PATENT DOCUMENT

Publication Number: US-8679201-B2
Application Number: US-201113273113-A
Country: US
Kind Code: B2

Title: Increasing the stiffness of battery cells for portable electronic devices

Abstract:
The disclosed embodiments relate to the manufacture of a battery cell. The battery cell includes a set of layers including a cathode with an active coating, a separator, and an anode with an active coating. The battery cell also includes a pouch enclosing the layers, wherein the pouch is flexible. The layers may be wound to create a jelly roll prior to sealing the layers in the flexible pouch. The stiffness of the battery cell may be increased by applying a pressure of at least 0.13 kilogram-force (kgf) per square millimeter and a temperature of about 85° C. to the layers.

Claims:
What is claimed is: 
     
       1. A method for manufacturing a battery cell, comprising:
 obtaining a set of layers for the battery cell, wherein the set of layers comprises a cathode with an active coating, a separator, and an anode with an active coating; 
 sealing the layers in a pouch to form the battery cell, wherein the pouch is flexible; and 
 increasing a stiffness of the battery cell by applying a pressure of at least 0.13 kilogram-force (kgf) per square millimeter and a temperature of about 85° C. to the layers, wherein the pressure and the temperature are applied to the layers for about eight hours. 
 
     
     
       2. The method of  claim 1 , further comprising:
 winding the layers to create a jelly roll prior to sealing the layers in the flexible pouch. 
 
     
     
       3. The method of  claim 1 , further comprising:
 performing a formation charge on the battery cell prior to applying the pressure and the temperature to the layers; and 
 degassing the battery cell after applying the pressure and the temperature to the layers. 
 
     
     
       4. The method of  claim 3 , wherein degassing the battery cell involves:
 puncturing a portion of the pouch that does not contact the layers to release gas generated during the formation charge by the battery cell; and 
 resealing the pouch along a line that is closer to the layers than the punctured portion; and 
 removing extra pouch material associated with the punctured portion from the battery cell. 
 
     
     
       5. The method of  claim 1 , wherein the layers further comprise a binder coating that laminates the layers together upon applying the pressure and the temperature to the layers. 
     
     
       6. The method of  claim 5 , wherein the binder coating is approximately 1 micron thick. 
     
     
       7. The method of  claim 5 , wherein the binder coating is applied to at least one of the cathode, the anode, and the separator. 
     
     
       8. The method of  claim 7 , wherein the binder coating is applied using at least one of a dip-coating technique and a spray-coating technique. 
     
     
       9. The method of  claim 3 , further comprising:
 prior to performing the formation charge on the battery cell, flattening the battery cell by applying a pressure for about a minute to the layers.

Description:
BACKGROUND 
     1. Field 
     The present embodiments relate to batteries for portable electronic devices. More specifically, the present embodiments relate to techniques for increasing the stiffness of battery cells for portable electronic devices. 
     2. Related Art 
     Rechargeable batteries are presently used to provide power to a wide variety of portable electronic devices, including laptop computers, tablet computers, mobile phones, personal digital assistants (PDAs), digital music players and cordless power tools. The most commonly used type of rechargeable battery is a lithium battery, which can include a lithium-ion or a lithium-polymer battery. 
     Lithium-polymer batteries often include cells that are packaged in flexible pouches. Such pouches are typically lightweight and inexpensive to manufacture. Moreover, these pouches may be tailored to various cell dimensions, allowing lithium-polymer batteries to be used in space-constrained portable electronic devices such as mobile phones, laptop computers, and/or digital cameras. For example, a lithium-polymer battery cell may achieve a packaging efficiency of 90-95% by enclosing rolled electrodes and electrolyte in an aluminized laminated pouch. Multiple pouches may then be placed side-by-side within a portable electronic device and electrically coupled in series and/or in parallel to form a battery for the portable electronic device. 
     Conversely, the lack of a rigid, sealed battery enclosure may increase the susceptibility of lithium-polymer batteries to faults caused by mechanical stress. Such faults may occur during assembly of the batteries, installation of the batteries in portable electronic devices, and/or use of the portable electronic devices. For example, the dropping of an object onto a lightweight portable electronic device may dent the portable electronic device&#39;s enclosure, as well as a lithium-polymer battery underneath the enclosure. The dent may deform, weaken, and/or compress the battery&#39;s electrodes and/or separator, thus compromising the integrity of the battery and potentially causing a short circuit and/or another fault in the battery. 
     Hence, the use of portable electronic devices may be facilitated by mechanisms that improve the resistance of lithium-polymer battery cells to mechanical stress. 
     SUMMARY 
     The disclosed embodiments relate to the manufacture of a battery cell. The battery cell includes a set of layers including a cathode with an active coating, a separator, and an anode with an active coating. The battery cell also includes a pouch enclosing the layers, wherein the pouch is flexible. The layers may be wound to create a jelly roll prior to sealing the layers in the flexible pouch. The stiffness of the battery cell may be increased by applying a pressure of at least 0.13 kilogram-force (kgf) per square millimeter and a temperature of about 85° C. to the layers. 
     In some embodiments, the pressure and the temperature are applied to the layers for about eight hours. The combination of pressure, temperature, and time may increase the stiffness of the battery cell and improve the resistance of the battery cell to mechanical stress. 
     In some embodiments, the set of layers further includes a binder coating that laminates the layers together upon applying the pressure and the temperature to the layers. The binder coating may be approximately 1 micron thick and include polyvinylidene fluoride (PVDF) and/or another binder material. The binder coating may be applied as a continuous coating and/or non-continuous coating to the separator, cathode, and/or anode. For example, the binder coating may be applied as a continuous coating on the separator using a dip-coating technique. On the other hand, the binder coating may be applied as a non-continuous coating on the cathode and/or anode using a spray-coating technique. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  shows the placement of a battery in a computer system in accordance with an embodiment. 
         FIG. 2  shows a battery cell in accordance with an embodiment. 
         FIG. 3  shows a set of layers for a battery cell in accordance with an embodiment. 
         FIG. 4  shows the degassing of a battery cell in accordance with an embodiment. 
         FIG. 5  shows an exemplary plot in accordance with an embodiment. 
         FIG. 6  shows a flowchart illustrating the process of manufacturing a battery cell in accordance with an embodiment. 
         FIG. 7  shows a portable electronic device in accordance with an embodiment. 
     
    
    
     In the figures, like reference numerals refer to the same figure elements. 
     DETAILED DESCRIPTION 
     The following description is presented to enable any person skilled in the art to make and use the embodiments, and is provided in the context of a particular application and its requirements. Various modifications to the disclosed embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the present disclosure. Thus, the present invention is not limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. 
     The data structures and code described in this detailed description are typically stored on a computer-readable storage medium, which may be any device or medium that can store code and/or data for use by a computer system. The computer-readable storage medium includes, but is not limited to, volatile memory, non-volatile memory, magnetic and optical storage devices such as disk drives, magnetic tape, CDs (compact discs), DVDs (digital versatile discs or digital video discs), or other media capable of storing code and/or data now known or later developed. 
     The methods and processes described in the detailed description section can be embodied as code and/or data, which can be stored in a computer-readable storage medium as described above. When a computer system reads and executes the code and/or data stored on the computer-readable storage medium, the computer system performs the methods and processes embodied as data structures and code and stored within the computer-readable storage medium. 
     Furthermore, methods and processes described herein can be included in hardware modules or apparatus. These modules or apparatus may include, but are not limited to, an application-specific integrated circuit (ASIC) chip, a field-programmable gate array (FPGA), a dedicated or shared processor that executes a particular software module or a piece of code at a particular time, and/or other programmable-logic devices now known or later developed. When the hardware modules or apparatus are activated, they perform the methods and processes included within them. 
       FIG. 1  shows the placement of a battery  100  in a computer system  102  in accordance with an embodiment. Computer system  102  may correspond to a laptop computer, personal digital assistant (PDA), portable media player, mobile phone, digital camera, tablet computer, and/or other portable electronic device. Battery  100  may correspond to a lithium-polymer battery and/or other type of power source for computer system  102 . For example, battery  100  may correspond to a lithium-polymer battery that includes one or more cells packaged in flexible pouches. The cells may then be connected in series and/or in parallel and used to power computer system  102 . 
     In one or more embodiments, battery  100  is designed to accommodate the space constraints of computer system  102 . For example, battery  100  may include cells of different sizes and thicknesses that are placed side-by-side, top-to-bottom, and/or stacked within computer system  102  to fill up the free space within computer system  102 . The use of space within computer system  102  may additionally be optimized by omitting a separate enclosure for battery  100 . For example, battery  100  may include non-removable pouches of lithium-polymer cells encased directly within the enclosure for computer system  102 . As a result, the cells of battery  100  may be larger than the cells of a comparable removable battery, which in turn may provide increased battery capacity and weight savings over the removable battery. 
     On the other hand, the elimination of a separate, sealed enclosure for battery  100  may increase the susceptibility of battery  100  to contamination and/or damage. First, battery  100  may be physically vulnerable until battery  100  is encased within the enclosure for computer system  102 . In addition, the enclosure for computer system  102  may provide limited protection against mechanical stress on battery  100 . For example, the dropping of computer system  102  onto a hard surface and/or a hard object onto computer system  102  may dent both the enclosure for computer system  102  and one or more cells of battery  100 . In turn, the dent may deform, compress, and/or weaken the electrodes within the cell(s), potentially resulting in shortened cycle life, reduced capacity, an electrical short, and/or other fault or failure in battery  100 . Battery  100  may thus be susceptible to physical damage during assembly, installation in computer system  102 , and/or use of computer system  102 . 
     In one or more embodiments, the resistance of battery  100  to mechanical stress is improved by increasing the stiffness of battery  100  during manufacturing of battery  100 . As discussed in further detail below, the increased stiffness may be provided by obtaining a set of layers for a battery cell, including a cathode with an active coating, a separator, an anode with an active coating, and a binder coating applied to the cathode, anode, and/or separator. The layers may be sealed in a pouch to form the battery cell. To increase the stiffness of battery  100 , a pressure of at least 0.13 kilogram-force (kgf) per square millimeter and a temperature of about 85° C. may be applied to the layers. Such application of pressure and/or temperature may compress the layers and/or melt the binder coating, thus laminating the layers together. The stiffness of the battery cell may additionally be increased by applying the pressure and/or temperature for about eight hours. 
       FIG. 2  shows a battery cell  200  in accordance with an embodiment. Battery cell  200  may correspond to a lithium-polymer cell that is used to power a portable electronic device. Battery cell  200  includes a jelly roll  202  containing a number of layers which are wound together, including a cathode with an active coating, a separator, and an anode with an active coating. More specifically, jelly roll  202  may include one strip of cathode material (e.g., aluminum foil coated with a lithium compound) and one strip of anode material (e.g., copper foil coated with carbon) separated by one strip of separator material (e.g., conducting polymer electrolyte). As discussed below with respect to  FIG. 3 , jelly roll  202  may also include a binder coating that is used to laminate and/or bond the layers together and increase the stiffness of battery cell  200 . The cathode, anode, and separator layers may then be wound on a mandrel to form a spirally wound structure. Jelly rolls are well known in the art and will not be described further. 
     During assembly of battery cell  200 , jelly roll  202  is enclosed in a flexible pouch, which is formed by folding a flexible sheet along a fold line  212 . For example, the flexible sheet may be made of aluminum with a polymer film, such as polypropylene and/or polyethylene. After the flexible sheet is folded, the flexible sheet can be sealed, for example by applying heat along a side seal  210  and along a terrace seal  208 . 
     Jelly roll  202  also includes a set of conductive tabs  206  coupled to the cathode and the anode. Conductive tabs  206  may extend through seals in the pouch (for example, formed using sealing tape  204 ) to provide terminals for battery cell  200 . Conductive tabs  206  may then be used to electrically couple battery cell  200  with one or more other battery cells to form a battery pack. For example, the battery pack may be formed by coupling the battery cells in a series, parallel, or series-and-parallel configuration. 
       FIG. 3  shows a set of layers for a battery cell (e.g., battery cell  200  of  FIG. 2 ) in accordance with an embodiment. The layers may include a cathode current collector  302 , cathode active coating  304 , separator  306 , anode active coating  308 , and anode current collector  310 . The layers may be wound to create a jelly roll for the battery cell, such as jelly roll  202  of  FIG. 2 . Alternatively, the layers may be used to form other types of battery cell structures, such as bi-cell structures. 
     As mentioned above, cathode current collector  302  may be aluminum foil, cathode active coating  304  may be a lithium compound, anode current collector  310  may be copper foil, anode active coating  308  may be carbon, and separator  306  may include polypropylene and/or polyethylene. The layers may also include a binder coating  312  between separator  306  and cathode active coating  304  and/or anode active coating  308 . Binder coating  312  may include polyvinylidene fluoride (PVDF) and/or another binder material. Binder coating  312  may be approximately 1 micron thick to facilitate optimal laminating of the layers without degrading the cycle life of the battery cell. 
     In addition, binder coating  312  may correspond to a continuous coating and/or non-continuous coating on separator  306 , cathode active coating  304 , and/or anode active coating  308 . For example, binder coating  312  may be applied as a continuous coating on separator  306  using a dip-coating technique. On the other hand, binder coating  312  may be applied as a non-continuous coating on cathode active coating  304  and/or anode active coating  308  using a spray-coating technique. 
     During manufacturing of the battery cell, layers may be sealed into a flexible pouch to form the battery cell. Next, a pressure of at least 0.13 kgf per square millimeter and a temperature of about 85° C. may be applied to the layers. The pressure and/or temperature may additionally be applied for a pre-specified period of time. For example, to create a battery cell for a tablet computer, a set of steel plates and a heater may be used to apply a pressure of 900 kgf and a temperature of 85° C. for about eight hours to the layers. The application of pressure and/or temperature to the layers may further melt binder coating  312  and laminate (e.g., bond) the layers together, creating a solid, compressed structure instead of a set of unbonded layers wound together. Such use of pressure, temperature, time, and/or binder coating  312  in manufacturing the battery cell may increase the stiffness of the battery cell, and in turn, increase the battery cell&#39;s resistance to mechanical stress, as discussed in further detail below with respect to  FIG. 5 . 
     Prior to applying the pressure and the temperature to the layers, a formation charge may be performed on the battery cell. The formation charge may electrochemically form the battery cell by leaving a voltage and polarity imprint on the layers. However, the formation charge may generate gas that accumulates within the pouch. As a result, the battery cell may be degassed after the pressure and temperature are applied to the layers to release the gas and prepare the battery cell for installation in a portable electronic device, as discussed in further detail below with respect to  FIG. 4 . 
       FIG. 4  shows the degassing of a battery cell  400  in accordance with an embodiment. As shown in  FIG. 4 , battery cell  400  is enclosed in a pouch  402 . In addition, pouch  402  contains extra material that does not contact the layers (e.g., cathode, anode, separator, binder coating) of battery cell  400 . 
     To degas battery cell  400 , a number of punctures  404 - 406  are made in the portion of the pouch not contacting the layers of battery cell  400  to release gas generated by battery cell  400  during a formation charge. Next, a new seal  408  is formed in pouch  402  along a line that is closer to the layers of battery cell  400  than punctures  406 . In other words, seal  408  may be formed to hermetically reseal battery cell  400  in pouch  402  after punctures  404 - 406  have been made. Finally, extra pouch material associated with the punctured portion of pouch  402  (e.g., to the left of seal  408 ) is removed to complete the manufacturing of battery cell  400 . Battery cell  400  may then be installed into a portable electronic device for use as a power source for the portable electronic device. 
       FIG. 5  shows an exemplary plot in accordance with an embodiment. In particular,  FIG. 5  shows a plot of battery cell deformation (in millimeters) as a function of force (in kgf) applied to the centers of different battery cells. Deformation  502  may be observed from a battery cell created without a binder coating and with a pressure of 750 kgf applied to the battery cell for four hours. Deformation  504  may be observed from a battery cell created using a 1-micron-thick binder coating (e.g., binder coating  312  of  FIG. 3 ) and a pressure of 750 kgf applied to the battery cell for four hours. Deformation  506  may be observed from a battery cell created without a binder coating and a pressure of 750 kgf applied to the battery cell for eight hours. As shown in  FIG. 5 , battery cells associated with deformations  504 - 506  may provide, at best, slight improvements in stiffness over the battery cell associated with deformation  502 . 
     On the other hand, deformations  508 - 510  may be observed from battery cells created by applying a pressure of 900 kgf to each battery cell for eight hours. The battery cell associated with deformation  508  does not include a binder coating, while the battery cell associated with deformation  510  includes a 1-micron-thick binder coating. Deformations  508 - 510  represent significant improvements in battery cell stiffness compared to deformations  502 - 506 , with deformation  510  representing the highest battery cell stiffness of all battery cells associated with deformations  502 - 510 . Consequently, battery cell stiffness may be improved the most by adding a binder coating to the layers of the battery cell and applying a pressure of 900 kgf (e.g., 0.13 kgf per square millimeter) for about eight hours to the battery cell. 
       FIG. 6  shows a flowchart illustrating the process of manufacturing a battery cell in accordance with an embodiment. In one or more embodiments, one or more of the steps may be omitted, repeated, and/or performed in a different order. Accordingly, the specific arrangement of steps shown in  FIG. 6  should not be construed as limiting the scope of the embodiments. 
     First, a set of layers for the battery cell is obtained (operation  602 ). The layers may include a cathode with an active coating, a separator, and an anode with an active coating. The layers may also include a binder coating applied to the cathode, anode, and/or separator as a continuous and/or non-continuous coating. In addition, the binder coating may be approximately 1 micron thick to enable the laminating of the cathode, anode, and separator layers without degrading the cycle life of the battery cell. 
     Next, the layers are wound to create a jelly roll (operation  604 ). The winding step may be skipped and/or altered if the layers are used to create other battery cell structures, such as bi-cells. The layers are then sealed in a pouch to form the battery cell (operation  606 ). For example, the battery cell may be formed by placing the layers into the pouch, filling the pouch with electrolyte, and forming side and terrace seals along the edges of the pouch. The battery cell may then be left alone for 1-1.5 days to allow the electrolyte to distribute within the battery cell. 
     After the layers are sealed in the pouch, pressure is applied for a short period of time to flatten the battery cell (operation  608 ), and a formation charge is performed on the battery cell (operation  610 ). For example, the pressure may be applied for about a minute using a set of steel plates on either side of the battery cell. The formation charge may then be performed at one or more charge rates until the battery&#39;s voltage reaches a pre-specified amount. 
     The stiffness of the battery cell is then increased by applying a pressure of at least 0.13 kgf per square millimeter and a temperature of about 85° C. to the layers (operation  612 ). In addition, the pressure and temperature may be applied to the layers for about eight hours. Such application of pressure, temperature, and/or time may melt the binder coating and laminate the cathode, anode, and separator layers together, thus forming a solid structure that resists mechanical stress better than a set of unbonded layers of a jelly roll and/or other battery cell structure. 
     Finally, the battery cell is degassed (operation  614 ). To degas the battery cell, a portion of the pouch that does not contact the layers is punctured to release gas generated during the formation charge by the battery cell. Next, the pouch is resealed along a line that is closer to the layers than the punctured portion. Finally, extra pouch material associated with the punctured portion is removed from the battery cell. 
     The above-described rechargeable battery cell can generally be used in any type of electronic device. For example,  FIG. 7  illustrates a portable electronic device  700  which includes a processor  702 , a memory  704  and a display  708 , which are all powered by a battery  706 . Portable electronic device  700  may correspond to a laptop computer, mobile phone, PDA, tablet computer, portable media player, digital camera, and/or other type of battery-powered electronic device. Battery  706  may correspond to a battery pack that includes one or more battery cells. Each battery cell may include a set of layers sealed in a pouch, including a cathode with an active coating, a separator, an anode with an active coating, and/or a binder coating. During manufacturing of the battery cell, a pressure of at least 0.13 kilogram-force (kgf) per square millimeter and a temperature of about 85° C. may be applied to the layers. In addition, the pressure and/or temperature may be applied to the layers for about eight hours. The combination of pressure, temperature, and time may increase the stiffness of the battery cell and improve the resistance of the battery cell to mechanical stress. 
     The foregoing descriptions of various embodiments have been presented only for purposes of illustration and description. They are not intended to be exhaustive or to limit the present invention to the forms disclosed. Accordingly, many modifications and variations will be apparent to practitioners skilled in the art. Additionally, the above disclosure is not intended to limit the present invention.

Metadata:
Filing Date: 20111013
Publication Date: 20140325
Grant Date: 20140325
Priority Date: 20111013
Inventors: BHARDWAJ RAMESH C.
DEVAN SHEBA
HWANG TAISUP
Assignee: APPLE INC
CPC Classifications: [{"code": "Y02P70/50", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/0431", "inventive": true, "first": true, "tree": "[]"}, {"code": "Y10T29/4911", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/0565", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/0436", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01M10/044", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T29/49108", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y02E60/10", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/0587", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/052", "inventive": false, "first": false, "tree": "[]"}, {"code": "Y10T29/49114", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M2220/30", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/0431", "inventive": true, "first": true, "tree": "[]"}, {"code": "H01M10/0436", "inventive": true, "first": false, "tree": "[]"}, {"code": "H01M10/0565", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M2220/30", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/044", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/0587", "inventive": false, "first": false, "tree": "[]"}, {"code": "H01M10/052", "inventive": false, "first": false, "tree": "[]"}]
Family ID: 46800364