Abstract:
A cell undergoing a thermal runaway process is characterized by high local temperatures at the cell. Thermal insulator prevents the high temperatures from dissipating to nearby cells such that thermal runaway is triggered in the nearby cells. In addition, thermal conductors are provided that form conduction paths that draw heat from a cell undergoing thermal runaway and distribute the heat to other cells in manner that thermal runaway is not triggered in cells that are near the failing cell. If sufficient heat is drawn away from a failing cell, temperatures of the cells surrounding the failed cell can remain low enough to prevent the surrounding cells from undergoing the thermal runaway process.

Description:
BACKGROUND 
       [0001]    The described embodiments relate generally to batteries and in particular to preventing thermal runaway propagation within a battery. 
         [0002]    Cells in a battery may fail in the form of an exothermal process called thermal runaway. A thermal runaway process in a cell may be caused by manufacturing defects, mishandling or abuse of cells or any factor that raises a cell&#39;s temperature, or exposes the cell to high temperatures from an external source. The high temperatures often cause an increase in reaction rates in the cells, thereby causing a further increase in their temperature and therefore a further increase in the reaction rate. As a result of this runaway process, cells in a battery release a large amount of heat into areas surrounding the cell. 
         [0003]    Multiple cells are often needed to reach higher voltages and store sufficient energy to make the battery effective for its intended use. Since cells of a battery are often packed very closely together, if one cell in a part of an assembly of cells experiences thermal runaway, the high temperature of that failed cell can trigger thermal runaway of nearby cells. Such a process may cause the nearby cells to release heat and propagate the thermal runaway process throughout the remaining cells in the battery, causing a cascading failure of the battery and releasing a large amount of energy. 
       SUMMARY 
       [0004]    Embodiments of the invention enable components of a battery to distribute heat away from a cell that is experiencing thermal runaway. In one embodiment, a battery includes thermal conductors that draw heat away from a cell experiencing thermal runaway and thermal insulation that protects other cells from heat exposure. The thermal conductors and the thermal insulators form conduction paths that draw heat from a cell undergoing thermal runaway and distribute the heat across other cells in contact with the thermal conductors. By drawing heat from the failing cell, temperatures of the surrounding cells remain low enough to prevent the surrounding cells from undergoing the thermal runaway process. 
         [0005]    The configuration of conduction paths drawing heat away from cells undergoing thermal runaway may vary. In one embodiment, two thermal buses are used, with one bus located on each side of a cell assembly. Each cell in the assembly is in direct contact with the bus opposite to the bus coupled to the neighboring cells—that is, adjacent cells are in direct contact with alternate buses. The area in between and surrounding the cells comprises an insulating material. 
         [0006]    In another embodiment, an insulator surrounds each cell, and a single thermal bus conducts heat away from a runaway cell and distributes the heat across each of the other cells. Such a configuration slows the rate of heat conduction from a failing cell to the thermal bus and from the thermal bus to the cells surrounding the failing cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  illustrates a battery configured to dissipate heat from a failing cell during a thermal runaway process. 
           [0008]      FIG. 2  illustrates a configuration of thermal conductors and insulators for dissipating heat from a failing cell during a thermal runaway process in a first configuration, in accordance with an embodiment of the invention. 
           [0009]      FIG. 3  illustrates a configuration of a thermal conductor and insulators for dissipating heat from a failing cell during a thermal runaway process in a second configuration, in accordance with an embodiment of the invention. 
           [0010]      FIG. 4  is a graph illustrating temperature variation over time among a first cell experiencing thermal runaway and its neighboring cells in accordance with one embodiment. 
       
    
    
       [0011]    The figures depict various embodiments of the present invention for purposes of illustration only. One skilled in the art will readily recognize from the following discussion that alternative embodiments of the structures and methods illustrated herein may be employed without departing from the principles of the invention described herein. 
       DETAILED DESCRIPTION 
       [0012]    A battery includes a cell assembly housing one or more electrochemical cells. Each cell converts stored chemical energy to electrical energy. The battery includes electrical connections which connect two or more cells together such that a higher voltage, and/or greater capacity, may be output by the battery. During a process of converting chemical energy to electrical energy, cells may generate heat, and a sufficiently high amount of heat may cause the cell to fail and trigger a thermal runaway process. Embodiments of the invention provide a mechanism to dissipate heat away from a cell in a cell assembly of a battery to prevent thermal runaway. 
         [0013]      FIG. 1  illustrates a battery configured to dissipate heat from a failing cell during a thermal runaway process and avoid propagation of the thermal runaway process. The illustrated battery  100  includes a number of cell assemblies  102 , each of the cell assemblies including a number of cells  106 , a conductor  108  and an insulator  110 . Battery  100  outputs an electrical signal to one or more devices connected to the battery such that the battery may electrically power the devices. 
         [0014]    Each cell  106  converts stored chemical energy to electrical energy. A cell  106  may be a primary cell that irreversibly transforms chemical energy to electrical energy, or a secondary cell that is rechargeable. A cell type may include, but is not limited to lithium, lithium-ion, lithium-sulfur, nickel-metal hydride, nickel-cadmium, alkaline. In one embodiment, the cell  106  is a lithium-ion cell comprising of a polymer electrolyte; in an alternative embodiment, a liquid electrolyte is used. The cell mechanical packaging may be a pouch, a metallic can, or a plastic or composite structure. The cell&#39;s chemistry may include any combination capable of producing electrical energy. Each cell may also comprise electrodes including a cathode and an anode of varying chemistries. A cathode may comprise, but is not limited to: lithium cobalt-oxide, lithium nickel manganese cobalt, lithium iron phosphate. Anode materials may include, but are not limited to: carbon, silicon etc. 
         [0015]    Thermal conductor  108  conducts heat away from a cell undergoing a thermal runaway process and diffuses the heat across the remaining cells in contact with the conductor  108 . As described in greater detail below, the thermal conductor  108  may or may not make contact with a particular cell  106  based on a configuration employed. Material of the thermal conductor  108  may be selected based on several factors, including but not limited to, thermal conductivity of the metal, melting point of the metal, characteristics of a cell&#39;s chemistry, materials property of electrodes within the cell, weight, cost and ease of manufacture. Thermal conductors  108  may be composed of a variety of materials capable of conducting heat. In one embodiment, the thermal conductor  108  has a melting point that is higher than a peak temperature likely to be generated during a thermal runaway process. For example, if a maximum expected temperature of a cell undergoing thermal runaway is expected to be 160 degrees Celsius, a conductor material that has a melting point higher than 160 degrees Celsius may be used. In one embodiment, the thermal conductor  108  includes commercial grade Aluminum 1100-O, which is pliable and corrosion resistant but does not provide structural strength. In other embodiments, the thermal conductor  108  material may include but is not limited to, graphite, graphene, carbon fiber, carbon nanotubes, copper, aluminum alloy, or silver. In one embodiment, thermal conductor  108  terminates at the end of each cell assembly  102 ; alternatively it connects to an additional thermal bus associated with another cell assembly; or to a heat exchanger. 
         [0016]    In one embodiment, the length of the thermal conductor  108  between two cells is determined according to a temperature that triggers thermal runaway in a cell. For example, if thermal runaway in a cell is triggered at a higher temperature, the length of the thermal conductor  108  between two cells may be shorter, since each cell can reach a higher temperature before a runaway is triggered in that cell. In contrast, if a cell experiences thermal runaway at lower temperatures, the length of the thermal conductor  108  between two cells may be longer. 
         [0017]    The thermal insulator  110  insulates cells  106  from heat generated by other cells. The configuration of the thermal insulator  110  around cells may vary, as described below. Additionally, materials used for a thermal insulator  110  may include a ceramic fiber such as an aluminum oxide fiber. In other embodiments, silica aerogel material or a fiberglass fabric may be used as a thermal insulator  110  in the battery  100 . 
         [0018]      FIG. 2  illustrates a configuration of thermal conductors and insulators for dissipating heat from a failing cell during a thermal runaway process in a first configuration, in accordance with an embodiment of the invention. A cell assembly  200  of  FIG. 2  includes cells  206   a - 206   e,  two thermal conductor buses  208   a  and  208   b,  and a thermal insulator  210 . 
         [0019]    In the embodiment illustrated in  FIG. 2 , each thermal conductor bus  208   a,    208   b  makes contact with alternate cells  206  housed in the cell assembly  200 . For example, the conductor bus  208   a  connects and makes contact with cells  206   b  and  206   d.  Similarly, the conductor bus  208   b  connects and makes contact with cells  206   a,    206   c  and  206   e.  Direct contact between the conductor bus  208  and the cells  206  permits the conductor bus  208  to conduct heat more quickly from a cell experiencing thermal runaway. Connecting each bus  208   a,    208   b  to alternating cells  206  helps keep higher temperatures away from cells immediately adjacent to a cell experiencing runaway. For example, if cell  206   c  is experiencing thermal runaway, thermal bus  208   b  conducts heat away not only from cell  206   c,  but also from adjacent cells  206   b  and  206   d.    
         [0020]    In the configuration illustrated in  FIG. 2 , a thermal insulator  210  insulates each cell  206  and conductor bus  208 . The insulator prevents heat generated at a cell experiencing thermal runaway from dissipating to nearby cells. For example, if a cell  206   a  is experiencing thermal runaway, the insulator  210  prevents heat from dissipating to cell  206   b.  As such, the insulator  210  allows more of the heat generated at  206   a  to dissipate along the conduction path provided by the conductor bus  208   b.    
         [0021]      FIG. 3  illustrates a configuration of a thermal conductor and insulators for dissipating heat from a failing cell during a thermal runaway process in a second configuration, in accordance with an embodiment of the invention. A cell assembly  300  of  FIG. 3  includes cells  306   a - 306   e,  one thermal conductor bus  308 , and thermal insulators  310 . 
         [0022]    In the illustrated configuration, the thermal conductor bus  308  does not make direct contact with cells  306  housed in the cell assembly  300 . Thermal insulator  310  insulates each cell  306  such that heat generated during a thermal runaway event in a failed cell is conducted slowly to the conductor bus  308  at points near the failed cell, and then dissipated across the entire bus and absorbed by insulator  310  and the additional cells. By distributing the heat of the thermal runaway across the thermal mass of the system, including the thermal bus and the thermal insulator, the temperature of the other cells is raised, but to a level lower than the critical value that would trigger thermal runaway in the other cells. The critical temperature value depends on cell chemistry as understood by those of skill in the art. As such, the length and material of the thermal conductor bus  308  and the amount and material of the thermal insulator  310  is selected based on the critical temperature of each one or more cell in the cell assembly. As described in reference to  FIG. 1 , the length and material of the thermal insulator and thermal conductor may be changed responsive to the critical temperature. In the configuration described above, there is no direct conduction path between the cells  306  and the conductor bus  308 . However, such a configuration permits conduction to occur at a slower rate at the conductor bus  308 . An advantage of such a configuration is that a single conductor bus  308  dissipates heat more equally among cells that have not failed. 
         [0023]      FIG. 4  is a graph illustrating temperature variation over time among a first cell T 1  experiencing thermal runaway and its neighboring cells T 2 , T 3 , T 4  and T 5  in accordance with one embodiment having a configuration illustrated in  FIG. 2 . In the example of  FIG. 4 , each cell was 0.31″ thick, with 0.005″ thick aluminum thermal conductors and 0.1″ ceramic insulation between cells. The 0.005″ thick aluminum conductors were attached to a 0.015″ thick thermal bus. As illustrated in the graph, although the failing cell T 1  experienced temperatures nearing 400 degrees Celsius, its neighboring cells did not reach temperatures of even 100 degrees. In  FIG. 4 , cells T 2  and T 3  neighbor the failing cell T 1 ; cells T 4  and T 5  neighbor cells T 2  and T 3  but not the failing cell T 1 . As illustrated in  FIG. 4 , the neighboring cells T 2  and T 3  experience temperatures nearing 100 degrees Celsius as heat dissipates to these cells from the failing cell T 1 . However, the thermal bus effectively discharges heat from the cells T 2  and T 3  to prevent thermal runaway propagation in these cells. Additionally, cells T 4  and T 5  are further away from the failing cell T 1  and reach temperatures nearing 50 degrees Celsius. As such, the thermal bus effectively dissipates heat from the failing cell T 1  to those further away in a cell assembly. 
         [0024]    The foregoing description of the embodiments of the invention has been presented for the purpose of illustration; it is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Persons skilled in the relevant art can appreciate that many modifications and variations are possible in light of the above disclosure. 
         [0025]    Some portions of this description describe the embodiments of the invention in terms of algorithms and symbolic representations of operations on information. These algorithmic descriptions and representations are commonly used by those skilled in the data processing arts to convey the substance of their work effectively to others skilled in the art. These operations, while described functionally, computationally, or logically, are understood to be implemented by computer programs or equivalent electrical circuits, microcode, or the like. Furthermore, it has also proven convenient at times, to refer to these arrangements of operations as modules, without loss of generality. The described operations and their associated modules may be embodied in software, firmware, hardware, or any combinations thereof 
         [0026]    Any of the steps, operations, or processes described herein may be performed or implemented with one or more hardware or software modules, alone or in combination with other devices. In one embodiment, a software module is implemented with a computer program product comprising a computer-readable medium containing computer program code, which can be executed by a computer processor for performing any or all of the steps, operations, or processes described. 
         [0027]    Embodiments of the invention may also relate to an apparatus for performing the operations herein. This apparatus may be specially constructed for the required purposes, and/or it may comprise a general-purpose computing device selectively activated or reconfigured by a computer program stored in the computer. Such a computer program may be stored in a tangible computer readable storage medium or any type of media suitable for storing electronic instructions, and coupled to a computer system bus. Furthermore, any computing systems referred to in the specification may include a single processor or may be architectures employing multiple processor designs for increased computing capability. 
         [0028]    Embodiments of the invention may also relate to a computer data signal embodied in a carrier wave, where the computer data signal includes any embodiment of a computer program product or other data combination described herein. The computer data signal is a product that is presented in a tangible medium or carrier wave and modulated or otherwise encoded in the carrier wave, which is tangible, and transmitted according to any suitable transmission method. 
         [0029]    Finally, the language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the invention be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.