Abstract:
This invention includes a tapered electrode assembly for providing a crumple zone in a rechargeable cell. The taper, which may be curvilinear, angular or piecewise linear, allows a void to exist between the corner of the metal can and the electrode assembly. This “crumple zone” prevents any external damage to the can from damaging the internal electrode assembly. The invention facilitates passage of common OEM drop testing without compromising cell performance or energy storage capacity. The invention increases the reliability of the cell by allowing the cell to resist external impacts.

Description:
BACKGROUND  
       [0001]     1. Technical Field  
         [0002]     This invention relates generally to rechargeable electrochemical battery cells, and more particularly to impact resistant designs for such cells.  
         [0003]     2. Background Art  
         [0004]     Portable, battery-operated, electronic devices seem to be everywhere. From handheld games, to compact disc players, to radios, to personal data assistants (PDAs), to phones, to pagers, it is becoming rare to encounter a person who does not carry at least one portable electronic device with them at all times. People carry the devices for entertainment, for organizational purposes, and for staying connected with others. A common characteristic shared by each of these devices is that they all rely on batteries for portability.  
         [0005]     Batteries are manufactured by taking two electrically opposite electrodes and stacking them together, with each electrode being physically separate from the other. A common way to manufacture the electrochemical cells used in the batteries is known as the “jellyroll” technique, where the inner parts of the cell are rolled up and placed inside an aluminum can, thereby resembling an old-fashioned jellyroll cake. Aluminum is the preferred metal for the can due to its light weight and favorable thermal properties. To understand the jellyroll technique, consider the following example:  
         [0006]     Cells are made of a positive electrode (cathode), a negative electrode (anode). A separator prevents these two electrodes from touching, while allowing electrons to pass through. Referring now to  FIG. 1 , illustrated therein is a cross-sectional side view of a typical electrode layer assembly. The electrode  10  includes a separator  12  having a top and bottom  14  and  16 . Disposed on the top  14  of the separator  12  is a first layer  18  of an electrochemically active material. For example, in a nickel metal hydride battery, layer  18  may be a layer of a metal hydride charge storage material as is known in the art. Alternatively, layer  18  may be a lithium or a lithium intercalation material as is commonly employed in lithium batteries.  
         [0007]     Disposed atop layer  18 , is a current collecting layer  20 . The current collecting layer may be fabricated of any of a number of metals known in the art. Examples of such metals include, for example, nickel, copper, stainless steel, silver, and titanium. Disposed atop the current collection layer  20  is a second layer  22  of electrochemically active material.  
         [0008]     Referring now to  FIGS. 2 and 3 , illustrated therein is stack of electrodes like that in  FIG. 1  assembled in the jellyroll configuration so as to make a rechargeable cell. In  FIGS. 2 and 3 , two electrodes  40  and  60  are provided as described above. Electrode  40  is fabricated with two layers of, for example, negative/active electrochemical material while electrode  60  is fabricated with two layers of positive electrode material. Each electrode  40 , 60  is provided with a current collecting region  20 . The current collecting region  20  is disposed on the current collector, and allows for electrical communication between the electrode itself and a terminal on the outside of the cell can into which the electrode stack of  FIG. 2  may be inserted. While the current collecting region  20  is disposed on the top and bottom of the jellyroll in this exemplary embodiment, note that they may equally be located at the leading and trailing edges of the jellyroll as well.  
         [0009]     The electrodes  40  and  60  are arranged in stacked relationship with the current collecting regions  20  disposed on opposite edges of the stack. Thereafter, the stack is rolled into a roll  70  for a subsequent insertion into an electrochemical cell can. The cans are generally oval, rectangular or circular in cross section with a single opening and a lid. This is similar to the common trashcan.  
         [0010]     Referring now to  FIG. 3 , illustrated therein is a cross-sectional cut-away view of the stacked configuration shown in  FIG. 2 . Here, electrodes  40  and  60  can be seen in stacked orientation. Electrode  40  comprises substrate  42  first layer of negative active material  44 , current collecting layer  46 , and second layer of active material  48 . Disposed immediately atop layer  48  is the separator  62  of electrode  60 . Thereafter the first layer of active material  64  is disposed atop the separator  62  with current collecting layer  66  disposed there over and second layer of active material  68  disposed atop the current collecting layer.  
         [0011]     As the configuration is rolled into roll  70 , the outer membrane layer is rolled into contact with the membrane substrate layer  42  of electrode  40  is rolled into contact with the second layer of active material  68  of electrode  60 . In this way, the membrane substrate layers act as a separator to electrically isolate the positive and negative electrodes from one another. Moreover, as the membranes are porous, they may be filled with a liquid electrolyte such as is known in the art. Accordingly, the membrane allows for deposition of ultra-thin electrode layers, and current collecting layers, while providing the function of both electrolyte reservoir and separator. The result is ultra-thin electrodes having extremely high capacity.  
         [0012]     Once the jellyroll is complete, it is inserted into a metal can  122  as shown in  FIG. 4 . The metal can  122  includes a first metal connector  24  that may serve as the cathode and a second metal connector  26  capable of serving as the anode. Looking to the jellyroll, the various layers can be seen: separator  34 , first electrode  34 , and second electrode  36 . Depending upon the construction, an electron or current collector or grid  38  may be added to the device if desired. The current collector  38  is typically formed from a metal such as cobalt, copper, gold, iron, manganese, nickel, platinum, silver, tantalum, titanium, or zinc.  
         [0013]     Traditionally, such metal-can type batteries were inserted into plastic battery housings that included circuitry like protection circuits, charging circuits, fuel gauging circuits and the like. The plastic battery housings were then used with electronic host devices. However, as electronic devices have gotten smaller and smaller, manufacturers have begun putting the associated battery circuitry in the host device. Thus, they use just the metal-can battery, without a protective plastic housing, in their devices.  
         [0014]     This creates a problem in that, as stated above, the metal cans are generally made from soft metals like aluminum. Thus, when the metal-can battery is dropped, the can may dent, bend and deform. Recall from above that it is important in battery construction that the cathode and anode be kept apart by the separator or membrane layer. If the metal can bends or dents, this may cause the cathode and anode to touch either the inside of the can or each other, thereby creating a short circuit condition in the can. Short circuit conditions can lead to high currents that generate high temperatures and seriously compromise reliability of the battery.  
         [0015]     There is thus a need for an improved metal-can battery assembly that prevents short circuit conditions caused by impact related deformations in the metal can. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  is a cross-sectional side view of a typical prior art electrode layer assembly.  
         [0017]      FIG. 2  is a prior art stack of electrodes assembled in the jellyroll configuration so as to make a rechargeable cell.  
         [0018]      FIG. 3  is a cross-sectional cut-away view of the stacked configuration shown in  FIG. 2 .  
         [0019]      FIG. 4  is cut away, cross sectional view of a prior art jellyroll inserted into a metal can.  
         [0020]      FIG. 5  is a cross sectional view of a prior art metal-can battery that has been repeatedly dropped on a hard surface as is typical in OEM quality and qualification practice.  
         [0021]      FIG. 6  is a perspective view of one preferred embodiment of an electrode assembly in accordance with the invention.  
         [0022]     FIGS.  7 A-C are comparisons of cross-sectional views various electrodes that may be used in accordance with the invention.  
         [0023]      FIG. 8  illustrates an electrode in accordance with the invention being rolled into a cinnamon bun shape.  
         [0024]      FIG. 9  illustrates a cell assembly in accordance with the invention.  
         [0025]      FIG. 10  illustrates a comparison of the prior art cell and a cell in accordance with the invention.  
         [0026]      FIG. 11  illustrates an alternate embodiment of an electrode assembly in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0027]     A preferred embodiment of the invention is now described in detail. Referring to the drawings, like numbers indicate like parts throughout the views. As used in the description herein and throughout the claims, the following terms take the meanings explicitly associated herein, unless the context clearly dictates otherwise: the meaning of “a,” “an,” and “the” includes plural reference, the meaning of “in” includes “in” and “on.” 
         [0028]     Referring now to  FIG. 5 , illustrated therein is a cross sectional view of a prior art metal-can battery that has been repeatedly dropped on a hard surface as is typical in OEM quality and qualification practice. For example, a typical qualification test may require the battery withstand 30 five-foot drops to a concrete surface. Testing was done on common lithium-ion metal-can cells in the lab. Test results showed that on average 7 batteries in 500 failed this test, with an average of 4 failing within the first 18 drops. Nothing would be more frustrating for a consumer than to pay $200 for a new personal organizer only to drop it a couple of times and have it stop working! As shown in  FIG. 5 , the failure is caused by deformation  502  of the metal can  500  causing damage  503  to the inner jellyroll  501 . As stated above, this damage  503  can cause short circuits within the cell.  
         [0029]     The present invention prevents such a deformed jellyroll situation by employing an electrode assembly that provides a “crumple zone” into which a can or housing may deform without contacting the electrode assembly itself. The electrode assembly of the present invention has a central length that is longer than an exterior length. In other words, at least one end of the electrode assembly is tapered from the center to the exterior. When the electrode assembly is inserted into a can, the tapered profile shape leaves a void or air gap between the electrode assembly and the can. This void provides the crumple zone that allows the cell to keep functioning, even after it has been dropped.  
         [0030]     Commonly assigned U.S. Pat. No. 6,574,111, entitled “Impact Resistant Rechargeable Battery Cell with Crumple Zone” teaches the utilization of the spacer to create a crumple zone. While the &#39;111 patent works well in practice, the present invention eliminates the need for a spacer, thereby both saving cost and increasing the total amount of energy that may be stored within the cell (by increasing the amount of active material within the cell).  
         [0031]     The invention may be manufactured in several different ways. In one embodiment, the electrode assembly is wound, as in the traditional jellyroll process. The shape of the unwound electrode is such that when the assembly is wound, the height of the electrode becomes shorter. Expressed differently, the jellyroll, when viewed from a cross-section, has a radiused or tapered end.  
         [0032]     Turning now to  FIG. 6 , illustrated therein is one example of an electrode assembly  601  in accordance with the invention. The embodiment of  FIG. 6  is formed by wrapping the electrode into a jellyroll structure. As will be seen in the discussion of  FIG. 11 , the invention may also be formed by stacking layers of electrode material atop each other.  
         [0033]     The electrode assembly  601  is created by rolling a shaped, elongated electrode. Such an electrode layer may include the constituent layers of material as described in  FIG. 1 . The elongated electrode layer has a first end  602  and a second end  603 . The layer is shaped so that the first end  602  is wider than the second end  603 . As such, when the layer is rolled starting with the first end  602 , the resulting electrode assembly  601  will be taller in the center (i.e. the central height) than at the outer edges (i.e. the exterior height). Using this unique electrode shape, the resulting electrode assembly  601  resembles more the appearance of a baked cinnamon bun (with a tapered top) than the traditional jelly roll (with planar ends that extend perpendicularly from the sides). The taper is laterally transverse to the winding of the overall shape.  
         [0034]     Turning now to FIGS.  7 A-C, illustrated therein are various forms of electrodes that may be used to create the cinnamon bun shaped electrode assembly in accordance with the invention. Each electrode has a profile shape that is defined by a predetermined length between a first longitudinal end  701  and a second longitudinal end  702 . Each electrode likewise has a height defined by an upper side  703  and a lower side  704 . While the plan view of FIGS.  7 A-C is two dimensional, the actual electrodes also have a finite width defined by a first lateral side  705  and a second lateral side  706 .  
         [0035]     At least one of the upper side  703  and the lower side  604  includes a taper. In FIGS.  7 A-C, for the purposes of discussion, the lower side  704  is shown to include the taper.  FIG. 7A  illustrates a taper that is curvilinear.  FIG. 7B  illustrates a taper that is angular.  FIG. 7C  illustrates a taper that is piecewise linear. In each of FIGS.  7 A-C, the height between the upper side  703  and the lower side  704  differs from one longitudinal end  701  to the other longitudinal end  702 . In the embodiments of FIGS.  7 A-C, longitudinal end  702  is shorter than longitudinal end  701 . Experimental results have shown that the electrode is most effective when one longitudinal end  702  is at least 2% shorter than the other longitudinal end  701 .  
         [0036]     Turning now to  FIG. 8 , illustrated therein is an electrode  800 , such as any of the ones illustrated in FIGS.  7 A-C, being rolled so as to form the cinnamon bun shape. Starting at the wide end  801 , the electrode  800  is rolled at an appropriate speed, attempting to keep the edge that will become the top of the electrode assembly even, such that a substantially planar end will result. When the roll gets to the narrow end  803 , the tapered side  802  causes the exterior height to be shorter than the central height at the wide end  801 . The roll is effectively wound in a spiral having a perimeter determined by the length of the profile shape of the electrode  800 , the winding beginning at one of the longitudinal ends such that one lateral side of the profile shape substantially contacts the other lateral side of the profile shape in adjacent layers of the spiral.  
         [0037]     Referring now to  FIG. 9 , illustrated therein is a cell assembly in accordance with the invention. A cinnamon bun electrode assembly  900  with cathode  901  and anode  902  is provided. The cinnamon bun  900  will be inserted into a metal can (not shown). The assembly includes a first metal connector  903  that serves as the external cathode and a tab  904  for coupling the first metal connector  903  to the cathode  901 . An optional insulator  905  is provided to isolate the first metal connector  903  from the anode  902 . Flat, top insulators, at one end of the cinnamon bun  900 , are known in the art as recited in U.S. Pat. No. 6,317,335 to Zayatz.  
         [0038]     In accordance with the invention, the cinnamon bun electrode assembly  900 , which would normally be substantially planar and would contact the can across the bottom of the can, has been tapered on at least one end. The electrode assembly  900  has a central height  907  and an exterior height  908 . The central height  907  of the electrode assembly  900  is preferably at least 2% longer than the exterior height  908 .  
         [0039]     When the electrode assembly  900  is inserted into a housing or can (not shown), the central height will be roughly equivalent to the interior height of the can, neglecting space required for tabs  904 , insulators,  905  and other components, including current interrupt devices. The exterior height  908  of the electrode assembly  900  will generally be at least 2% shorter than the effective interior height of the housing.  
         [0040]     The electrode assembly  900  of  FIG. 9  has a first end  906 . The first end  906  has a cross sectional or profile shape that includes a taper. As mentioned in the discussion of FIGS.  7 A-C, the taper may be curvilinear, like an exponential or parabolic curve for example. The taper may also be angular or piecewise linear. For cinnamon bun construction, the taper is laterally transverse to a winding of the electrode assembly. When the electrode assembly  900  is inserted into the can, the taper provides at least one void between the first end and a corner of the housing.  
         [0041]     Turning now to  FIG. 10 , illustrated therein is a comparison of cross-sectional views of the prior art cell  1000  and a cell in accordance with the invention  1001 . In the prior art cell  1000 , the jellyroll  1002  mounts flush against the metal can  1003 . However, in the cell in accordance with the invention  1001 , the taper  1004  leaves a void  1007  between the cinnamon bun  1005  and the metal can  1006 . This void allows the can  1006  to deform, or “crumple”, when dropped on a corner, while the cinnamon bun  1005  remains unharmed. With the taper  1004 , test results have shown that zero batteries in 250 failed as a result of the 30 drops to concrete.  
         [0042]     Turning now to  FIG. 11 , illustrated therein is an alternate construction of an electrode assembly in accordance with the invention. In this embodiment, layers of electrode  1101 , 1102  are stacked to form an electrode assembly  1103 . In this stacked method, each layer, e.g.  1101 , has some form of taper  1104  or radius on at least one end. This particular taper  1104 , which may be curvilinear, angular or piecewise linear, begins at a first lateral side and extends outward from the center of the electrode. The taper  1104  then reaches a predetermined length somewhere near the middle of one end of the electrode, and then tapers back to the opposite lateral side. These layers  1101 ,  1102  may then be adhered together with a binder, gel, polymer or electrolyte to form a stacked electrode assembly  1103 .  
         [0043]     While the preferred embodiments of the invention have been illustrated and described, it is clear that the invention is not so limited. Numerous modifications, changes, variations, substitutions, and equivalents will occur to those skilled in the art without departing from the spirit and scope of the present invention as defined by the following claims. For example, while one preferred embodiment of an electrode assembly illustrated herein had a taper on one end of the assembly, the electrode assembly may have tapers at both ends or on the sides.