Abstract:
An electrochemical cell in one embodiment includes a first negative electrode including a form of lithium, a positive electrode, and a first separator positioned between the first negative electrode and the positive electrode, wherein the positive electrode includes a plurality of coated small grains of Li 2 S.

Description:
CROSS REFERENCE 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 61/866,089 filed Aug. 15, 2013, and U.S. Provisional Application No. 61/866,080 filed Aug. 15, 2013, the entire contents of which are both herein incorporated by reference. 
     
    
     FIELD OF THE DISCLOSURE 
       [0002]    The present disclosure relates to batteries and more particularly to lithium-metal 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 lithium-sulfur electrochemical cell which addresses one or more of the above identified issues. 
       SUMMARY 
       [0012]    An electrochemical cell in one embodiment includes a first negative electrode including a form of lithium, a positive electrode, and a first separator positioned between the first negative electrode and the positive electrode, wherein the positive electrode includes a plurality of coated small grains of Li 2 S. 
         [0013]    In one or more embodiments the coating of each of the plurality of coated small grains of Li 2 S comprises no more than 20% of the mass of each of the plurality of coated small grains of Li 2 S. 
         [0014]    In one or more embodiments an electrochemical cell includes a liquid electrolyte within pores in the positive electrode. 
         [0015]    In one or more embodiments an electrochemical cell includes a second negative electrode including a form of lithium, and a second separator positioned between the second negative electrode and the positive electrode. 
         [0016]    In one or more embodiments each of the grains in the plurality of coated small grains of Li 2 S has a diameter of about 100 nm to 10 μm. 
         [0017]    In one or more embodiments each of the grains in the plurality of coated small grains of Li 2 S has a diameter of 1 μm. 
         [0018]    In one or more embodiments each of the grains in the plurality of coated small grains of Li 2 S is coated with carbon. 
         [0019]    In one or more embodiments each of the grains in the plurality of coated small grains of Li 2 S is coated with Al 2 O 3 . 
         [0020]    A method of forming an electrochemical cell in one embodiment includes forming a plurality of small Li 2 S grains, coating the plurality of small Li 2 S grains, forming a positive electrode using the coated plurality of small Li 2 S grains, and locating a separator between the positive electrode and a negative electrode including a form of lithium. 
         [0021]    In one or more embodiments, a method of forming an electrochemical cell includes exposing the coated plurality of small Li 2 S grains and an uncoated plurality of small Li 2 S grains to an organic solvent including dissolved sulfur, dissolving the uncoated plurality of small Li 2 S grains with the organic solvent, and rinsing the coated plurality of small Li 2 S grains after exposing the coated plurality of small Li 2 S grains to the organic solvent and prior to forming the positive electrode. 
         [0022]    In one or more embodiments dissolving the uncoated plurality of small Li 2 S grains includes dissolving a first portion of the uncoated plurality of small Li 2 S grains, rinsing a second portion of the uncoated plurality of small Li 2 S grains after dissolving the first portion, and dissolving the second portion of the uncoated plurality of small Li 2 S grains. 
         [0023]    In one or more embodiments dissolving the uncoated plurality of small Li 2 S grains includes dissolving the uncoated plurality of small Li 2 S grains into a lithium polysulfide. 
         [0024]    In one or more embodiments, a method of forming an electrochemical cell includes forming a plurality of coating fragments by dissolving the uncoated plurality of small Li 2 S grains, wherein forming the positive electrode includes forming the positive electrode using the coated plurality of small Li 2 S grains and the plurality of coating fragments. 
         [0025]    In one or more embodiments, a method of forming an electrochemical cell includes forming a plurality of coating fragments by dissolving the uncoated plurality of small Li 2 S grains, and separating the plurality of coating fragments and the coated plurality of small Li 2 S grains, wherein forming the positive electrode includes forming the positive electrode using the separated coated plurality of small Li 2 S grains. 
         [0026]    In one or more embodiments, separating the plurality of coating fragments includes separating the plurality of coating fragments using a sedimentation process. 
         [0027]    In one or more embodiments, separating the plurality of coating fragments includes separating the plurality of coating fragments using a centrifugation process. 
         [0028]    In one or more embodiments, a method of forming an electrochemical cell includes exposing the coated plurality of small Li 2 S grains and an uncoated plurality of small Li 2 S grains to toluene, dissolving the uncoated plurality of small Li 2 S grains with the toluene, and rinsing the coated plurality of small Li 2 S grains after exposing the coated plurality of small Li 2 S grains to the toluene and prior to forming the positive electrode. 
         [0029]    In one or more embodiments, a method of forming an electrochemical cell includes forming a plurality of coating fragments by dissolving the uncoated plurality of small Li 2 S grains, wherein forming the positive electrode includes forming the positive electrode using the coated plurality of small Li 2 S grains and the plurality of coating fragments. 
         [0030]    In one or more embodiments, a method of forming an electrochemical cell includes forming a plurality of coating fragments by dissolving the uncoated plurality of small Li 2 S grains, and separating the plurality of coating fragments and the coated plurality of small Li 2 S grains, wherein forming the positive electrode includes forming the positive electrode using the separated coated plurality of small Li 2 S grains. 
         [0031]    In one or more embodiments, separating the plurality of coating fragments includes separating the plurality of coating fragments using a sedimentation process. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0032]      FIG. 1  depicts a plot showing the relationship between battery weight and vehicular range for various specific energies; 
           [0033]      FIG. 2  depicts a chart of the specific energy and energy density of various lithium-based cells; 
           [0034]      FIG. 3  depicts a simplified schematic of a solid state electrochemical cell including a positive electrode with nano-sized Li 2 S particles with a conductive coating; and 
           [0035]      FIG. 4  depicts a simplified schematic of an electrochemical cell including a positive electrode with nano-sized Li 2 S particles with a conductive coating. 
       
    
    
     DESCRIPTION 
       [0036]    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. 
         [0037]      FIG. 3  depicts a solid state lithium-sulfur cell  100  with improved cycle life and energy density. The cell  100  includes a negative electrode  102 , a separator  104 , and a positive electrode  106 . The negative electrode  102  contains Li metal or some other Li-insertion material that can reversibly insert and extract Li ions electrochemically. The negative electrode  102  in some embodiments includes a current collector (not shown), such as copper metal. In the embodiment of  FIG. 3 , the negative electrode  102  uses Li metal to conduct electrons to and from the electrode. 
         [0038]    The separator  104  is electronically insulating, and in some embodiments includes a Li-conducting solid electrolyte with one or more components, that may include LiPON, Li-conducting garnet, Li-conducting sulfide (e.g., Li 2 S—P 2 S 5 ), Li-conducting polymer (e.g., polyethylene oxide), Li-conducting metal-organic frameworks (e.g., 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), Li 3 N, Li 3 P, thio-LISiCONs, Li-conducting NaSICONs, Li 10 GeP 2 S 12 , lithium polysulfidophosphates, or other solid Li-conducting material. 
         [0039]    The positive electrode  106  contains a coated form of sulfur and/or lithium sulfide (Li 2 S). In some embodiments, the positive electrode  106  includes additional Li-insertion materials, an electronically conducting material (e.g., carbon fragments, graphite, and/or carbon black), and a solid-electrolyte Li-conducting phase (including those materials that could be used for the separator). The positive electrode  106  is connected to an electronically-conductive current collector  108  (e.g., Al metal). 
         [0040]    The cell  100  described above (also referred to as a “cell sandwich”) is double sided in some embodiments. In other words, the cell  100  is symmetric about the Al current collector as follows: negative electrode/separator/positive electrode/positive current collector/positive electrode/separator/negative electrode/(negative current collector) and stacked (as a “stack”) or wound (as a “jellyroll”) so as to increase the capacity per unit volume. In some embodiments, the cell  100  is stacked in a bipolar design (negative electrode/separator/positive electrode/positive current collector/bipolar plate/negative electrode . . . ) so as to increase the cell voltage. This electrochemically active volume is enclosed in a cell housing (not shown), the terminals of which are wired to the negative and positive poles of the stack or jellyroll. 
         [0041]    The Li 2 S in the positive electrode  106  may be obtained using commercially available Li 2 S that is milled into small grains (about 100 nm to 10 um diameter), or it may be obtained from synthesized nano-Li 2 S (&lt;1 μm diameter). The Li 2 S in the positive electrode  106  is coated with a material that prevents it from reacting with the electrolyte, even after Li is extracted electrochemically from the Li 2 S. The coating material is preferably a mixed electronic and Li+ conductor, such as carbon or Ti 2 S, but it may also be an insulator such as Al 2 O 3 . 
         [0042]    The coating is preferably very thin such that it comprises no more than 20% of the mass of the coated Li 2 S. The Li 2 S particles may be any shape, but are preferably spherical. The coating may be accomplished via a chemical vapor deposition (CVD), atomic layer deposition (ALD), physical vapor deposition (PVD) or other coating process. When carbon is the coating, it is preferably deposited on the surface of the Li 2 S particles by a CVD method developed by Prof. Elton Cairns of the University of California, Berkeley (UC Berkeley Case no. B14-011) using gaseous C 2 H 2  as a precursor. The CVD is carried out at approximately 400° C. under slowly-flowing Ar. The coating procedure may be carried out several times in order to ensure complete coverage of the particles. 
         [0043]    In one embodiment, the nano-Li 2 S is synthesized using a method developed by Prof. Elton Cairns of the University of California, Berkeley (UC Berkeley Case no. B14-011). In this method, a solution-based reaction of elemental sulfur with 1M Li(CH 2 CH 3 ) 3 BH solution in THF is used. Following the preparation of nano-Li 2 S, the carbon coating is deposited on the surface of the particles using CVD. The nano-Li 2 S is fully covered by a thin layer of carbon, which enhances conductivity and prevents a polysulfide shuttle process. The gaseous precursor for carbon is C 2 H 2 . The temperature used for CVD is 400° C., and the atmosphere is slowly-flowing Ar. The coating procedure can be repeated 3-4 times to achieve full coverage of carbon on the Li 2 S particles. 
         [0044]    Sometimes the above described methods for synthesizing coated nano-Li 2 S result in a mixture of completely coated (i.e., pinhole-free) Li 2 S and some uncoated and/or partially coated Li 2 S. The presence of the latter components (jointly referred to herein as “uncoated” Li 2 S particles) would limit the cycle life of the Li/S cell  100 . Specifically, the uncoated Li 2 S is available to react with the electrolyte or dissolve into the electrolyte. Subsequently, this dissolved sulfide may form a dissolved lithium polysulfide (Li 2 S x , 1&lt;x≦8) that reacts with the negative electrode, resulting in loss of active sulfur and therefore a reduction in the capacity of the cell. Hence, when one of the foregoing methods is used which results in both coated and uncoated Li2S, the uncoated Li 2 S is separated from the well-coated Li 2 S, and only the well-coated Li 2 S is used in fabricating the positive electrode  106  of the cell  100 . 
         [0045]    In one embodiment, the coated Li 2 S is obtained from a mixture of coated and uncoated Li 2 S by dissolving the uncoated Li 2 S. Specifically, when uncoated Li 2 S is exposed to a solution containing an organic solvent (e.g., DOL) and dissolved sulfur, the sulfur reacts with the Li 2 S to form soluble lithium polysulfides. The introduction of the dissolved polysulfide species results in an intense change in color of the solution. 
         [0046]    Accordingly, in one embodiment, a mixture of coated and uncoated Li 2 S is mixed with a sulfur solution. After allowing time for dissolution of any uncoated Li 2 S, the remaining Li 2 S particles are collected from the solution and washed with organic solvent that does not contain sulfur. The washed Li 2 S particles are predominantly coated Li 2 S particles. Some uncoated Li 2 S particles, however, may still remain. Accordingly, the process is repeated several times until there is no color change observed upon mixing the Li 2 S particles with the sulfur solution, indicating that all of the imperfectly coated Li 2 S has been dissolved and removed. 
         [0047]    The above described process may be modified so as to continuously flow a sulfur solution over a mixture of coated and uncoated Li 2 S particles until there is no color change observed. Moreover, rather than visually observing the color change of the solution, a chemical sensor may be used to determine when the solution no longer contains polysulfide species. 
         [0048]    In some embodiments, the dissolved lithium polysulfide is electrochemically oxidized to form sulfur, which in turn can be recycled and used again in the Li2S purification process. A byproduct of this part of the process can include the plating of Li metal. 
         [0049]    Once the uncoated Li 2 S particles have been dissolved, the remaining coated Li 2 S particles are rinsed in organic solvent that does not contain sulfur. Afterwards the coated Li 2 S particles are dried, possibly at elevated temperature (80 to 120° C. for up to 12 hours), in order to remove any residual solvent. The process results in a collection of well-coated Li 2 S and potentially some fragments of coating material that no longer contain Li 2 S. 
         [0050]    In one embodiment, the positive electrode  106  is fabricated directly from this collection of coated Li 2 S and coating material fragments. For example, in embodiments wherein the coating material is a mixed electronic and ionic conductor, the presence of the coating material fragments can enhance the power capability of the composite electrode. 
         [0051]    In another embodiment, the fragments are separated from the coated Li 2 S in a second process. For example, by sedimentation in an inert liquid or by centrifugation. Both of these methods operate on the principle that the coated Li 2 S has a much higher density than the coating material fragments, many of which may be partially incomplete hollow spheres. 
         [0052]    Further process steps for completing the manufacture of the positive electrode  106  preferably follow a standard method for fabricating battery electrodes. Thus, in some embodiments the coated Li 2 S is mixed with conductive additives, binder, and optionally solid electrolyte powders (e.g., lithium-conducting garnet or lithium-conducting sulfide glasses or ceramics) in a carrier solvent. The mixture forms a slurry that can be coated onto the positive current collector. The electrode is then heated so that the solvent evaporates. The electrode may then be densified using a roll press. 
         [0053]    In another embodiment, toluene is used to dissolve the uncoated Li 2 S particles. 
         [0054]    The use of toluene to dissolve Li 2 S in TiO 2  particles is discussed by Seh et al., “Sulphur-TiO 2  yolk-shell nanoarchitecture with internal void space for long-cycle lithium-sulphur batteries,” Nature Communications 4, Article number: 1331 doi:10.1038/ncomms2327, the entire contents of which are herein incorporated by reference. In the Seh article, the toluene is transported to the core through porous TiO 2  where the sulfur is dissolved. The present process simply introduces the toluene into the mixture of coated and uncoated Li 2 S where the uncoated Li 2 S is dissolved while the coated Li 2 S is not. The remaining coated Li 2 S particles are then rinsed in an inert organic solvent and dried, and the electrode fabrication procedure outlined in the embodiment above is subsequently followed. 
         [0055]    While the embodiment of  FIG. 3  is a solid state embodiment, the disclosure is further applicable to embodiments including a liquid electrolyte. By way of example,  FIG. 4  depicts a lithium-sulfur cell  140  with improved cycle life and energy density. The cell  140  includes a negative electrode  142 , a separator  144 , and a positive electrode  146 . The negative electrode  142  contains Li metal or some other Li-insertion material that can reversibly insert and extract Li ions electrochemically. The negative electrode  142  in some embodiments includes a current collector (not shown), such as copper metal. In the embodiment of  FIG. 4 , the negative electrode  142  uses Li metal to conduct electrons to and from the electrode. 
         [0056]    The separator  144  is electronically insulating, and in some embodiments includes a Li-conducting solid electrolyte and/or a porous material with Li-conducting liquid electrolyte in the pores. 
         [0057]    The positive electrode  146  contains a coated form of sulfur and/or lithium sulfide (Li 2 S). In some embodiments, the positive electrode  146  includes additional Li-insertion materials, an electronically conducting material (e.g., carbon fragments, graphite, and/or carbon black), and a Li-conducting phase (e.g., liquid electrolyte and/or solid electrolyte). The positive electrode  146  is connected to an electronically-conductive current collector  148  (e.g., Al metal). 
         [0058]    The cell  140  is constructed of similar materials and in substantially the same manner as the cell  100 . Thus, the Li 2 S in the positive electrode  146  is provided and coated in the same manner as described above with respect to the cell  100 . Some modifications are implicated, however, since a liquid electrolyte is included in the cell  140 . For example, the coating of the Li 2 S in the positive electrode  146  must be configured to prevent the Li 2 S from dissolving into the electrolyte even after Li is extracted electrochemically from the Li 2 S. In some embodiments, the coating material may be the same as the solid state cell. The coating material may thus be a mixed electronic and Li+ conductor, such as carbon or Ti 2 S, but it may also be an insulator such as Al 2 O 3 . 
         [0059]    One other difference is that during densification described above with respect to cell  100 , some pores are left in the electrode such that they can be filled with liquid electrolyte during cell fabrication of the cell  140 . 
         [0060]    The above described embodiments thus provide a lithium-sulfur cell with improved cycle life and energy density which in some embodiments is a solid state cell. The embodiments avoid reaction of Li 2 S, S, and lithium polysulfides (Li 2 S x , 1&lt;x&lt;=8) with the electrolyte. By providing a conductive coating and nanosized Li 2 S particles the rate capability of the cell is increased. The disclosed embodiments provide for increased sulfur loading and utilization of a cell while providing increased safety by avoiding the use of a volatile organic electrolyte. Additionally, the charging time of the cell is decreased while increasing the power delivered from the cell. 
         [0061]    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.