Patent Publication Number: US-10790493-B2

Title: Lithium-sulfur battery separator

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims all benefits accruing under 35 U.S.C. § 119 from China Patent Application No. 201710971522.X, filed on Oct. 18, 2017, in the China Intellectual Property Office, the contents of which are hereby incorporated by reference. The application is also related to copending applications entitled, “LITHIUM-SULFUR BATTERY”, with application number of 16/143500 filed on Sep. 27, 2019; “LITHIUM-SULFUR BATTERY SEPARATOR”, with application number of 16/143481, filed on Sep. 27,2019; “LITHIUM-SULFUR BATTERY SEPARATOR”, with application number of 16/143502, filed on Sep. 27, 2019; “LITHIUM-SULFUR BATTERY”, with application number of 16/143502,filed on Sep. 27, 2019; and “LITHIUM-SULFUR BATTERY”, with application number of 16/143506, filed on Sep. 27, 2019. 
     FIELD 
     The present disclosure relates to a lithium-sulfur battery separator. 
     BACKGROUND 
     In a lithium-sulfur battery, the cathode is made of sulfur and the anode is made of elemental lithium. During electrical discharge process, the elemental lithium loses electrons to become lithium-ion, and the sulfur reacts with the lithium-ion and electrons to produce lithium sulfides. A reaction equation is: S 8 +16Li + +16e −1 =8Li 2 S. A lithium-sulfur battery has advantages of low-cost, environmental friendliness, good safety, and high theoretical specific capacity. 
     A separator is an important component in the lithium-sulfur battery. The separator separates the cathode and the anode to avoid an internal short-circuit. A conventional lithium-sulfur battery separator is polypropylene (PP), polyethylene (PE) or other non-polar film. However, polysulfides (Li 2 S x , 4≤x≤8) formed during an electrical discharge process can be easily dissolved into an electrolyte, which affects a cyclic performance and a coulombic efficiency of the lithium-sulfur battery. It is difficult for conventional lithium-sulfur battery separators to inhibit polysulfide diffusion. With a great loss of active sulfur, a “shuttle effect” occurs between electrodes. Thus the specific capacity and cycling stability of the lithium-sulfur battery would be limited. 
     Therefore, there is room for improvement within the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Implementations of the present technology will now be described, by way of example only, with reference to the attached figures, wherein: 
         FIG. 1  is a view of one embodiment of a lithium-sulfur battery separator. 
         FIG. 2  is a schematic view of one embodiment of a lithium-sulfur battery using the lithium-sulfur battery separator of  FIG. 1 . 
         FIG. 3  is a graph showing cyclic performance of the lithium-sulfur battery using the lithium-sulfur battery separator (battery with MoP 2 /CNT separator) with functional layer (FL) having different MoP 2  nanoparticles surface density. 
         FIG. 4  is a graph comparing between the battery with MoP 2 /CNT separator and a lithium sulfur battery with pristine separator (PSL) (battery with PSL) of a comparative example showing cyclic performance at certain current test result. 
         FIG. 5  is a graph comparing the battery with MoP 2 /CNT separator and the battery with PSL showing the cyclic performance at a current density of 0.2 C. 
         FIG. 6  is a graph comparing the battery with MoP 2 /CNT separator and the battery with PSL showing a cyclic performance at high plateau capacity at 0.2 C. 
         FIG. 7  is a graph showing In-situ Raman spectroscopy of the battery with PSL at 2.08 V. 
         FIG. 8  is a graph showing In-situ Raman spectroscopy of the battery with MoP 2 /CNT separator at 2.08 V. 
         FIG. 9  a schematic view of another embodiment of a lithium-sulfur battery separator. 
         FIG. 10  a schematic view of yet another embodiment of a lithium-sulfur battery separator. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “another,” “an,” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean “at least one.” 
     It will be appreciated that for simplicity and clarity of illustration, where appropriate, reference numerals have been repeated among the different figures to indicate corresponding or analogous elements. In addition, numerous specific details are set forth in order to provide a thorough understanding of the embodiments described herein. However, it will be understood by those of ordinary skill in the art that the embodiments described herein can be practiced without these specific details. In other instances, methods, procedures, and components have not been described in detail so as not to obscure the related relevant feature being described. Also, the description is not to be considered as limiting the scope of the embodiments described herein. The drawings are not necessarily to scale, and the proportions of certain parts have been exaggerated to illustrate details and features of the present disclosure better. 
     Several definitions that apply throughout this disclosure will now be presented. 
     The term “substantially” is defined to be essentially conforming to the particular dimension, shape, or other feature which is described, such that the component need not be exactly or strictly conforming to such a feature. The term “comprise,” when utilized, means “include, but not necessarily limited to”; it specifically indicates open-ended inclusion or membership in the so-described combination, group, series, and the like. 
       FIGS. 1 and 2  show one embodiment of a lithium-sulfur battery  10 . The lithium-sulfur battery  10  includes a cathode  20 , an anode  30 , a lithium-sulfur battery separator  100 , and an electrolyte (not shown). The electrolyte is located between the cathode  20  and the anode  30 . The lithium-sulfur battery separator  100  is located between the cathode  20  and the anode  30  and is located in the electrolyte. The lithium-sulfur battery separator  100  includes a pristine separator (hereinafter PSL)  102  and a functional layer (hereinafter FL)  104  stacked with each other. The FL  104  is disposed opposite the cathode  20 , that is, the FL  104  is located between the cathode  20  and the PSL  102 . In this embodiment, the FL  104  is sandwiched by the cathode  20  and the PSL  102 , and the FL  104  is located on the surface of the cathode  20 . That is, the FL  104  and one surface of the cathode  20  are in direct contact with each other. 
     The cathode  20  can be made of sulfur or composite material containing sulfur. The anode  30  can be metallic lithium. In one embodiment, the anode  30  is a lithium sheet. The electrolyte may be a solid electrolyte or a liquid electrolyte. A material of the electrolyte can be selected from the electrolyte materials commonly, currently used in lithium-sulfur batteries. In one embodiment, the electrolyte is prepared by magnetically stirring 1 Mol bis-trifluoromethane sulfonamide lithium (LiTFSI) and 0.1 Mol lithium nitrate (LiNO 3 ) in a solvent with mixed dioxolane (DOL) and dimethyl ether (DME) for 2 hours. In the mixed solvent, a volume ratio between DOL and DME is 1:1. 
       FIG. 2  shows the lithium sulfur battery separator  10  including a PSL  102  and a FL  104 . The PSL  102  is a planar structure and is a film. The PSL  102  defines a first surface and a second surface disposed opposite the first surface. The first surface is opposite with the cathode  20  and the second surface is opposite with the anode  30 . The FL  104  is located on the first surface. The FL  104  is located between the cathode  20  and the first surface of PSL  102 . 
     The PSL  102  can be a polyolefin microporous film, such as a polypropylene (PP) membrane, a polyethylene (PE) membrane, or a multilayer composite membrane of PP and PE. The PSL  102  defines a plurality of micropores through the first surface and the second surface. In this embodiment, the PSL  102  is a porous polypropylene film 25 microns thick. 
     The FL  104  includes a carbon nanotube structure  106  and a plurality of MoP 2  nanoparticles  108 . That is, the FL  104  is a CNT/MoP 2  layer. The carbon nanotube structure  106  includes a plurality of carbon nanotubes, and the plurality of MoP 2  nanoparticles  108  are adsorbed on surfaces of the plurality of carbon nanotubes and supported by the carbon nanotube structure  106 . In some embodiments, the FL  104  only includes carbon nanotubes and MoP 2  nanoparticles  108 , and does not contain other materials. The FL  104  has 3 microns to 5 microns thick. 
     The carbon nanotube structure  106  is a net-like structure having a plurality of micropores. The carbon nanotube structure  106  includes a plurality of carbon nanotubes. The plurality of carbon nanotubes are uniformly distributed in the carbon nanotube structure  106 . The carbon nanotubes in the carbon nanotube structure  106  can be joined with each other by van der Waals attractive force therebetween. The carbon nanotubes can be disorderly or orderly arranged in the carbon nanotube structure  106 . The term ‘disorderly’ describes the carbon nanotubes being arranged along many different directions, such that the number of carbon nanotubes arranged along each different direction can be almost the same (e.g. uniformly disordered), and/or entangled with each other. The term ‘orderly’ describes the carbon nanotubes being arranged in a consistently systematic manner, e.g., the carbon nanotubes are arranged approximately along a same direction and or have two or more sections within each of which the carbon nanotubes are arranged approximately along a same direction (different sections can have different directions). The carbon nanotubes in the carbon nanotube structure  106  can be single-walled, double-walled, or multi-walled carbon nanotubes. The thickness of the carbon nanotube structure  106  is not limited, and can be in a range from about 0.5 nanometers to about 1 centimeter. In one embodiment, the carbon nanotube structure  106  is 1 micrometer to about 10 micrometers thick. The carbon nanotube structure  106  can include at least one drawn carbon nanotube film, flocculated carbon nanotube film or pressed carbon nanotube film. 
     The drawn carbon nanotube film includes a plurality of successive and oriented carbon nanotubes joined end-to-end by van der Waals attractive force therebetween. The carbon nanotubes in the carbon nanotube film can be substantially aligned in a single direction. The drawn carbon nanotube film can be formed by drawing a film from a carbon nanotube array that is capable of having a film drawn therefrom. The plurality of carbon nanotubes in the drawn carbon nanotube film are arranged substantially parallel to a surface of the drawn carbon nanotube film. A large number of the carbon nanotubes in the drawn carbon nanotube film can be oriented along a preferred orientation, meaning that a large number of the carbon nanotubes in the drawn carbon nanotube film are arranged substantially along the same direction. An end of one carbon nanotube is joined to another end of an adjacent carbon nanotube arranged substantially along the same direction, by van der Waals attractive force. A small number of the carbon nanotubes are randomly arranged in the drawn carbon nanotube film, and has a small if not negligible effect on the larger number of the carbon nanotubes in the drawn carbon nanotube film arranged substantially along the same direction. The drawn carbon nanotube film is capable of forming a free-standing structure. The term “free-standing structure” includes, but is not limited to, a structure that does not have to be supported by a substrate. For example, a free-standing structure can sustain the weight of itself when it is hoisted by a portion thereof without any significant damage to its structural integrity. So, if the drawn carbon nanotube film is placed between two separate supporters, a portion of the drawn carbon nanotube film, not in contact with the two supporters, would be suspended between the two supporters and yet maintain film structural integrity. The free-standing structure of the drawn carbon nanotube film is realized by the successive carbon nanotubes joined end to end by van der Waals attractive force. Microscopically, in the drawn carbon nanotube film, the carbon nanotubes oriented substantially along the same direction may not be perfectly aligned in a straight line, and some curve portions may exist. It can be understood that a contact between some carbon nanotubes located substantially side by side and oriented along the same direction can not be totally excluded. The drawn carbon nanotube film can be a pure structure only including the carbon nanotubes. The drawn carbon nanotube film has 0.5 nanometers to 100 micrometers. The width and length of the drawn carbon nanotube film is not limited. When the carbon nanotube structure  106  includes a plurality of drawn carbon nanotube films, an angle between the aligned directions of the carbon nanotubes in at least two drawn carbon nanotube films can be in a range from about 0 degrees to about 90 degrees, for example can be equal to about 0 degrees, 15 degrees, 45 degrees, 60 degrees, or 90 degrees. In one embodiment, the carbon nanotube structure  106  includes 20 layers of drawn carbon nanotube films overlapped with each other, and carbon nanotube structure  106  is 2 micrometers thick. 
     The flocculated carbon nanotube film can include a plurality of long, curved, disordered carbon nanotubes entangled with each other. The length of the carbon nanotube film can be greater than 10 centimeters. The carbon nanotubes can be randomly arranged and curved in the flocculated carbon nanotube film. The carbon nanotubes can be substantially uniformly distributed in the flocculated carbon nanotube film. The adjacent carbon nanotubes are acted upon by the van der Waals attractive force there between, thereby forming an entangled structure with micropores defined therein. Because the carbon nanotubes in the flocculated carbon nanotube film are entangled with each other, the flocculated carbon nanotube film has excellent durability, and can be fashioned into desired shapes with a low risk to the integrity of flocculated carbon nanotube film. The flocculated carbon nanotube film can be a free-standing structure due to the carbon nanotubes being entangled and adhered together by van der Waals attractive force there between. The flocculated carbon nanotube film is 1 micrometer to about 1 millimeter thick. It is also understood that many of the embodiments of the carbon nanotube structure are flexible and do not require the use of a structural support to maintain their structural integrity. The flocculated carbon nanotube film can be a pure carbon nanotube film only including carbon nanotubes. 
     The pressed carbon nanotube film can be formed by pressing a carbon nanotube array to slant the carbon nanotubes in the carbon nanotube array. The pressed carbon nanotube film can be a free-standing carbon nanotube film. The carbon nanotubes in the pressed carbon nanotube film are arranged along a same direction, along more than one predetermined different directions, or randomly arranged. The carbon nanotubes in the pressed carbon nanotube film can rest upon each other. Adjacent carbon nanotubes are attracted to each other and joined by van der Waals attractive force. An angle between a primary alignment direction of the carbon nanotubes and a surface of the pressed carbon nanotube film is about 0 degrees to approximately 15 degrees. In some embodiments, the angle between a primary alignment direction of the carbon nanotubes and a surface of the pressed carbon nanotube film is between 0 degrees and 15 degrees. The greater the pressure applied, the smaller the angle obtained. The pressed carbon nanotube film is about 1 micrometer to about 1 millimeter thick. The pressed carbon nanotube film can be pure carbon nanotube film only including carbon nanotubes. The length and width of the pressed carbon nanotube film depend on the carbon nanotube array that is pressed. If the length and width of the carbon nanotube array is large, the pressed carbon nanotube film can have large length and width. 
     The MoP 2  nanoparticles  108  are not limited in shape and are nanometer in size. The plurality of MoP 2  nanoparticles  108  are attached to the carbon nanotube structural structure  106 , attached on the surfaces of the carbon nanotubes and filled in the micropores of the carbon nanotube structure  106 . Preferably, the plurality of MoP 2  nanoparticles  108  are evenly distributed on the carbon nanotube structure  106 . Diameters of the MoP 2  nanoparticles  108  may be between 10 nm-500 nm. In one embodiment, the plurality of MoP 2  nanoparticles  108  have an average diameter of 100 nm. 
     In the lithium-sulfur battery separator  100 , an areal density of the plurality of MoP 2  nanoparticles  108  (a mass of the MoP 2  nanoparticles  108  in per unit area of the lithium-sulfur battery separator  100 ) is not limited, and can be adjusted according to actual needs. Preferably, the areal density of the plurality of MoP 2  nanoparticles  108  are ranged from 0.1 mg/cm 2  to 0.6 mg/cm 2 . In one embodiment, the areal density of the plurality of MoP 2  nanoparticles  108  is about 0.3 mg/cm 2, a mass ratio between the plurality of MoP 2  nanoparticles  108  and the carbon nanotube structure  106  in the lithium-sulfur battery separator  100  is 7.5:1. 
     Depending on the embodiment, certain of the steps of a method described may be removed, others may be added, and the sequence of steps may be altered. The description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps. 
     A method for preparing the lithium-sulfur battery separator  100  may include at least the following steps: providing the PSL  102 ; laying the carbon nanotube structure  106  on a surface of the PSL  102 ; and uniformly depositing a suspension of the MoP 2  nanoparticle  108  on the carbon nanotube structure  106 , after drying, a plurality of MoP 2  nanoparticles  108  are attached on the surfaces of the carbon nanotubes in the carbon nanotube structure  106 . The plurality of MoP 2  nanoparticles  108  are located on the surface of the carbon nanotube structure  106  or in the micropores of the carbon nanotube structure  106 . 
     The lithium-sulfur battery  10  provided by the present invention uses a special separator, that is, the lithium-sulfur battery separator  100 . The lithium-sulfur battery separator  100  includes a PSL  102  and a FL  104 . The FL  104  includes a carbon nanotube structure  106  and a plurality of MoP 2  nanoparticles  108 , and the FL  104  is disposed opposite the cathode  20  of the lithium sulfur battery  10 . During the charging and discharging process of the lithium-sulfur battery  10 , a plurality of MoP 2  nanoparticles  108  act as a fixing agent to chemically react with the dissolved polysulfide to achieve the effect of adsorbing polysulfide; meanwhile, the plurality MoP 2  nanoparticles  108  also act as a catalyst to improve redox reaction of the polysulfide to reduce the polysulfide content in the electrolyte. Additionally, the carbon nanotube structure  106  serves as a supporting skeleton for supporting the plurality of MoP 2  nanoparticles  108  to ensure the plurality of MoP 2  nanoparticles  108  is uniformly distributed. The carbon nanotube structure  106  also physically hinders polysulfide migration. Further, the carbon nanotube structure  106  is in contact with one surface of the cathode  20 . Because of the good electrical conductivity of the carbon nanotubes, the carbon nanotube structure  106  can also be regarded as a current collector for providing the cathode  20  a rich electron passage. As such, the internal charge transfer resistance of the lithium-sulfur battery  10  is reduced. Test results show that the cyclic performance of the lithium-sulfur battery  10  using the lithium-sulfur battery separator  100  is significantly improved. A capacity attenuation per cycle is 0.152% at 0.2 C in 100 cycles; the capacity attenuation per cycle is 0.025% at 1 C in 500 cycles. The in-situ Raman spectroscopy also prove the adsorption effect of polysulfide by the FL  104  in the lithium-sulfur battery separator  100  and the catalysis effect of the MoP2 nanoparticles during charge and discharge. 
       FIG. 3  shows the areal density of the MoP 2  nanoparticles  108  is also an important parameter affecting the electrochemical performance of the lithium-sulfur battery  10 . The initial capacitance of the cathode  20  is the same. After 100 cycles, when the surface density of the MoP 2  nanoparticles  108  in the lithium-sulfur battery separator  100  is low (0.1 mg/cm 2 ), the capacitance of the cathode  20  is attenuated to 783.9 mAh/g; when the areal density of MoP 2  nanoparticles  108  in the sulfur battery separator is high (0.6 mg/cm 2 ), the capacity of the cathode  20  is attenuated to 657.5 mAh/g; when the areal density of MoP 2  nanoparticles  108  in the lithium-sulfur battery separator  100  is at an intermediate value (0.3 mg/cm 2 ), the capacity of the cathode  20  is attenuated to 905.4 mAh/g. The above testing data indicates that, the areal density of the MoP 2  nanoparticles  108  is not the higher the better or worse. When the areal density is too low, the polysulfide may not be completely adsorbed; when the areal density is too high, the adsorbed polysulfide may be difficult to be released because of the strong interaction. As such, the areal density of the MoP 2  nanoparticles is ranged from 0.1 mg/cm 2  to 0.6 mg/cm 2 . 
     Comparing tests between lithium-sulfur battery  10  using lithium-sulfur battery separator  100  (battery with MoP 2 /CNT separator) and a conventional lithium-sulfur battery using PSL (battery with PSL) are provided to prove the effect of the lithium-sulfur battery separator  100 . In  FIGS. 4-8 , the curve corresponding to the MoP 2 /CNT separator represents the battery with MoP 2 /CNT separator provided by the present invention, and the curve corresponding to the PSL represents the battery with PSL provided by a comparative example. The diaphragm of the lithium-sulfur battery  10  provided by the present invention adopts the lithium-sulfur battery separator  100  including the PSL  102  and the FL  104 ; the conventional lithium-sulfur battery provided adopts a conventional lithium-sulfur battery separator only including the PSL  102 . The conventional lithium-sulfur battery separator does not contain a FL. Other features of the lithium-sulfur battery  10  and the conventional lithium-sulfur battery are the same. 
       FIG. 4  is a graph comparing between the battery with MoP 2 /CNT separator and a lithium sulfur battery with PSL (battery with PSL) of a comparative example showing cyclic performance at certain current test result.  FIG. 4  shows the battery with MoP 2 /CNT separator exhibits a higher cycle retention ratio than the battery with PSL. The initial specific discharge capacity of the battery with MoP 2 /CNT separator is 1223.2 mAh/g, and after 100 cycles, the specific discharge capacity is attenuated to 905.4 mAh/g. That is, the capacity retention rate is 74.02%, wherein, after the second discharge cycle, each cycle capacity decay rate is only 0.152%. The battery with PSL provided in the comparative example has a battery capacity retention rate of 33.2% (360 mAh/g) after 100 cycles. Accordingly, the battery with MoP 2 /CNT separator has a better cyclic performance, sufficient to demonstrate the effect of the sulfur-sulfur battery separator with the FL on the cyclic performance of the entire battery. 
       FIG. 5  is a graph comparing the battery with MoP 2 /CNT separator and the battery with PSL showing the cyclic performance at a current density of 0.2 C (1 C=1600 mA/g). In the voltage characteristic curves, a voltage hysteresis ΔE between a platform at the time of charging and a platform at the time of discharge is an important indicator of the electrochemical reversibility in the battery system. It can be seen from the figure that, the ΔE value of the lithium-sulfur battery with PSL is 0.32 V; the ΔE value of the lithium-sulfur battery with MoP 2 /CNT separator is 0.25 V. The lower the ΔE value of the lithium-sulfur battery, the weaker the internal polarization of the battery, and the better the rate performance of the battery. The decrease in the ΔE value of the lithium-sulfur battery with MoP 2 /CNT separator is mainly due to that the presence of MoP 2  nanoparticles in the lithium-sulfur battery separator improves the polysulfide conversion activity. 
       FIG. 6  is a graph comparing between the battery with MoP 2 /CNT separator and the battery with PSL showing a cyclic performance at high platform capacity at 0.2 C. In the lithium-sulfur battery, the high platform capacity corresponds to a process of conversion of elemental sulfur S to Li 2 S 4 . The higher the high platform capacity, the better the inhibition effect of the lithium-sulfur battery separator on the shuttle effect of polysulfide.  FIG. 6  shows that, in the lithium-sulfur battery with MoP 2 /CNT separator, the lithium sulfur battery separator has a better inhibitory effect on the shuttle effect of polysulfide. 
       FIG. 7  is a graph showing In-situ Raman spectroscopy of the battery with PSL at 2.08 V.  FIG. 8  is a graph showing In-situ Raman spectroscopy of the battery with MoP 2 /CNT separator at 2.08 V. For the lithium-sulfur battery with PSL.  FIG. 7  shows peaks of Li 2 S 6  and Li 2 S 2  can be observed on both sides of the cathode and the anode, indicating that Li 2 S 6  and Li 2 S 2  substances on both sides of the cathode and anode. As such, the PSL does not inhibit the shuttle effect of polysulfide. For the lithium-sulfur battery with MoP 2 /CNT separator, the peaks of Li 2 S 6  and Li 2 S 2  are observed only on the cathode side, and no polysulfide peak appears on the anode side, which indicates that the MoP 2 /CNT separator the lithium-sulfur battery has a good inhibitory effect on the shuttle effect of polysulfide. 
     Another embodiment of the present invention provides a lithium sulfur battery comprising a cathode, an anode, a lithium-sulfur battery separator  200  and an electrolyte. 
       FIG. 9  shows the lithium-sulfur battery separator  200  provided in the second embodiment includes a PSL  202  and a FL  204 . The PSL  202  is a film. The PSL  202  has a first surface and a second surface opposite with the first surface. The first surface is located opposite with the cathode, and the second surface is located opposite with the anode. The FL  204  is disposed on the first surface. The FL  204  is located between the cathode and the first surface of PSL  202 . 
     The FL  204  includes a plurality of MoP 2  nanoparticles  208  and a plurality of carbon nanotubes  210 . The plurality of MoP 2  nanoparticles  208  and the plurality of carbon nanotubes  210  are uniformly mixed with each other. The plurality of carbon nanotubes  210  can be entangled with each other. The MoP 2  nanoparticles  208  can also be entangled by the carbon nanotubes  210 . The plurality of MoP 2  nanoparticles  208  and the plurality of carbon nanotubes  210  can be joined by van der Waals attractive force to form an integrity structure. In the lithium-sulfur battery separator  200 , an areal density of the plurality of MoP 2  nanoparticles  108  is not limited and can be adjusted according to practical needs. In some embodiments, the plurality of MoP 2  nanoparticles  208  have an areal density ranged from 0.1 mg/cm 2  to 0.6 mg/cm 2 . 
     In some embodiments, the FL  204  only includes MoP 2  nanoparticles  208  and carbon nanotubes  210 , and does not contain other materials. In other embodiments, the FL  204  can further include conductive particles, such as, carbon black and graphene. Alternatively, the FL  204  can further include a binder for fixing the plurality of MoP 2  nanoparticles  208  and the plurality of carbon nanotubes  210 . 
     A preparation method of the lithium-sulfur battery separator  200  includes at least the following steps: preparing a liquid mixture of MoP 2  nanoparticles  208  and carbon nanotubes  210 , and stirring the mixture uniformly; providing a PSL  102 , coating the liquid mixture of MoP 2  nanoparticles  208  and carbon nanotubes  210  on the first surface of the PSL  102 , and to form the FL  204  after drying. 
     Other characteristics of the lithium-sulfur battery separator  200  is the same as the lithium-sulfur battery separator  100  discussed above. 
     A lithium sulfur battery comprising a cathode, an anode, a lithium-sulfur battery separator  300  and an electrolyte according to yet another embodiment is provided. 
       FIG. 10  shows the lithium-sulfur battery separator  300  includes a PSL  302  and a FL  304 . The PSL  302  is a film having a certain thickness. The PSL  302  has a first surface and a second surface opposite with the first surface. The first surface is located opposite with the cathode, and the second surface is located opposite with the anode. The FL  304  is disposed on the first surface. The FL  304  is located between the cathode and the first surface of PSL  302 . 
     The FL  304  includes a plurality of MoP 2  nanoparticles  308  and a plurality of graphene sheets  312 . The plurality of MoP 2  nanoparticles  308  and the plurality of graphene sheets  312  are uniformly mixed with each other. The plurality of graphene sheets  312  can be overlapped or laminated to form an integral graphene layer. The plurality of MoP 2  nanoparticles  308  are attached to surfaces of the plurality of graphene sheets  312 . In some embodiments, the FL  304  only includes MoP 2  nanoparticles  308  and graphene sheets  312 , and does not contain other materials. 
     The graphene sheets  312  in the graphene layer are joined with each other by Van der Waals attractive force. The graphene sheets  312  in the graphene layer can be arranged side by side or overlapped with each other. Graphene sheets have good electrical conductivity. Thickness of graphene sheets  312  is less than 100 nm. In an embodiment, the graphene sheets  312  have a thickness ranged from 0.5 nm to 100 nm. The graphene layer is a single layer graphene sheet to 1 mm thick. 
     In the lithium-sulfur battery separator  300 , an areal density of the plurality of MoP2 nanoparticles  108  is not limited, and can be adjusted according to actual needs. In some embodiments, the plurality of MoP 2  nanoparticles  208  have an areal density ranged from 0.1 mg/cm 2  to 0.6 mg/cm 2 . 
     A method for preparing the lithium-sulfur battery separator  300  includes at least the following steps: providing a certain amount of graphene oxide sheets, placing the graphene oxide sheets into a solvent to form a mixture; ultrasonically shaking the mixture to make the graphene oxide sheets dispersed uniform to form a graphene oxide sheet dispersion; forming the graphene layer by suction filtration; transferring the graphene layer to the surface of the PSL  302 ; dropping a solvent including MoP 2  nanoparticles  108  on the graphene layer and then drying them. 
     In the lithium-sulfur battery separator  300 , the plurality of MoP2 nanoparticles  308  and the plurality of graphene sheets  312  are uniformly distributed; the plurality of graphene sheets  312  are stacked to form a graphene layer to allow the electrolyte to pass smoothly, and also to hinder polysulfide migration. 
     Other characteristics of the lithium-sulfur battery separator  300  are the same as the lithium-sulfur battery separator  100  discussed above. 
     It is to be understood that the above-described embodiments are intended to illustrate rather than limit the present disclosure. Variations may be made to the embodiments without departing from the spirit of the present disclosure as claimed. Elements associated with any of the above embodiments are envisioned to be associated with any other embodiments. The above-described embodiments illustrate the scope of the present disclosure but do not restrict the scope of the present disclosure. 
     Depending on the embodiment, certain of the steps of a method described may be removed, others may be added, and the sequence of steps may be altered. The description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.