241  respectively. The current application means  302  is capable of applying a current. 
     SECOND EMBODIMENT 
     This embodiment is identical with the first embodiment except for the following point. 
       FIG. 5  illustrates a schematic circuit diagram of a lithium ion secondary battery system in this embodiment,  FIG. 6  is an upper plan view of a lithium ion secondary battery in this embodiment,  FIG. 7  is a B-B′ cross sectional view of  FIG. 6 , and  FIG. 8  is a B″-B′″ cross sectional view of  FIG. 7 . 
     This embodiment has a feature that positive electrode connection opening contacts  220  and negative electrode connection opening contacts  240  are in the inside of a battery container  101 . As illustrated in  FIG. 6 , a positive and negative electrode terminal group  260  having terminals assembled for measuring the potential of each of the electrode groups  103  when the connection opening contacts are opened is provided at the exterior of the battery container  101  in addition to the positive electrode charge/discharge terminals  225  and negative electrode charge/discharge terminals  245 . 
     In this embodiment, the positive electrode connection opening contacts  220  and the negative electrode connection opening contacts  240  can disconnect between the positive electrode and negative electrode terminal group  260  in a predetermined state. 
     The positive electrode connection opening contact  220  in this embodiment includes a terminal plate  227  connected to a positive electrode bus bar  201 , a fastening bolt  226 , and a counter nut (not illustrated). The positive electrode bus bar  201  is a cylindrical column extended in a direction perpendicular to the plane shown in  FIG. 7  and can rotate within a plane parallel to  FIG. 7  by a magnetic force from the outside of the battery container  101 . The terminal plate  227  is connected to the positive electrode bus bar  201  and moves as shown by an arrow in the drawing in accordance with the rotation of the positive electrode bus bar  201 . 
     The terminal plate  227  has a structure in which a recess thereof fits the fastening bolt  226  when the terminal plate  227  is at a position illustrated by a solid line in  FIG. 7 . On the other hand, the positive electrode bus bar  201  is completely disconnected from the positive electrode of the electrode group  103  when the terminal plate is at a position illustrated by the dotted line. 
     On the other hand, also the fastening bolt  226  is a bolt extended in a direction perpendicular to  FIG. 7  like the positive electrode bus bar  201  and rotated by a magnetic force from the outside. By fastening the terminal plate  227  provided on every electrode group  103  together with a counter nut (not illustrated) provided on every electrode group  103 , the positive electrode of each of the electrode groups  103  and the positive electrode bus bar  201  are connected. 
     Further, the positive electrode  251  and the negative electrode  252  of each of the electrode groups  103  are connected to each of the terminals of the positive and negative electrode terminal group  260 . When the positive electrode connection opening contacts  220  and the negative electrode connection opening contacts  240  are opened, the potential of the positive electrode and the negative electrode of each of the electrode groups  103  can be measured through the positive and negative electrode terminal group  260 . 
     As has been described above, the present embodiment makes it possible to open connection of each of the terminals for the positive electrodes and the negative electrodes on every electrode groups. Thus it can be confirmed whether the battery is safe or not by providing a maintenance period in usual use and inspecting the potential difference between each of the terminals for the maintenance period by using the positive and negative electrode terminal group  260 . Further, even if a potential difference is generated inside the battery and the safety is deteriorated, the potential difference can be eliminated by applying the current between the terminals by using positive electrode charge/discharge terminal  225  or the negative electrode charge/discharge terminal  245 . Thus, local potential difference in the battery can be eliminated and the safety can be recovered; therefore, a battery less undergoing deterioration of capacitance, deterioration of a positive electrode material, and deposition of metallic lithium can be provided. 
     A configuration in which the secondary battery includes a measuring means  301  and a current application means  302  is referred to as a secondary battery system. The measuring means  301  is capable of measuring the potential differences between the positive electrode terminals  221  and between the negative electrode terminals  241  respectively. The current application means  302  is capable of applying a current. In this embodiment, the measuring means  301  includes the positive and negative electrode terminal group  260  and the current application means  302  includes the positive electrode charge/discharge terminal  225  or the negative electrode charge/discharge terminal  225 . 
     THIRD EMBODIMENT 
     This embodiment is identical with the first embodiment except for the following point. 
       FIG. 9  illustrates a schematic circuit diagram of a lithium ion secondary battery system in this embodiment,  FIG. 10  is an upper plan view of the lithium ion secondary battery in this embodiment,  FIG. 11  is a C-C′ cross sectional view of  FIG. 10 , and  FIG. 12  is a C″-C′″ cross sectional view of  FIG. 12 . 
     This embodiment has a feature that a third electrode  270  is disposed in a battery container  101  so as to be in contact with an electrolyte  102  identical with that for the electrode group  103 . The third electrode comprises metallic lithium and a third electrode terminal  271  is disposed outside of the battery container  101  so that the potential can be measured. 
     This embodiment can measure not only the potential of the positive electrode and the negative electrode of each of the electrode groups  103  as the difference voltage of each of the electrodes as in the first embodiment but also can measure the potential as the potential with reference to the metal lithium. In this case, even if the deterioration proceeds uniformly in all of the electrode groups  103  due to the solid electrolyte interphase formation attributable to the side reaction during charge/discharge, the potential change thereof can be detected. 
     As has been described above, the present embodiment makes it possible to open connection of each of the terminals for the positive electrodes and the negative electrodes on every electrode groups. Thus it can be confirmed whether the battery is safe or not by providing a maintenance period in usual use and inspecting the potential difference between each of the terminals for the maintenance period. Further, even if a potential difference is generated inside the battery and the safety is deteriorated, the potential difference can be eliminated by applying the current across the terminals. Thus, local potential difference in the battery can be eliminated and the safety can be recovered; therefore, a battery less undergoing deterioration of capacitance, deterioration of a positive electrode material, and deposition of metallic lithium can be provided. 
     Further, according to this embodiment, even if the inside of the battery deteriorates uniformly and the potential change occurs, since this can be detected by measuring the potential difference relative to the third electrode. Thus the battery with higher safety can be provided. 
     FOURTH EMBODIMENT 
     This embodiment is identical with the second embodiment except for the following point. 
       FIG. 13  illustrates a schematic circuit diagram of a lithium ion secondary battery system in this embodiment,  FIG. 14  is an upper plan view of a lithium ion secondary battery in this embodiment,  FIG. 15  is a D-D′ cross sectional view of  FIG. 14 , and  FIG. 15  is a D″-D′″ cross sectional view of  FIG. 16 . 
     This embodiment has a feature that a third electrode  270  is disposed in a battery container  101  so as to be in contact with an electrolyte  102  identical with that for the electrode group  103 . The third electrode comprises metallic lithium. The third electrode and one of the positive and negative electrodes and a third electrode terminal group  261  are connected so that the potential can be measured. 
     This embodiment can measure not only the potential of the positive electrode and the negative electrode of each of the electrode groups  103  as the difference voltage of each of the electrodes as in the second embodiment but also can measure the potential as the potential with reference to the metal lithium. In this case, even when the deterioration proceeds uniformly in all of the electrode groups  103  due to the solid electrolyte interphase formation attributable to the side reaction during charge/discharge, the potential change thereof can be detected. 
     As has been described above, the present embodiment makes it possible to open connection of each of the terminals for the positive electrodes and the negative electrodes on every electrode groups. Thus it can be confirmed whether the battery is safe or not by providing a maintenance period in usual use and inspecting the potential difference between each of the terminals for the maintenance period. Further, even if a potential difference is generated inside the battery and the safety is deteriorated, the potential difference can be eliminated by applying the current across the terminals. Thus, local potential difference in the battery can be eliminated and the safety can be recovered; therefore, a battery less undergoing deterioration of capacitance, deterioration of a positive electrode material, and deposition of metallic lithium can be provided. 
     Further, according to this embodiment, even if the inside of the battery deteriorates uniformly and the potential change occurs, since this can be detected by measuring the potential difference relative to the third electrode. Thus the battery with higher safety can be provided. 
     DESCRIPTION OF REFERENCE NUMERALS 
     
         
           101  battery container 
           102  electrolyte 
           103  electrode group 
           201  positive electrode bus bar 
           202  negative electrode bus bar 
           220  positive electrode connection opening contact 
           221  positive electrode terminal 
           222  bolt for attachment 
           223  nut 
           224  gasket 
           225  positive electrode charge/discharge terminal 
           226  fastening bolt 
           227  terminal plate 
           240  negative electrode connection opening contact 
           241  negative electrode, terminal 
           245  negative electrode charge/discharge terminal 
           251  positive electrode 
           252  negative electrode 
           253  separator 
           260  positive and negative electrode terminal group 
           261  positive and negative electrode and third electrode terminal group 
           270  third electrode 
           271  third electrode terminal 
           301  measuring means 
           302  current application means