Abstract:
The instant invention is directed to a secondary lithium battery. The battery includes a negative electrode, a positive electrode, a separator sandwiched between the electrodes, an electrolyte impregnating the separator and being in a fluid communication with the electrodes, and a metal package adapted for containing the electrodes, the separator, and the electrolyte. One of the electrodes is in thermal contact with the package.

Description:
FIELD OF THE INVENTION  
         [0001]    This invention is directed to the construction of a secondary, lithium battery.  
         BACKGROUND OF THE INVENTION  
         [0002]    Lithium ion or secondary lithium batteriers are known. See: Linden, D., Ed.,  Handbook of Batteries  2 nd    Edition,  McGraw Hill Inc., New York, N.Y. (1995), Chapter 36; and Besenhard, J. O. Ed.,  Handbook of Battery Material , Wiley-VCH Verlag GmbH, New York, N.Y. (1999). These batteries are the state of the art power sources for portable electronic devices, such as: laptop computers, cellular phones, and the like. While these batteries have enjoyed an excellent safety record, efforts to improve their safety continues.  
           [0003]    The safety concern arises from the threat of, for example, cell rupture arising from a thermal runaway situation. The cell&#39;s components, electrolyte and lithium containing electrodes, are packaged in a sealed metal can. In thermal runaway, heat is generated within the cell that could raise the temperature of the electrolyte and lithium electrodes above their ignition temperature. See: Hatchard, T. D. et al, “Importance of Heat Transfer by Radiation in Li-ion Batteries during Thermal Abuse,”  Electrochemical and Solid State Letters , vol. 3, no. 7, pages 305-308 (2000), incorporated herein by reference.  
           [0004]    Thermal runaway may arise from several different situations, but those of concern here arise from “abuse” (or “thermal abuse”). Abuse is qualified by several standard tests including the “nail penetration” test, the “crush” test, and the “short circuit” test. See, for example, UL1642—Standard for Lithium Batteries (Underwriters Laboratories Inc., 1st Edition 10/1985 and 2nd Edition 11/1992); and “Guideline for Safety Evaluation on Secondary Lithium Cells,” Japan Storage Battery Association, Tokyo, Japan (1995), both are incorporated herein by reference. In the first two mentioned tests, the cell is physically damaged thereby bringing about contact of the anode and cathode (a short circuit) which leads to thermal runaway. In the latter test, the anode and cathode are externally electrically coupled (a short circuit) which leads to thermal runaway.  
           [0005]    In the short circuited battery, a localized heat spot begins forming within the cell. This heat accelerates the chemical reactions (between anode and cathode via electrolyte) going on within the cell which creates an escalating heat producing situation (the heat production is also rapid, e.g. seconds) that should be avoided because of the potential adverse consequences. The potential adverse consequences and the importance of heat transfer out of a cell is known. See: Hatchard,  Ibid . In Hatchard, a label on the exterior of the package (can) is used to improve the heat transfer from the can. The label is used to regulate the internal temperature of the can.  
           [0006]    In a conventional secondary lithium cell (either cylindrical or prismatic), the microporous separator membrane between the anode and cathode is wrapped several times around the exterior of the wound anode, cathode, separator, prior to its insertion into the package (can). These additional wraps of the separator act as an insulator (thermal and electrical). The electrode comprises an electrode active mix and a current collector. The current collector for the negative electrode (anode) is a copper foil. The current collector for the positive electrode (cathode) is an aluminum foil. The cans are made of iron-based materials (e.g. steel) or aluminum. In a conventional cylindrical cell, for example, an 18650 (18 mm diameter and 65 mm long), the electrode closest to the interior surface of the can is the negative electrode (anode), having a copper foil current collector, the can body which is made of the iron-based material is the negative terminal of the battery, while the lid of the can is the positive terminal. In the conventional prismatic cell, two constructions are recognized. First, the negative electrode (i.e. copper current collector) is closest to the interior surface of the can (i.e. iron-based). Second, the positive electrode (i.e. aluminum current collector) is closest to the interior surface of the can (i.e. aluminum).  
         SUMMARY OF THE INVENTION  
         [0007]    The instant invention is directed to a secondary lithium battery. The battery includes a negative electrode, a positive electrode, a separator sandwiched between the electrodes, an electrolyte impregnating the separator and being in a fluid communication with the electrodes, and a metal package adapted for containing the electrodes, the separator, and the electrolyte. One of the electrodes is in thermal contact with the package. 
       
    
    
     DESCRIPTION OF THE DRAWINGS  
       [0008]    For the purpose of illustrating the invention, there is shown in the drawings a form which is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown.  
         [0009]    [0009]FIG. 1 is a cross-section view of a prior art cell.  
         [0010]    [0010]FIG. 2 is a cross-sectional view of a cell made according to the present invention.  
         [0011]    [0011]FIG. 3 is an exploded view of a cell made according to the present invention.  
         [0012]    [0012]FIG. 4 is a graphical illustration of the performance (voltage and temperature) of a prior art cell and an inventive cell as a function of time. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0013]    With regard to electrodes, electrolytes, and separators referred to herein, each is of conventional design or construction. Such information is known to the skilled artisan. For example, see: Linden, D., Ed.,  Handbook of Batteries  2 nd Edition , McGraw Hill Inc., New York, N.Y. (1995), Clip 36, and Besenhard, J. O., Ed.,  Handbook of Battery Materials , Wiley-VCH Verlag GmbH (1999), e.g., § 2.6, 2.7, and the like, both are incorporated herein by reference.  
         [0014]    The instant invention is disclosed with reference to a cylindrical cell for convenience, but it is not so limited and may be applied to prismatic cells as well.  
         [0015]    Referring to the drawings wherein like numerals indicate like elements as shown in FIG. 1, a prior art cell  10  comprising a can  12  being closed by lid  14 . A separator  16  is wrapped around a “jelly roll” and is in contact with the interior surface of can  12 . A seam  18 , created by overlapping the separator  16  upon itself as the battery is wound, is secured by a conventional piece of tape  20 .  
         [0016]    In FIG. 2, the instant invention is illustrated. Cell  30  consists of a can  12  sealed with a lid  14 . The metal sheet  32 , typically a copper foil (i.e., the current collector) upon which the negative electrode material or electrode active mix (not shown, but conventional) has been spread in conventional manner, is in thermal contact (e.g., direct physical contact) with the interior surface of can  12 .  
         [0017]    Referring to FIG. 3, the inventive cell  30  is illustrated in an exploded view. Jelly roll  36  is made in a conventional fashion with the exception that negative electrode  38  is longer than separator  40  or positive electrode  42 . Thus, when the jelly roll is completely wound, the copper current collector  32  of the negative electrode  38  is visible. This jelly roll  36  is inserted into can  12 , so that collector  32  of electrode  38  is in thermal contact with the interior surface of the can  12 . Tab  44  may be welded, in a known manner, to can  12  and thereby provides good electrical contact, just as tab  46  may be welded to lid  14  to provide good electrical contact. In a prismatic cell, a copper current collector would be in contact with the can or an aluminum current collector would be in contact with the can.  
         [0018]    The current collector  32  is an excellent heat conductor, so that heat generated in the interior of the jelly roll  36  may be efficiently transferred to the exterior surface of the metal can  12 .  
         [0019]    The improvement in heat transfer is best understood with reference to FIG. 4. In FIG. 4, the performance (voltage and temperature) of the prior art cell and inventive cell is illustrated. The left-hand vertical axis indicates voltage (volts) as a function of time (seconds on the horizontal axis). The right-hand vertical axis illustrates temperature (centigrade) as a function of time (seconds on the horizontal axis).  
         [0020]    The prior art cell&#39;s performance is illustrated with lines  50 ,  52  and  54 . Line  50  illustrates the voltage as a function of time after the cell has suffered nail penetration (e.g., test method UL1642). Line  52  illustrates the temperature at the center of the jelly roll as a function of time. Line  54  illustrates the temperature at the exterior surface of the can as a function of time. Note, that as the voltage  50  drops, both temperature lines rise, but the interior temperature  52  rises more sharply in comparison to the exterior temperature  54 . The difference between lines  52  and  54  shows that heat does not dissipate well from the cell. Remember FIG. 1, where separator  16 , a plastic insulator, is in contact with can  12 .  
         [0021]    The inventive cell&#39;s performance is illustrated with lines  60 ,  62 , and  64 . Line  60  is the voltage, line  52  is the interior temperature, and line  64 , the exterior temperature. Note that the difference between lines  62  and  64  is smaller than the difference shown with the prior art cell. The small temperature difference of the inventive cell shows that heat is dissipated better from the cell when the current collector is in thermal contact with the can.  
         [0022]    The present invention may be embodied into others specific forms without departing from the attributes thereof and, accordingly, reference should be made to the pending claims rather than to the foregoing specification as indicating the scope of the invention.