Abstract:
In accordance with one embodiment, a bipolar solid state battery includes a first cell stack including a first solid-electrolyte separator positioned between a first cathode and a first anode, a first base layer including a first base portion positioned directly beneath the first anode, a second cell stack including a second solid-electrolyte separator positioned between a second cathode and a second anode, a second base layer including a second base portion positioned directly beneath the second anode, and a thermally insulating medium surrounding the first cell stack and the second cell stack.

Description:
CROSS REFERENCE 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/870,836 filed Aug. 28, 2013, the entire contents of which is herein incorporated by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to batteries and more particularly to solid state batteries. 
       BACKGROUND 
       [0003]    Rechargeable lithium-ion batteries are attractive energy storage systems for portable electronics and electric and hybrid-electric vehicles because of their high specific energy compared to other electrochemical energy storage devices. A typical Li-ion cell contains a negative electrode, a positive electrode, and a separator region between the negative and positive electrodes. Both electrodes contain active materials that insert or react with lithium reversibly. In some cases the negative electrode may include lithium metal, which can be electrochemically dissolved and deposited reversibly. The separator contains an electrolyte with a lithium cation, and serves as a physical barrier between the electrodes such that none of the electrodes are electronically connected within the cell. 
         [0004]    Typically, during charging, there is generation of electrons at the positive electrode and consumption of an equal amount of electrons at the negative electrode, and these electrons are transferred via an external circuit. In the ideal charging of the cell, these electrons are generated at the positive electrode because there is extraction via oxidation of lithium ions from the active material of the positive electrode, and the electrons are consumed at the negative electrode because there is reduction of lithium ions into the active material of the negative electrode. During discharging, the exact opposite reactions occur. 
         [0005]    Batteries with a lithium metal negative electrode afford exceptionally high specific energy (in Wh/kg) and energy density (in Wh/L) compared to batteries with conventional carbonaceous negative electrodes. However, the cycle life of such systems is rather limited due to (a) significant volume changes in the cell sandwich during every cycle as the Li metal is stripped and plated, (b) formation of dendrites during recharge that may penetrate the separator and short the cell and/or result in fragmentation and capacity loss of the negative electrode; (c) morphology changes in the metal upon extended cycling that result in a large overall volume change in the cell; and (d) changes in the structure and composition of the passivating layer that forms at the surface of the metal when exposed to certain electrolytes, which may isolate some metal and/or increase the resistance of the cell over time. 
         [0006]    When high-specific-capacity negative electrodes such as a metal are used in a battery, the maximum benefit of the capacity increase over conventional systems is realized when a high-capacity positive electrode active material is also used. For example, conventional lithium-intercalating oxides (e.g., LiCoO 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 , Li 1.1 Ni 0.3 Co 0.3  Mn 0.3 O 2 ) are typically limited to a theoretical capacity of ˜280 mAh/g (based on the mass of the lithiated oxide) and a practical capacity of 180 to 250 mAh/g, which is quite low compared to the specific capacity of lithium metal, 3863 mAh/g. The highest theoretical capacity for which some practical cycling has been achieved for a lithium-ion positive electrode is 1168 mAh/g (based on the mass of the lithiated material), which is shared by Li 2 S and Li 2 O 2 . Other high-capacity materials include BiF 3  (303 mAh/g, lithiated), FeF 3  (712 mAh/g, lithiated), LiOH.H 2 O (639 mAh/g), and others. Unfortunately, all of these materials react with lithium at a lower voltage compared to conventional oxide positive electrodes, hence limiting the theoretical specific energy; however, the theoretical specific energies are still very high (&gt;800 Wh/kg, compared to a maximum of ˜500 Wh/kg for a cell with lithium negative and conventional oxide positive electrodes). 
         [0007]      FIG. 1  depicts a chart  2  showing the range achievable for a vehicle using battery packs of different specific energies versus the weight of the battery pack. In the chart  10 , the specific energies are for an entire cell, including cell packaging weight, assuming a 50% weight increase for forming a battery pack from a particular set of cells. The U.S. Department of Energy has established a weight limit of 200 kg for a battery pack that is located within a vehicle. Accordingly, only a battery pack with about 600 Wh/kg or more can achieve a range of 300 miles. 
         [0008]    Lithium-based batteries have a sufficiently high specific energy (Wh/kg) and energy density (Wh/L) that they are now being used in electric-powered vehicles. However, in order to power a full-electric vehicle with a range of several hundred miles, a battery with a higher specific energy than the present state of the art (an intercalation system with a graphite anode and transition-metal oxide cathode) is necessary. 
         [0009]    Some options which provide higher specific energy as compared to the currently utilized batteries are possible. For example,  FIG. 2  depicts a chart  4  which identifies the specific energy and energy density of various lithium-based chemistries. In the chart  4 , only the weight of the active materials, current collectors, binders, separator, and other inert material of the battery cells are included. The packaging weight, such as tabs, the cell can, etc., are not included. As is evident from the chart  4 , the lithium-sulfur battery, which uses a lithium metal negative electrode and a positive electrode that reduces sulfur to form lithium sulfide, has a significantly higher specific energy than the present state of the art. 
         [0010]    There are significant challenges that must be addressed for the lithium-sulfur system to become commercially viable. Important challenges include increasing the cycle life (current state of the art is 100 to several hundred cycles; target is &gt;500, preferably &gt;2000), increasing the utilization of sulfur (typical utilization is below 75% due to passivation of the positive electrode by Li 2 S or Li 2 S 2 , which are electronically insulating), increasing the mass fraction of sulfur in the positive electrode (typically the mass fraction is below 50%), and increasing the rate capability of the cell (target discharge rate is 1 C or higher). While some Li/S cells described in the literature fulfill some of the objectives for cycle life, specific energy, and specific power, none of these cells adequately address all of the issues as would be needed to realize a commercial cell. 
         [0011]    What is needed, therefore, is a solid state electrochemical cell which addresses one or more of the above identified issues. 
       SUMMARY 
       [0012]    In accordance with one embodiment, a bipolar solid state battery includes a first cell stack including a first solid-electrolyte separator positioned between a first cathode and a first anode, a first base layer including a first base portion positioned directly beneath the first anode, a second cell stack including a second solid-electrolyte separator positioned between a second cathode and a second anode, a second base layer including a second base portion positioned directly beneath the second anode, and a thermally insulating medium surrounding the first cell stack and the second cell stack. 
         [0013]    In one or more embodiments the thermally insulating medium includes a polymer. 
         [0014]    In one or more embodiments the thermally insulating medium includes a thermally conductive and electronically insulating solid. 
         [0015]    In one or more embodiments the thermally insulating medium comprises a fluid. 
         [0016]    In one or more embodiments the thermally insulating fluid is in fluid contact with the first cell stack and the second cell stack. 
         [0017]    In one or more embodiments the battery includes an inner packaging layer surrounding the first cell stack and the second cell stack, and an outer packaging layer spaced apart from the inner packaging layer, and surrounding the first cell stack and the second cell stack, wherein the thermally insulating fluid is contained between the inner packaging layer and the outer packaging layer. 
         [0018]    In one or more embodiments the thermally insulating fluid includes a thermally conductive and electronically insulating solid. 
         [0019]    In one or more embodiments the insulating fluid includes a non-flammable oil. 
         [0020]    In one or more embodiments the battery includes a cooling plate in thermal communication with the outer packaging layer. 
         [0021]    In one or more embodiments the cooling plate includes a thermally controlled fluid within a cooling coil. 
         [0022]    In one embodiment, a method of forming a bipolar solid state battery includes providing a first cell stack including a first solid-electrolyte separator positioned between a first cathode and a first anode, positioning a first base portion of a first base layer directly beneath the first anode, providing a second cell stack including a second solid-electrolyte separator positioned between a second cathode and a second anode, positioning a second base portion of a second base layer directly beneath the second anode, and surrounding the first cell stack and the second cell stack with a thermally insulating medium. 
         [0023]    In one or more embodiments surrounding the first cell stack and the second cell stack includes surrounding the first cell stack and the second cell stack with a polymer. 
         [0024]    In one or more embodiments surrounding the first cell stack and the second cell stack includes surrounding the first cell stack and the second cell stack with a non-flammable fluid. 
         [0025]    In one or more embodiments surrounding the first cell stack and the second cell stack includes placing the thermally insulating fluid in fluid contact with the first cell stack and the second cell stack. 
         [0026]    In one or more embodiments the method includes surrounding the first cell stack and the second cell stack with an inner packaging layer, and surrounding the inner packaging with an outer packaging layer spaced apart from the inner packaging layer, wherein surrounding the first cell stack and the second cell stack with a thermally insulating medium includes placing the thermally insulating fluid in a space between the inner packaging layer and the outer packaging layer. 
         [0027]    In one or more embodiments the method includes placing a thermally conductive and electronically insulating solid in the thermally insulating fluid. 
         [0028]    In one or more embodiments the method include positioning a cooling plate in thermal communication with the outer packaging layer. 
         [0029]    In one or more embodiments the method includes filling a cooling coil in the cooling plate with a thermally controlled fluid. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0030]      FIG. 1  depicts a plot showing the relationship between battery weight and vehicular range for various specific energies; 
           [0031]      FIG. 2  depicts a chart of the specific energy and energy density of various lithium-based cells; 
           [0032]      FIG. 3  depicts a simplified cross sectional view of a stacked battery with some components omitted and with a bipolar design that includes an insulating/cooling packaging; and 
           [0033]      FIG. 4  depicts a partial side perspective view of one of the cells of  FIG. 3  showing a separator with an open cell microstructured composite separator with solid-electrolyte components in the form of columns which inhibits dendrite formation while allowing flexing of the anodes. 
       
    
    
     DESCRIPTION 
       [0034]    For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the embodiments illustrated in the drawings and described in the following written description. It is understood that no limitation to the scope of the disclosure is thereby intended. It is further understood that the present disclosure includes any alterations and modifications to the illustrated embodiments and includes further applications of the principles of the disclosure as would normally occur to one of ordinary skill in the art to which this disclosure pertains. 
         [0035]      FIG. 3  depicts an electrochemical battery  100 . The electrochemical battery  100  includes a number of cells or cell stacks  102   X  within a packaging  104  or other surrounding environment that is both electrically insulating and (optionally) thermally conductive. The packaging  104  improves the safety of the electrochemical battery  100 . 
         [0036]    Each of the cells  102   X  includes an anode  106   X , a separator  108   X , and a cathode  110   X . A base layer  112   X , which is typically metal such as copper and can serve as a current collector is positioned adjacent to the anode  106   X  and between the anode  106   X  and an adjacent cathode. For example, the base layer  112   1  is located between the anode  106   1  and the cathode  110   2 . 
         [0037]    The anodes  106   X  include lithium metal or a lithium alloy metal. The anodes  106   X  are sized such that they have at least as much capacity as the associated cathode  110   X , and preferably at least 10% excess capacity and up to greater than 50% capacity in some embodiments. 
         [0038]    The cathodes  110   X  in one embodiment are a dense layer of active Li-insertion material. In some embodiments the cathodes  110   X  include a sulfur or sulfur-containing material (e.g., PAN-S composite or Li 2 S); an air electrode; Li-insertion materials such as NCM, LiNi 0.5 Mn 1.5 O 4 , Li-rich layered oxides, LiCoO 2 , LiFePO 4 , LiMn 2 O 4 ; Li-rich NCM, NCA, and other Li intercalation materials, or blends thereof or any other active material or blend of materials that react with and/or insert Li cations and/or electrolyte anions. The cathodes  110   X  include Li-conducting polymer, ceramic or other solid, non-polymer electrolyte. The cathode Li-insertion materials may additionally be coated (e.g., via spray coating) with a material such as LiNbO 3  in order to improve the flow of ions between the Li-insertion materials and the solid electrolyte, as described in T. Ohtomo et al., Journal of Power Sources 233 (2013) 231-235. Solid electrolyte materials in the cathodes  110   X  may further include lithium conducting garnets, lithium conducting sulfides (e.g., Li 2 S—P 2 S 5 ) or phosphates, Li 3 P, LIPON, Li-conducting polymer (e.g., PEO), Li 7-x La 3 Ta x Zr 2-x O 12 , wherein 0≦X≦2, Li-conducting metal-organic frameworks such as described by Wiers et al. “A Solid Lithium Electrolyte via Addition of Lithium Isopropoxide to a Metal-Organic Framework with Open Metal Sites,” Journal of American Chemical Society, 2011, 133 (37), pp 14522-14525, the entire contents of which are herein incorporated by reference, thio-LISiCONs, Li-conducting NaSICONs, Li 10 GeP 2 S 12 , lithium polysulfidophosphates, or other solid Li-conducting material. Other solid electrolyte materials that may be used are described in Christensen et al., “A critical Review of Li/Air Batteries”, Journal of the Electrochemical Society 159(2) 2012, the entire contents of which are herein incorporated by reference. Other materials in the cathodes  110   X  may include electronically conductive additives such as carbon black, and a binder material. The cathode materials are selected to allow sufficient electrolyte-cathode interfacial area for a desired design. 
         [0039]    In some embodiments, the separators  108   X  are microstructured composite separators which conduct lithium ions between the anodes  106   X  and the cathodes  110   X  while blocking electrons. For example,  FIG. 4  depicts a partial perspective view of the cell  102   1  which includes a layer  120  adjacent to the anode  106   1  and a layer  122  adjacent to the cathode  110   1 . A current collector  124  is also shown which may be made of aluminum and is provided in some embodiments, and may be separated from an adjacent base layer  112   X  by a layer of electrically conductive but chemically inactive material such as graphite. A number of solid-electrolyte components in the form of columns  126  extend between the layer  120  and the layer  122  defining microstructure cavities  128  therebetween. 
         [0040]    The microstructured composite separator  108   X  thus consists of regularly spaced solid-electrolyte components  126  which provide sufficient ionic transport (i.e., by providing a sufficiently high volume fraction of conducing material and by limiting the thickness of the structure between the anode and cathode) and provide mechanical resistance to suppress the formation and growth of lithium dendrites in the anode  106   X . In the embodiment of  FIG. 4 , solid-electrolyte components  108   X  are flexible so as to accommodate volume change of the electrodes. 
         [0041]    While three columns  126  are shown in  FIG. 4 , there are more or fewer solid-electrolyte components in other embodiments. In other embodiments, the solid-electrolyte components may be configured in other forms. In some embodiments, the microstructure cavities  128  may be filled with different compositions to provide a desired flexibility and/or to otherwise modify mechanical properties of the microstructured composite separator. More details regarding the microstructured composite separator  108   X , and other alternative separator configurations, are provided in U.S. application Ser. No. 14/460,798, filed Aug. 15, 2014, the entire contents of which are herein incorporated by reference. 
         [0042]    By stacking the cells  102   X  in the bipolar design of  FIG. 3 , the operating voltage of the battery  100  can be modified to the desired voltage. By way of example, if each cell  102   X  has an operating voltage of ˜4 V, 100 cells  102   X  can be stacked to produce a device that has an operating voltage of ˜400 V. In this way, a given power can be achieved while passing a low current through each of the cells  102   X . Therefore, wiring of the cells  102   X  can be achieved with small-diameter electrical conductors while maintaining high energy efficiency. The battery  100  thus provides an operating voltage greater than 5 V, and in some embodiments, greater than 50 V. 
         [0043]    The packaging  104  is electrically insulating and includes an inner packaging layer  142  and an outer packaging layer  144 . An insulating polymer or liquid  146  (such as nonflammable oil) fills the space in between the two layers of packaging. Examples of nonflammable oils include DuPont&#39;s Krytox® Fluorinated oils for lubrication, Halocarbon&#39;s inert lubricants, etc. In general, these lubricants (described as oils, greases, and or waxes) are halogenated. The insulation  146  may optionally contain thermally conductive solids such as oxides (e.g., magnesium oxide) that are electronically insulating in order to improve the rate of heat transfer to and from the cell stack during operation of the energy storage device. 
         [0044]    In some embodiments, the liquid or polymer  146  may contact the cell stack directly (i.e., there is no inner layer  142  of cell packaging). In such cases, the liquid/polymer  146  and thermally conductive additive is inert to the materials of the cell stack that form an interface with the fluid. 
         [0045]    In the embodiment of  FIG. 3 , one side of the packaging  104  is connected to a cooling medium or cooling plate  150  (e.g., aluminum block). In other embodiments, more sides of the packaged cell stack are connected to a cooling medium or cooling plate while in some embodiments the cooling plate is omitted. In one embodiment an oxide layer (not shown) is positioned in between the cell stack and the cooling medium or cooling plate. 
         [0046]    The cooling plate  150  is thermally controlled by a fluid (air, water, or other heat-transfer fluid, such as ethylene glycol) which flows through a cooling coil  152 . The coil  152  is used to remove heat via a radiator or other heat exchanger and/or may absorb heat from a heater. In some embodiments, the fluid is air which is incorporated into the heating/air-conditioning system of the vehicle or device that uses the energy storage system. In some embodiments, a fan or blower forces air through or along the surface of the cooling plate and/or battery. Seals (e.g., polymer O-rings) may be incorporated into the cell packaging such that the electronically insulating fluid does not evaporate or otherwise escape from the cell packaging. 
         [0047]    While  FIG. 3  depicts a single stack of cells  102   X , a number of cell stacks can be connected to one another electrically, either in parallel, or series, or both. The cell stacks may be packaged separately and/or may use a common packaging, including the electronically insulating/cooling material. Each cell or cell stack includes a positive and negative terminal (not shown) to which an electronic conductor can be connected, in order to connect the cell stack to other cell stacks and/or to the terminals of the battery. 
         [0048]    The above described embodiments provide a safe energy-storage system with high voltage enabled by multiple cell sandwiches stacked in series and an electronically insulating material or medium surrounding the cell stack or incorporated into the cell packaging. Improved cooling and heating of the energy storage system is enabled by thermally conductive material incorporated into the packaging or medium surrounding the cell stack and/or a cooling/heating plate and/or a cooling/heating fluid. 
         [0049]    While the disclosure has been illustrated and described in detail in the drawings and foregoing description, the same should be considered as illustrative and not restrictive in character. It is understood that only the preferred embodiments have been presented and that all changes, modifications and further applications that come within the spirit of the disclosure are desired to be protected.