Patent Publication Number: US-2016248084-A1

Title: Durable carbon-coated li2s core-shell materials for high performance lithium/sulfur cells

Description:
STATEMENT OF GOVERNMENT SUPPORT 
     The invention described and claimed herein was made in part utilizing funds supplied by the U.S. Department of Energy under Contract No. DE-ACO2-05CH11231 between the U.S. Department of Energy and the Regents of the University of California for the management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention. 
    
    
     CROSS REFERENCE TO RELATED APPLICATIONS 
     Not Applicable 
     TECHNICAL FIELD 
     The disclosure provides methods for producing uniformly sized lithium sulfide materials which are coated with one or more durable and conductive carbon shells that impede the polysulfide shuttle. The disclosure further provides for the carbon coated lithium sulfide materials made therefrom, and the use of these materials in lithium sulfide batteries. 
     BACKGROUND 
     Lithium sulfide (Li 2 S) is an attractive cathode material with a high theoretical specific capacity (1166 mAh g-1). However, the poor cycle life and rate capability have remained significant challenges, preventing its practical application. 
     SUMMARY 
     Disclosed herein are methods to synthesize Li 2 S materials. Based upon the reaction conditions, the resulting size of the Li 2 S materials can be controlled. Further disclosed herein, are methods (e.g., a chemical vapor deposition (CVD)) for carbon-coating the Li 2 S materials with a stable, durable conductive carbon layer to form carbon-coated Li 2 S materials (Li 2 S@C). The Li 2 S@C materials of the disclosure provided specific capacities and cycling performance with a high initial discharge capacity of at least 972 mAh g −1  Li 2 S (1394 mAh g −1 S) at a 0.2C rate. Further, electrodes composed of 98 wt % of 1 μm Li 2 S@C material and 2 wt % binder, demonstrated stable cycling performance with little morphology change after 400 cycles at a 0.5C rate. 
     In a particular embodiment, the disclosure provides a method to synthesize a Li 2 S core material having a uniform size, comprising: adding a first solution comprising elemental sulfur in a nonpolar organic solvent to a second solution comprising a strong lithium based reducing agent to make a reaction mixture; and precipitating the Li 2 S core material from the reaction mixture by heating the reaction mixture at an elevated temperature (e.g., about 90° C.) for 2 to 30 minutes (e.g., 7 minutes or 10 minutes). In a further embodiment, the method further comprises: collecting the precipitated Li 2 S core material from the reaction mixture; washing the Li 2 S core material; and drying the Li 2 S core material. 
     In another embodiment, the disclosure provides methods to synthesize a Li 2 S core material comprising using a nonpolar organic solvent selected from pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, chloroform, 1,4-dioxane and diethyl ether. In yet another embodiment, the disclosure provides methods to synthesize a Li 2 S core material comprising using a strong lithium based reducing agent is selected from lithium triethylborohydride, n-butyl-lithium, and lithium aluminum hydride. In a further embodiment, the disclosure provides methods to synthesize a Li 2 S core material comprising using 1.0 M lithium triethylborohydride in 4.2 mL of tetrahydrofuran. In yet a further embodiment, the disclosure provides methods to synthesize a Li 2 S core material comprising dissolving 64 mg of sulfur in 6 mL of the nonpolar organic solvent, and further adding a portion of this solution (e.g., 3 mL, 3.5 mL, or 6 mL) to a solution comprising a strong lithium based reducing agent. 
     In another embodiment, the disclosure provides that the Li 2 S core material is substantially spherical in shape. In yet another embodiment, the Li 2 S core material is uniformly size particles from 500 nm to 2 μm in diameter (e.g., 1 μm in diameter). 
     In a particular embodiment, the disclosure provides for uniformly sized Li 2 S core material core material made by the methods disclosed herein. 
     In a further embodiment, the disclosure also provides a method for coating a Li 2 S core material core material with one or more durable and conductive carbon shells comprising: placing the uniformly sized Li 2 S core material under an atmosphere comprising a carbon based precursor compound and an inert gas; and pyrolyzing the precursor compound by heating at an elevated temperature for a period between 15 minutes to 3 hours. In a further embodiment, the carbon based precursor compound is selected from methane, ethylene, acetylene, benzene, xylene, carbon monoxide, or combinations thereof. In yet a further embodiment, the inert gas is argon. In another embodiment, the inert gas to the carbon based precursor compound is introduced at a Standard Cubic Centimeters per Minute (SCCM) flow rate ratio of 1:10 to 10:1. In yet another embodiment, the carbon based precursor compound is pyrolyzed by heating at a temperature between 400° C. to 700° C. In a particular embodiment, the carbon based precursor compound is pyrolyzed by heating at 450° C. for 1.5 hours. 
     In a certain embodiment, the disclosure provides for a Li 2 S core material comprising one or more carbon shells (Li 2 S@C material) made by any one of the methods disclosed herein. In a further embodiment, an electrode comprises the Li 2 S@C material disclosed herein. In yet a further embodiment, a lithium/sulfur battery comprises an electrode which comprises a Li 2 S@C material of the disclosure. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1A-B  provides SEM images of the Li 2 S spheres synthesized under different conditions. (A) Synthesis of Li 2 S with toluene, but no heat treatment; and (B) Synthesis of Li 2 S without toluene. 
         FIG. 2A-F  provides scanning electron microscope (SEM) images of the Li 2 S spheres before and after CVD coating. (A) 2 μm Li 2 S; (B) 1 μm Li 2 S; (C) 500 nm Li 2 S; (D) 2 μm Li 2 S@C; (E) 1 μm Li 2 S@C; and (F) 500 nm Li 2 S@C. 
         FIG. 3A-D  provides X-ray diffraction (XRD) and Raman characterization of as-synthesized Li 2 S spheres. (A) XRD patterns and (B) Raman spectrum of Li 2 S spheres, Li 2 S spheres treated at 500° C. and Li 2 S@C by further coated Li 2 S-500° C. sample with 10 wt % carbon from CVD method; (C) Magnification of the Raman spectrum between 300 cm −1  and 450 cm −1 . (D) Schematic of the coating process for the Li 2 S@C spheres. 
         FIG. 4A-B  presents electron energy loss spectroscopy (EELS) characterization of the Li 2 S@C sphere. (A) STEM image of a Li 2 S@C sphere and (B) normalized EELS intensity along the line in (A). 
         FIG. 5  provides SEM images of Li 2 S@C spheres after a polysulfide dissolution test. 
         FIG. 6A-B  presents cycle performance and cyclic voltammogram of the Li 2 S@C materials having different sizes. (A) Cycle performances of the 2 μm, 1 μm and 500 nm Li 2 S@C particles at 0.5C rates. (B) Cyclic voltammograms of the as-prepared Li 2 S@C cathodes at a scan rate of 0.025 mV/s. (Electrode composition: Li 2 S@C:C:PVP=66.7:28.3:5 by weight; 60 wt % Li 2 S, 1.0-1.5 mg Li 2 S/cm 2 ) 
         FIG. 7A-B  provides SEM images and characterization of commercial Li 2 S. (A) SEM image of commercial Li 2 S; and (B) its cycling performance at the 0.5C rate. (Electrode composition: Li 2 S:C:PVP=60:35:5 by weight; 1.2 mg Li 2 S/cm 2 ). 
         FIG. 8  provides a curve of the current versus potential voltage of commercial Li 2 S (c-Li 2 S) materials before and after ball-milling at a scan rate of 0.025 mV/s. (Electrode composition: Li 2 S:C:PVP=60:35:5 by weight; 1.0-1.5 mg Li 2 S/cm 2 ). 
         FIG. 9A-C  presents the electrochemical performance of the 1 μm Li 2 S@C spheres at different C-rates. (A) Cycling performances and coulombic efficiency of the 1 μm Li 2 S @C cathodes at 0.2C, 0.5C and 1C rates. (B) Rate capability of the 1 μm Li 2 S@C cathodes. (C) Voltage profile of the 1 μm Li 2 S@C spheres at different C-rates. (Electrode composition: Li 2 S@C:C:PVP=66.7:28.3:5 by weight; 60 wt % Li 2 S, 1.0-1.5 mg Li 2 S/cm 2 ) 
         FIG. 10A-B  provides cycling performance of a Li 2 S@C-based electrode and SEM images of the electrode. (A) Cycling performance of the 1 μm Li 2 S @C electrode with no added carbon and 98 wt % Li 2 S@C spheres at 0.5C. (B) SEM images of the electrode after 400 cycles. (Electrode composition: Li 2 S@C:SBR=98:2 by weight; 88 wt % Li 2 S, 1.1 mg Li 2 S/cm 2 ) 
     
    
    
     DETAILED DESCRIPTION 
     As used herein and in the appended claims, the singular forms “a,” “and,” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a Li 2 S material” includes a plurality of such materials and reference to “the carbon coating” includes reference to one or more carbon coatings and equivalents thereof known to those skilled in the art, and so forth. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art. Although many methods and reagents similar or equivalent to those described herein, the exemplary methods and materials are presented herein. 
     As current cathode materials for lithium ion cells approach an intrinsic specific capacity limit of less than 300 mAh g −1 , new generations of rechargeable cathodes with high capability are needed. Sulfur is a promising candidate which has a high theoretical specific capacity of 1675 mAh g −1  as well as being low cost, abundant and environmentally benign. Significant progress has been made to improve the utilization of sulfur and alleviate the capacity fading by size control of the sulfur particles, coatings on sulfur particles, use of sulfur-carbon composites, trapping of polysulfides and electrolyte modification. However, for use with the sulfur cathode, lithium metal or a light-weight lithiated anode is important for high specific energy. Lithium metal typically forms dendrites in conventional organic solvent-based electrolytes, causing safety concerns. Furthermore, some of the protective coatings on sulfur particles may be easily destroyed because sulfur undergoes a volume expansion of ˜76% when totally converted to Li 2 S. 
     Considering the challenges for the sulfur cathode, fully lithiated sulfur, lithium sulfide (Li 2 S), is an attractive cathode material for lithium/sulfur (Li/S) cells, with a theoretical specific capacity of 1166 mAh g −1 . It can be paired with different kinds of lithium metal free materials, such as the high capacity silicon anode. Moreover, compared with sulfur, Li 2 S has a higher melting point and is in the maximum volume state, so modifications on Li 2 S materials can be performed at a higher temperature and the surface coating can be more stable. Nevertheless, the problems of low electronic conductivity, and the solubility of polysulfides in many electrolytes still exist for Li 2 S cathodes. Thus, the use carbon-containing composites, controlling particle size and providing protection for the Li 2 S active materials are important considerations to be taken into account. A protective and conductive shell on the surface of small Li 2 S particles, which is not broken by expansion during discharge, provides tangible advantages for the Li 2 S materials disclosed herein in Li/S cells. Commercial Li 2 S powder or commercial Li 2 S is subjected to ball-milling, and because of which, the particle size and morphology are random and may not be at an ideal configuration for surface modifications. 
     Any number of methods known in the art may be used to produce the Li 2 S core material disclosure herein. In a particular embodiment, a Li 2 S core material of the disclosure can be prepared by a solution-based reaction of elemental sulfur with a strong lithium based reducing agent such as, superhydride (i.e., Li(CH 2 CH 3 ) 3 BH), n-butyl-lithium, or lithium aluminum hydride, in any number of different weak acids (e.g., formic acid, acetic acid, and nitrous acid) and collecting the precipitate. In a preferred embodiment, the strong lithium based reducing agent is Li(CH 2 CH 3 ) 3 )BH. In a certain embodiment, a method of synthesizing a Li 2 S material comprises: dissolving elemental sulfur in a nonpolar organic solvent (e.g., toluene) to form a sulfur containing solution; adding the sulfur containing solution to a solution comprising a strong lithium based reducing agent so as to form a reaction mixture; and precipitating the Li 2 S materials from the reaction mixture by heating the mixture at an elevated temperature (e.g., 90° C.) for 2-30 minutes. In a particular embodiment, the reaction is heated at 90° C. for up to 2 minutes, for up to 3 minutes, for up to 4 minutes, for up to 5 minutes, for up to 6 minutes, for up to 7 minutes, for up to 8 minutes, for up to 9 minutes, for up to 10 minutes, for up to 11 minutes, for up to 12 minutes, for up to 13 minutes, for up to 14 minutes or for up to 15 minutes. In another embodiment, the nonpolar organic solvent is selected from pentane, cyclopentane, hexane, cyclohexane, benzene, toluene, chloroform, 1,4-dioxane and diethyl ether. In a preferred embodiment, the nonpolar organic solvent is toluene. In a further embodiment, the method further comprises collecting the precipitated Li 2 S core material and washing the precipitated Li 2 S core material. 
     In another embodiment, the chemical reaction for the synthesis of Li 2 S is indicated below. 
       S+2LiEt 3 BH=Li 2 S+2Et 3 B+H 2    
     Sulfur was first dissolved in toluene and then added into a solution of lithium triethylborohydride (LiEt 3 BH) in tetrahydrofuran (THF). After the THF was removed by heat treatment, stable Li 2 S spheres were formed. For example, the S-toluene solution was then added to 1.0 M lithium triethylborohydride in tetrahydrofuran (4.2 mL). After stirring for 2 min at ambient temperature, the solution was heated to 90° C. for 10 minutes under continuous stirring. 
     When synthesizing these Li 2 S spheres, both the heat treatment and the use of toluene played very important roles. The particles assume a spherical shape due to the combination of small droplet size of the liquid dispersion, the elevated temperature, and the limited solubility of Li 2 S. We have observed that, without the elevated temperature, spherical particles are not formed. Toluene dissolves S and thus a liquid droplet dispersion is formed. According to our observations, THF has a higher solubility for Li 2 S than does toluene, so when the heat treatment is applied, the THF slowly evaporates and stable Li 2 S spheres are formed in toluene. The sizes of the Li 2 S spheres were related to the amount of toluene used in the preparation. With the same amount of dissolved S (2 mmol), the largest amount of toluene (6 mL) gives the largest spherical Li 2 S size of 2 μm, while the 3.5 and 3 mL toluene amounts form 1 μm and 500 nm Li 2 S spheres respectively. When the obtained Li 2 S spheres are further converted to Li 2 S@C particles by CVD coating, volume shrinkage and roughening of the surface can be observed for these Li 2 S spheres, as shown in  FIG. 2 d - f   . For the 2 and 1 μm Li 2 S spheres, their morphologies do not change much after carbon coating. But for the 500 nm Li 2 S spheres, many Li 2 S particles are obviously linked together, showing some agglomeration after heat treatment. 
     The syntheses methods disclosed herein may be modified to produce particles having different sizes by adjusting the reaction times and/or increasing or decreasing the amount of sulfur added to the reaction mixture. For example, small size particles can be generated by using shorter reaction times and decreasing the amount of sulfur added to the reaction mixture; and vice versa, larger particles can be generated by using longer reaction times, and increasing the amount of sulfur added to the reaction mixture. In a certain embodiment, the disclosure provides one or more methods to synthesize the Li 2 S core material having a uniform particulate size. In a further embodiment, the Li 2 S core material is substantially spherical and/or substantially ovoid in shape. In another embodiment, the synthesis methods disclosed herein can generate uniformly size Li 2 S particles from 250 nm to 3 μm, from 500 nm to 2 μm, from 1 μm to 2 μm in size. In yet another embodiment, the synthesis methods disclosed herein can generate uniformly size Li 2 S particles greater than 500 nm, greater than 600 nm, greater than 700 nm, greater than 800 nm, greater than 900 nm, greater than 1 μm, greater than 1.2 μm, greater than 1.4 μm, greater than 1.6 μm, greater than 1.8 μm, greater than 2 μm, greater than 2.2 μm, greater than 2.4 μm, greater than 2.6 μm, or greater than 2.8 μm in size. In a further embodiment, the synthesis methods disclosed herein can generate uniformly size Li 2 S particles that are 200 nm, 300 nm, 400 nm, 500 nm, 600 nm 700 nm, 800 nm, 900 nm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, or 3.0 μm in size. 
     The disclosure further provides that the Li 2 S core material may be coated with one or more shells of carbon. In a particular embodiment, the carbon shells increase the conductivity of the Li 2 S core material in comparison to Li 2 S core material alone. The carbon coating may be applied so that the coating uniformly coats the Li 2 S core material or alternatively the coating is applied so that the coating does not uniformly coat the Li 2 S core material (i.e., portions in which the carbon shells are thicker and portions in which the carbon shells are thinner). Alternatively, the carbon shells can be patterned on the Li 2 S core material, such as by using lithography based methods. In a further embodiment, the carbon shell material is selected from carbon black, acetylene black carbon, pyrolytic carbon, pyrolytic graphene, or polyaniline polysulfide (SPan). 
     The term “carbon based precursor compound” as used herein refers to a saturated or unsaturated C 1  to C 20  compound that may be optionally substituted. 
     A carbon shell can be applied to the Li 2 S core material by using various techniques. For example, in one embodiment, a carbon-based coating can be applied to the Li 2 S core material by using a chemical vapor deposition process. In an alternate embodiment, a carbon-based coating can be applied to the Li 2 S core material by using a carbonization process. For example, Li 2 S core material can be carbon coated by preparing a mixture comprising a conductive carbon-based polymer, applying the mixture to the Li 2 S core material, and then carbonizing the carbon-based polymer by pyrolysis. The pyrolysis of the carbon based precursor compound is typically carried out in a non-oxygen environment and typically under a stream of inert gas such as, for example, Argon. 
     Accordingly, in a certain embodiment, a carbonization process is used to coat carbon on the Li 2 S core material by pyrolyzing a carbon based precursor compound. During the pyrolysis step, chemical and physical rearrangements occur, often with the emission of residual solvent and byproduct species, which can then be removed. As used in the disclosure, the term “carbon coating by carbonization” means that the carbon coating is generated from pyrolysis of a suitable carbon based precursor compound to amorphous pyrolytic carbon or pyrolytic graphene. A suitable precursor carbon compound (e.g., carbon based polymer) can be applied to the Li 2 S core material by any number of methods known in the art. For example, the Li 2 S core material can be immersed or soaked in a mixture, solution, or suspension comprising the carbon based precursor compound. Alternatively, a mixture, solution, or suspension comprising the carbon based precursor compound can be applied to the Li 2 S core material by spraying, dispensing, spin coating, depositing, printing, etc. The carbon based precursor compound can then be carbonized by heating the precursor compound at a suitable temperature and for a suitable time period so that the carbon based precursor compound undergoes thermal decomposition to carbon. 
     A carbon based first coating produced by carbonization can result from pyrolyzing a carbon based precursor compound at temperatures of about 300 to 800° C. in a reaction vessel, for example a crucible. Typically, pyrolyzation of the carbon based precursor compound is conducted at a temperature of at least 200° C. and up to 700° C. for a time period of up to 48 hours, wherein, generally, higher temperatures require shorter processing times to achieve the same effect. In a certain embodiment, carbonization of the carbon based precursor compound is conducted by pyrolyzing the at a temperature of at least 425° C. and up to 600° C. for a time period of up to 48 hours. In different embodiments, the temperature employed in the pyrolysis step is 200° C., 250° C., 300° C., 350° C., 400° C., 450° C., 500° C., 550° C., 600° C., 650° C., or 700° C., or within a temperature range bounded by any two of the foregoing exemplary values. For any of these temperatures, or a range therein, the processing time (i.e., time the carbon based precursor compound is processed at a temperature or within a temperature range) can be, for example, precisely, at least, or no more than 15 minutes, 30 minutes, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours, 12 hours, 18 hours, 24 hours, 30 hours, 36 hours, 42 hours, 48 hours, or within a time range bounded by any two of the foregoing exemplary values. Carbonization to produce carbon shells may include multiple, repeated steps of pyrolysis with the carbon based precursor compound. Between each step, aggregates can be milled to substantial homogeneity, followed by further pyrolysis with additional carbon based precursor compound. The thickness of the carbon shells can be modulated by any number of means, including (i) using repeated pyrolysis steps with carbon based precursor compound, (ii) increasing the amount of carbon based precursor applied to the Li 2 S core material, and (iii) the type of carbon based precursor compound. The amount of carbon deposited as a carbon shell may be determined by a measuring a change in weight before and after applying the coating to the Li 2 S core material. 
     Typical carbon based precursor compounds that can be used in the carbonization methods disclosed herein, includes carbon based polymers. For example, inexpensive carbon based polymers such as polystyrene (PS), polyacrylonitrile (PAN), and polymetylmetacrylate (PMMA) can be used. Carbon based polymers, unlike explosive gaseous raw carbon sources, are safe to handle and relatively inexpensive. 
     In a further embodiment, the pyrolysis step described above can be followed by a higher temperature sintering step to further encourage formation of a graphene product. In some embodiments, the sintering step is employed primarily to induce further crystallization, particularly when the product resulting from pyrolysis is found to retain an amorphous portion. Typically, the sintering step is conducted at a temperature greater than 700° C. and up to 1350° C. for a time of up to 48 hours, wherein, generally, higher temperatures require shorter processing times to achieve the same effect. In different embodiments, the temperature employed in the sintering step is up to 750° C., up to 800° C., up to 850° C., up to 900° C., up to 950° C., up to 1000° C., up to 1050° C., up to 1100° C., up to 1150° C., up to 1200° C., up to 1250° C., up to 1300° C., or up to 1350° C., or within a temperature range bounded by any two of the foregoing exemplary values. For any of these temperatures, or a range therein, the processing time can be, for example, any of the exemplary processing times or time ranges provided herein. 
     In a preferred embodiment, carbon shells can be produced by using a chemical vapor deposition (CVD) process. CVD is a chemical process used to produce high-purity, high-performance solid materials. For the Li 2 S@C materials disclosed herein, a carbon coating can be deposited onto a Li 2 S core material by placing the Li 2 S core material under an atmosphere comprising a carbon based precursor compound, such as acetylene, and heating at a temperature so as to pyrolyze the precursor compound. In a certain embodiment, the temperature employed in the CVD step is up to 400° C., up to 450° C., up to 500° C., up to 550° C., up to 600° C., up to 650° C., or up to 700° C., or within a temperature range bounded by any two of the foregoing exemplary values. In a further embodiment, the CVD step is performed over a period of 15 minutes, 30 minutes, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, or within a temperature range bounded by any two of the foregoing exemplary values. In a particular embodiment, a carbon coating can be deposited onto a Li 2 S core material by transferring the Li 2 S core material to a glove box and introducing an inert gas and carbon based precursor compound (e.g., a hydrocarbon) at a defined Standard Cubic Centimeters per Minute (SCCM) flow rate. In a further embodiment, the SCCM flow rate of the inert gas to carbon based precursor compound is introduced at a defined ratio. In a particular embodiment, the inert gas to the carbon based precursor compound is introduced at a SCCM flow rate ratio of 1:10 to 10:1, 1:9 to 9:1, 1:8 to 8:1, 1:7 to 7:1, 1:6 to 6:1, 1:5 to 5:1, 1:4 to 4:1, 1:3 to 3:1, or 1:2 to 2:1. For example, in a particular embodiment, Argon is introduced at 100 SCCM while acetylene is introduced at 10 SCCM resulting in a SCCM flow rate ratio of 10:1. In another embodiment, the CVD process utilizes a carbon based precursor compound selected from methane, ethylene, acetylene, benzene, xylene, carbon monoxide, or combinations thereof. Depending on the particular carbon based precursor compound, the flow rates can be adjusted to desired values using a mass flow controller. The thickness of the carbon coating can be modulated by adjusting the length of time the Li 2 S core material is exposed to the carbon based precursor compound, changing the flow rate of the carbon based precursor compound, and/or changing the deposition temperature. In order to achieve a more even carbon coating, the Li 2 S materials can be periodically removed from heat and milled to break up any agglomerations. The Li 2 S materials are then reheated with the carbon based precursor compound. The amount of carbon deposited can be determined by a change in weight of the Li 2 S materials. 
     To compensate for the poor ionic and electronic conductivity for the sulfur containing electrodes, a liquid electrolyte is conventionally employed, which has a high solubility of lithium polysulfides and sulfide. The utilization of sulfur in batteries containing liquid electrolyte depends on the solubility of these sulfur species in the liquid electrolyte. Further, the sulfur in the positive electrode, e.g., cathode, except at the full charge state, is generally present as a solution of polysulfides in the electrolyte. The concentration of polysulfide species S n   2−  with n greater than 4 at the positive electrode is generally higher than that at the negative electrode, e.g., anode, and the concentration of S n   2−  with n smaller than 4 is generally higher at the negative electrode than the positive electrode. The concentration gradients of the polysulfide species drive the intrinsic polysulfide shuttle between the electrodes. Polysulfide shuttle transports sulfur species back and forth between the two electrodes, in which the sulfur species may be migrating within the battery all the time. Polysulfide shuttle leads to poor cyclability, high current leakage, and low charge-discharge efficiency. Further, a portion of the polysulfide is transformed into lithium sulfide (Li 2 S), which is deposited on the negative electrode. The deposited polysulfide may cause a “chemical short” of the battery. The “chemical short” leads to the loss of active materials, corrosion of the lithium containing negative electrode, i.e., anode, and a low columbic efficiency. Further, the mobile sulfur species causes the redistribution of sulfur in the battery and imposes a poor cycle-life for the battery, in which the poor cycle life directly relates to micro-structural changes of the electrodes. This deposition process occurs in each charge/discharge cycle, and eventually leads to the complete loss of capacity of the sulfur positive electrode. The deposition of lithium sulfide also leads to an increase of internal cell resistance within the battery due to the insulating nature of lithium sulfide. Progressive increases in charging voltage and decreases in discharge voltage are common phenomena in lithium/sulfide (Li/S) batteries, because of the increase of cell resistance in consecutive cycles. Hence, the energy efficiency decreases with the increase of cycle number. It was found that the carbon shells of the Li 2 S@C materials disclosed herein were not only durable but inhibited the polysulfide shuttle. 
     The disclosure further provides that the Li 2 S@C materials disclosed herein or the composites made thereof can be used in a variety of applications, including for use in Li/S batteries. In comparison to conventional Li/S cells the Li/S cells comprising the Li 2 S@C materials have higher energy densities, lower material costs, and better cycling performance. Accordingly, Li/S cells comprising the Li 2 S@C materials could be used in high performance batteries in vehicles, electronic devices, electronic grids and the like. In a particular embodiment, a battery comprises the Li 2 S@C materials disclosed herein or the composites made thereof. In a further embodiment, the battery is a rechargeable Li/S battery. In yet a further embodiment, the battery comprising the Li 2 S@C materials disclosed herein or the composites made thereof is used in consumer electronics, electric vehicles, or aerospace applications. 
     The results provided herein demonstrate that the Li 2 S @C core-shell particles deliver both high specific capacity and stable cycling performance. The following examples are intended to illustrate but not limit the disclosure. While they are typical of those that might be used, other procedures known to those skilled in the art may alternatively be used. 
     Examples 
     Materials 
     Sulfur was purchased from International Sulphur Inc. All other materials were purchased from Sigma-Aldrich Co. All materials were of the highest grade possible. 
     Synthesis of the Li/S Materials. 
     As Li 2 S is highly sensitive to moisture, all of the synthesis work was carried out in an argon filled glove box with a moisture content below 0.1 ppm and oxygen levels below 5 ppm. The 2 μm Li 2 S spheres were prepared as follows: Sulfur (64 mg) was dissolved in toluene (6 mL) under stirring for 5 min at 40° C. to afford an S-toluene solution. The S-toluene solution was then added to 1.0 M lithium triethylborohydride in tetrahydrofuran (4.2 mL). After stirring for 2 min at ambient temperature, the solution was heated to 90° C. for 10 minutes under continuous stirring. The Li 2 S precipitate that formed was washed with tetrahydrofuran and hexane. The 1 μm Li 2 S and 500 nm-Li 2 S spheres were synthesized by the same process except a shorter reaction time of 7 minutes, and a different amount of toluene, 3.5 mL and 3 mL respectively were used. The chemical reaction for the synthesis of Li 2 S is indicated in Equation 1: 
       S+2LiEt 3 BH=Li 2 S+2Et 3 B+H 2   (1)
 
     Carbon Coating the Li/S Materials. 
     To obtain the Li 2 S @C core-shell particles with a 10 wt % carbon coating, the as-prepared Li 2 S spheres were heated at 500° C. for 0.5 hours under Argon, and then coated with carbon by a chemical vapor deposition (CVD) method at 450° C. for 1.5 hours, using acetylene and argon gas (100 SCCM (standard cubic centimeters per minute) Ar plus 10 SCCM C 2 H 2 ). In order to obtain a uniform coating, the powder was ground with a mortar and pestle in the glovebox for 10 min after each 30-minutes of CVD coating. 
     Preparing Electrodes Comprising Li 2 S@C Material: 
     For the 60 wt % Li 2 S electrodes: Li 2 S@C powder was ground with a mortar and pestle along with carbon black (Super P) and polyvinylpyrrolidone (PVP; M w ˜1,300K) in a weight ratio of 66.7:28.3:5 (Li 2 S:C:PVP=60:35:5 by weight) for 10 min, and then dispersed in N-methyl-2-pyrrolidinone (NMP) by sonication for 5 minutes and stirring for 4 hours to form a slurry. To make working electrodes, the slurry was drop-cast onto carbon fiber paper (Hesen Electrical Ltd, HCP010N; 0.1 mm thickness, 75% porosity), dried in vacuo, and heated at 60° C. overnight. The mass loading of the electrodes was around 1.0-1.5 mg/cm 2 . 
     For the 88 wt % Li 2 S electrodes: The electrodes were made by dispersing the 1 μm Li 2 S@C powder in NMP using 5 minutes sonication and 4 hours stirring, and then drop-casting the slurry onto a carbon paper current collector. In order to avoid powder dropping through the pores of the carbon paper, 20 uL of SBR-Toluene (Li 2 S@C:SBR=98:2 w/w) was added to both sides of the carbon paper. Considering the 10 wt % carbon coating on the 1 μm Li 2 S@C particles, the total content of Li 2 S in the cathodes was around 88 wt %. For the electrolyte, 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in N-methyl-N-butylpyrrolidinium bis(trifluoromethane sulfonyl)imide (PYR14TFSI)/DOL/DME (2:1:1, by volume) containing 1 wt % LiNO 3  was prepared. CR2032-type coin cells were fabricated with a porous polypropylene separator (2400, Celgard) and a lithium metal foil as counter/reference electrode in an Ar glove box. 
     Physical Characterization of the Li 2 S and Li 2 S@C Materials: 
     X-ray diffraction (XRD) patterns were collected on a Brucker XRD instrument (D2 Phaser) after the samples were protected by a Kapton film (Chemplex Industries, Inc.). Raman spectroscopy (Labram, ISA Groupe Horiba) was performed after the samples were sealed in a chamber with a glass window in a glovebox. The morphology of the samples was detected by a field emission scanning electron microscope (JEOL JSM-7500F). The EELS line scan was conducted by transmission electron microscope (TEM: 200 kV FEI monochromated F20 UT Tecnai) on a lacey silicon monoxide grid under the protection by an argon glove bag for transferring the samples. 
     Polysulfide Dissolution Test. 
     The Li 2 S and Li 2 S @C materials were tested for polysulfide dissolution according to the following: 1 mg Li 2 S@C spheres and 7 mg S were mixed in a 1.5 mL THF/toluene (2:1, v/v) solution for two weeks. The resulting precipitate was then washed with THF twice and dried for SEM. 
     Evaluating the Effect of Heat and Solvents on the Synthesis of the Li/S Materials: 
     For the synthesis of spherical Li 2 S particles, both the heat treatment and toluene played important roles. Without heat treatment, the morphology of the Li 2 S particles were not uniform and only a few spherical particles were detected (see  FIG. 1A ). When the synthesis was performed without toluene, no spherical Li2S particle was formed (see  FIG. 1B ). 
     Controlling the Size of the Li/S Materials: 
     SEM images of the three sizes of Li 2 S spheres, with typical sizes of ˜2 μm, 1 μm and 500 nm, and their corresponding Li 2 S@C core-shell particles are presented in  FIG. 2A-F . When synthesizing these Li 2 S spheres, the heat treatment and the use of toluene played very important roles (see  FIG. 1B ). The heat treatment helps to form uniform Li 2 S spheres, and spherically-shaped Li 2 S particles can be obtained only when toluene is present. More importantly, the sizes of the Li 2 S spheres correlated to the amount of toluene added (see  FIG. 2A-2C ). With less toluene, the concentration of sulfur was higher, which accelerated the nucleation speed of Li 2 S particles and thus resulted in smaller Li 2 S spheres. After converting to Li 2 S@C spheres by CVD coating, volume shrinkage and roughening of the surface was observed for these Li 2 S spheres (see  FIG. 2D-2F ). For 2 μm and 1 μm Li 2 S spheres, their morphologies don&#39;t change much. But for 500 nm Li 2 S spheres, agglomeration occurred and it was noticeable that many Li 2 S particles were linked together. 
     Physical Characterization of the Li 2 S Materials by X-Ray Diffraction (XRD) and Raman Spectroscopy. 
     XRD and Raman spectra of the as-prepared Li 2 S spheres are shown in  FIGS. 3A and 3B . The X-ray diffraction (XRD) spectrum shows that a pure Li 2 S powder was formed (JCPDS card no. 23-0369) by the reaction of sulfur with LiEt 3 BH. However, in the Raman spectrum, besides showing the characteristic peaks of Li 2 S at ˜370 cm −1 , several C—H, C—S, S—H and S—O bonds were detected between 700-1500 cm −1 , which reflect the existence of an organic residue in the powder. To remove the organics, the as-synthesized white Li 2 S spheres were heat treated in an argon atmosphere at 500° C. for 0.5 h (labeled as Li 2 S-500° C.). After the heat treatment, the white Li 2 S-spheres became a fine grey powder, and the sharp XRD peaks reveal an enhanced crystallization of the Li 2 S particles (see  FIG. 3A ). The Raman spectra show that after heat treatment all the peaks of the organic residue were gone (see  FIG. 3B ). Instead, new Raman peaks corresponding to the D band and G band of carbon were found. In addition, the intensity of the Li 2 S peak was found to be reduced. These spectra indicate that the organic residues were converted to carbon and covered some parts of the Li 2 S particles. In order to increase the conductivity of the Li 2 S powder as well as protect the Li 2 S particles from sulfur (polysulfide) loss during the electrochemical reaction, the Li 2 S-500° C. material was further coated with carbon by CVD at 450° C. for 1.5 hours using acetylene gas to form Li 2 S-10 wt % C core-shell particles (labeled as Li 2 S@C). Compared with the Li 2 S-500° C. material, no obvious change was found in the XRD patterns for the Li 2 S@C powder. However, the intensity of the Li 2 S peak in the Raman spectrum decreased. As shown in  FIG. 3C , it&#39;s hard to see the Li 2 S peak in the Li 2 S@C material, which demonstrates that the carbon shell from CVD covered most of the Li 2 S-core and blocked the Raman signal of Li 2 S. The steps in the procedure for preparing the Li 2 S@C core-shell particles are summarized in  FIG. 3D . 
     Confirming the Li 2 S-Core Carbon-Shell Structure by Electron Energy Loss Spectroscopy. 
     To further confirm the core-shell structure of the Li 2 S@C spheres, an electron energy loss spectroscopy (EELS) line scan across a typical Li 2 S@C sphere was performed, and the normalized EELS intensities of its S-L and C-K peaks were analyzed (see  FIG. 4 ). The results illustrate the Li 2 S@C sphere to be a typical core-shell structure, in which carbon covers the surface of the Li 2 S sphere. The stable core-shell structure was also demonstrated by a polysulfide dissolution test on a large number of Li 2 S@C spheres (see  FIG. 5 ). These observations support the conclusion that the CVD process effectively coats carbon on the Li 2 S spheres so as to form carbon-coated Li 2 S core-shell spheres. 
     Evaluating Polysulfide Dissolution of Li 2 S and Li 2 S@C Materials. 
     After the polysulfide dissolution test, unprotected Li 2 S spheres reacted with sulfur and were washed away by THF in the form of polysulfide. Therefore, the precipitate left would be comprised only of well protected Li 2 S@C spheres and broken carbon shells. The ratio of unreacted Li 2 S@C spheres would indicate the effects of the protection of the carbon-shells on Li 2 S@C spheres, and the broken carbon shells would give a direct idea of how the carbon-shells look. As shown in  FIG. 5 , most spherical Li 2 S@C particles remained undamaged after the polysulfide dissolution test and only a few particles changed to hollow spherical structures, which shows that the as-prepared Li 2 S@C spheres had a stable core-shell configuration and most Li 2 S particles were well protected by the carbon-shells. 
     Electrode and Electrochemical Performance of Different Sized Li 2 S@C Materials. 
     In coin cells under the same conditions, the effects of particle size on electrode performance were evaluated, along with the electrochemical performance of three Li 2 S@C materials.  FIG. 6A  shows the discharge capacities of the 2 μm, 1 μm, and 500 nm Li 2 S@C spheres at the 0.5C rate (1C=1166 mA g −1  Li 2 S). All of the electrodes were first charged to 4.0 V at the 0.05C rate to activate the cathodes and then cycled between 1.5 V and 2.8 V at the 0.5C rate. All cells show a high coulombic efficiency resulting from the small and uniform particle sizes as well as the protection provided by the stable carbon-shell of these Li 2 S@C spheres (Cycle performance of commercial Li 2 S powder under the same conditions is shown in  FIG. 7B ). Among these three synthesized Li 2 S@C materials, the electrochemical utilization of the 2 μm Li 2 S is the lowest, which gives the lowest specific capacity. The 500 nm Li 2 S@C and 1 μm Li 2 S@C electrodes show very similar performance during the first 30 cycles. However, the 500 nm Li 2 S@C spheres show a higher capacity fade rate than the 1 μm Li 2 S@C after 100 cycles. This may be caused by the change of morphology of the Li 2 S@C particles after heat treatment and CVD coating at high temperature. The 500 nm-Li 2 S@C particles undergo a more serious agglomeration as a result of heat treatment as shown in  FIG. 2F . 
     In order to analyze the effect of agglomeration of Li 2 S particles, the cyclic voltammetry (CV) curves of the three Li 2 S@C cathodes were compared as shown in  FIG. 6B . As for the commercial Li 2 S, the pristine particles with large particle sizes usually exhibit initial charge peaks at high voltages and after being ball-milled the charge peaks shift to a lower voltage because of reduced particle size (See  FIG. 8 ). By comparing the initial charge peaks of the three sizes of synthesized Li 2 S@C particles, it is possible to see the effect of the size changes resulting from agglomeration. For the as-synthesized 2 μm Li 2 S@C material, there are three peaks in the first charge curve. In addition to the one charge peak below 3.5 V, there are two other peaks above 3.5 V. As to the 1 μm Li 2 S@C and 500 nm Li 2 S@C material, although both of them show two peaks in the first charge curves, the 500 nm Li 2 S@C actually showed more capacity above 3.5 V. Since more energy is needed for the larger particles to overcome the barrier of lithium extraction, it is reasonable to conclude that the charge capacity at the higher voltage reflects bigger particle sizes. Therefore, for the 500 nm Li 2 S@C material, although the primary 500 nm Li 2 S spheres have the smallest particle size, this advantage was compromised because of significant agglomeration as a result of the high temperature treatment. It should also be noted that the cathodic and anodic peaks below 2.6 V are the sharpest for the 1 μm material, indicative of the highest rate of Li uptake and release. Finally, the 1 μm Li 2 S@C particles with less agglomeration have the best performance among the three synthesized Li 2 S@C materials. Considering all the electrochemical performance data of the three Li 2 S@C composites, it can be concluded that Li 2 S@C with small particle size and less agglomeration is the optimal configuration for Li/S cells. 
     The 1 μm Li 2 S@C cathodes were cycled at different C-rates (see  FIG. 9A ). The cells delivered good capacity retention. The initial specific capacity for the 1 μm Li 2 S@C spheres cathode at the 0.2C rate is 972 mAh g −1  Li 2 S (1394 mAh g −1  S), 821 mAh g −1  Li 2 S (1177 mAh g −1 5) at the 0.5C rate, and 793 mAh g −1  Li 2 S (1137 mAh g −1 5) at the 1C rate. After 100 cycles, the 1 μm particles can achieve a high specific capacity of 737 mAh g −1  Li 2 S (1056 mAh g −1 S) at the 0.2C rate, 683 mAh g −1  Li 2 S (979 mAh g −1 S) at 0.5C rate and 634 mAh g −1  Li 2 S (909 mAh g −1 S) at 1C rate with a Li 2 S content of 60 wt % in the electrodes. The rate capability of the 1 μm Li 2 S@C cathodes is shown in  FIG. 9B . A highly reversible capacity of 570 mAh g −1  Li 2 S could still be achieved even at the 2C rate and when the C-rate was switched back to 0.5C again, the capacity recovered, indicating fast reaction kinetics in the cathodes. Representative voltage profiles of the 1 μm Li 2 S@C cycled at different rates are shown in  FIG. 9C . All curves show highly reversible capacities. 
     The performance of an electrode with 1 μm Li 2 S@C material and a high Li 2 S weight percentage, electrodes with no added carbon black was evaluated. Even though no carbon black was added, the 98 wt % Li 2 S@C cells exhibited stable cycling performance with a high discharge capacity of 659 mAh g −1  Li 2 S (945 mAh g −1  S) in the second cycle and a capacity of 417 mAh g −1  Li 2 S (600 mAh g −1  S) after 400 cycles at 0.5C (see  FIG. 10A ). The 10 wt % carbon shell of the 1 μm Li 2 S@C material and the current collector of carbon paper helped with the conductivity of the electrode, the core-shell particles can provide good cycling performance at a relatively high rate of 0.5C without additional carbon. After the cycling test, the electrode was removed from the cell, and the morphology of the Li 2 S@C core-shell particles was examined by SEM as shown in  FIG. 10B . Some broken spheres can be found in the electrode, but most of the Li 2 S@C particles show little morphology change after 400 cycles which is consistent with the good cyclability of the cell. These results show that the 1 μm Li 2 S@C core-shell particles are stable in the cell and are suitable materials for Li/S cells. 
     A number of embodiments have been described herein. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of this disclosure. Accordingly, other embodiments are within the scope of the following claims.