Patent Document

RELATED APPLICATIONS 
       [0001]    The application is related to US Patent Publication No. 20120156364; US Patent Publication No. 20120153211; U.S. patent application Ser. No. 13/727,927; U.S. patent application Ser. No. 13/727,993; U.S. patent application Ser. No. 13/727,960; and U.S. patent application Ser. No. 13/728,016. 
     
    
     BACKGROUND 
       [0002]    A strong desire for battery systems with energy densities beyond conventional Lithium ion (Li-ion) chemistries exists. Lithium-sulfur batteries are a preferential choice because of their higher specific capacity as well as the abundance and low cost of elemental sulfur. A typical Li-S battery cell consists of lithium as the negative electrode, a sulfur-carbon composite as the positive electrode and an organic liquid electrolyte. Typically, Li—S batteries offer specific capacities up to 1675 Ah Kg −1  and energy densities up to 200 Wh L −1 . Specific capacity is typically the total Amp-hours (Ah) per kilogram available when the battery is discharged at a particular discharge current, and the energy density is the Watt-hours (Wh) per liter. These batteries currently deliver energy densities of 350 Wh/Kg already passing the densities of conventional Li-ion batters at 180 Wh/Kg. However, these batteries have issues with short cycle lives, low charging efficiency, high self-discharge rates, and safety concerns. 
         [0003]    Many of these problems stem from dissolution of lithium polysulfide (PS, Li 2 S n ), a family of sulfur reduction intermediates, in the liquid electrolyte. In spite of the problems of dissolution, the process is necessary to properly operate a Li—S battery. During the discharge step, lithium ion transport occurs through the liquid electrolyte from the anode to the cathode and yields Li 2 S 8  by reaction of lithium and sulfur around 2.2-2.3 Volts. Generally, both elemental sulfur and its reduction products are non-conductive, so that the conductive carbon surfaces must provide deposit sites for the reduction of sulfur and lithium polysulfides. Ideally, eventual dissolution of the lithium polysulfides re-exposes the conductive carbon surfaces. 
         [0004]    However, the lithium polysulfide species dissolved at the cathode electrode can also diffuse through the electrolyte to the lithium anode and form insoluble lithium polysulfide species. This parasitic reaction by what is sometimes referred to as ‘PS redox shuffle’ causes the loss of active material, corrosion of the lithium anode, and a shortened cycle life. Further, fire hazards exist during the battery cycling due to the presence of metastable lithium metal in flammable organic liquid electrolytes and lithium dendrites formed from the lithium that have penetrated the separator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1  shows a prior art embodiment of a battery. 
           [0006]      FIG. 2  shows a prior art embodiment of a long lithium transport path through tortuous solid electrolyte. 
           [0007]      FIG. 3  shows an embodiment of a lithium sulfur battery having interdigitated stripes of materials. 
           [0008]      FIG. 4  shows an embodiment of a short lithium transport path through channel filled by solid electrolyte. 
           [0009]      FIG. 5  shows an embodiment of a process for manufacturing a lithium sulfur battery. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0010]      FIG. 1  shows a prior art embodiment of a battery. Typically, batteries have a cathode  18  and an anode  20  separated by a separator  14 . Inactive components may consist of electrolyte, binder, and carbon. The battery may also include current collectors  12  and  16 . For current electric vehicle (EV) applications, large batteries are produced by stacking many layers of conventional thin electrodes. This results in a large proportion of inactive components, contributing to the costs and low volumetric energy density.  FIG. 2  shows how Li-ion transport path  22  occurs through a portion  20  of the electrolyte  24 . The path is torturous and affects the efficiency of the battery. 
         [0011]    As discussed in U.S. patent application Ser. No. 13,727,960, it is possible to form lithium structures having microstructures that allow for faster lithium transport using pore channels. This can be applied to the higher energy densities of Li—S batteries and solid electrolytes for safety considerations.  FIG. 3  shows an embodiment of such a battery  50 . 
         [0012]    The battery  50  has a current collector  56  adjacent an anode  60 , lithium anode. The separator  54  is arranged between the anode  60  and the cathode  58 . The cathode consists of interdigitated stripes or strips of material. Looking at the region of the material  70 , one can see that the first material has thicker stripes than the second material. 
         [0013]    The first material here contains sulfur, graphite, and solid electrolyte  62  in  FIG. 4 . The relatively small amount of solid electrolyte is added to the first material, which acts as a binder. In order for the material to form a lithium pore channel, the material will most likely be lithium sulfur such as lithium sulfur or lithium superionic sulfide (LSS). 
         [0014]    The second material will consists of a solid electrolyte  64  in  FIG. 4 . In some embodiments, the electrolyte is a polymer, in others it is a glass, ceramic, or a glass/ceramic blend. Polymer electrolytes are suitable for thin-film based devices and flexible battery designs, while inorganic ceramic electrolytes are suitable for rigid battery designs. The solid electrolytes are safer because they are non-flammable and also improve battery lifetime by reducing sulfur migration into the lithium anode. This prevents the formation of insoluble polysulfide species. 
         [0015]    The electrolytes may consist of several different types of materials. For example, the glass/ceramic materials may consist of: Li 2 S—P 2 S 5  glass; Li 2 S—P 2 S 5  glass-ceramic; Li 2 S-P 2 S 5 —Li 4 SiO 4 ; Li 2 S—SiS 2 +Li 2 SiO 4 ; and Li 2 S—Ga 2 S 5 -GeS 2 . The polymer electrolyte may consist of either a solid or a gel electrolyte. An example of a solid polymer electrolyte is poly(ethylene oxide). Examples of gel polymer electrolyte materials include poly(vinylidine fluoride), a room temperature ionic liquid, poly(methyl methacrylate), poly(acrylonitrile) and ethylene glycol based polymers. 
         [0016]    These materials are used to form the solid battery structures, either rigid or thin-film.  FIG. 5  shows an embodiment of a process to form Li—S batteries. As shown in  FIG. 5 , the active material, typically sulfur, carbon, and solid electrolyte is mixed with a solvent to form an extrudable paste or liquid at  80 . The relatively small amount of solid electrolyte is added into the first material, which acts as a binder. Similarly, the solid electrolyte material is also mixed with a solvent to allow it to be extruded at  82 . The two materials are then fed into a co-extrusion head and extruded in interdigitated, alternating stripes of materials at  84 . 
         [0017]    The solvent is then removed from the materials at  86 . The materials then solidify to form a solid battery cathode. Once the cathode is formed, a separator is placed adjacent the cathode at  88 . The lithium anode is then placed adjacent the anode to form a battery at  90 . 
         [0018]    The resulting batteries have better energy densities than traditional lithium cobalt oxide batteries, and are safer than batteries with liquid electrolytes. The solid electrolytes also reduces the migration of the sulfur species into the lithium anode electrode. The ionic conductivity is comparable to ionic conductivity of organic carbonate liquid electrolyte. 
         [0019]    It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be desirably combined into many other different systems or applications. Also that various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may be subsequently made by those skilled in the art which are also intended to be encompassed by the following claims.

Technology Category: 5