WO2020148707A1

WO2020148707A1 - Separator for an energy storage device

Info

Publication number: WO2020148707A1
Application number: PCT/IB2020/050355
Authority: WO
Inventors: Chengmeng CHEN; Liqin DAI; Fangyuan SU; Mohammed Awadh ALMAKER; Taghreed Adnan JALAL; Yunyang Liu
Original assignee: Sabic Global Technologies B.V.
Priority date: 2019-01-17
Filing date: 2020-01-16
Publication date: 2020-07-23

Abstract

A separator for an electrical energy storage device or battery has a separator body having a composite material coated thereon. The composite material comprises a reduced graphene oxide (rGO) having an oxygen content of from 5 wt.% to 40 wt.% by total weight of rGO, an electrically conductive carbon black, and a binder. In a method of forming a separator for an electrical energy storage device or battery, a reduced graphene oxide (rGO) having an oxygen content of from 5 wt.% to 40 wt.%, an electrically conductive carbon black, and a binder are combined to form a composite material. The composite material is coated on a separator body so that composite material is coated thereon.

Description

SEPARATOR FOR AN ENERGY STORAGE DEVICE

TECHNICAL FIELD

[0001] The invention relates to electrical energy storage devices, and in particular separators used in electrical energy storage devices.

BACKGROUND

[0002] High energy storage capacity and high energy density rechargeable batteries are among the highly sought after technologies for portable electronic devices and electric vehicles. Lithium-sulfur batteries are among the best candidates for these applications for several reasons. The sulfur cathode of these batteries has a high theoretical capacity of 1675 mAhg ¹, which is about five times that of currently used transition metal oxide cathode materials for lithium batteries. Additionally, sulfur is an abundant resource that may be obtained at a low cost. Sulfur is also non-poisonous and is environmentally benign.

[0003] Lithium-sulfur electrodes have certain drawbacks, however, so that they have not yet been commercialized. For one, sulfur has extremely low electrical conductivity at 5 xlO ³⁰ S/cm at 25 °C. Further, the migration of polysulfides into the electrolyte of the battery affects the battery’s cycle life and thus limits their applications. The diffusion of polysulfides and the corresponding shuttling effects are the primary reasons for the deterioration of the performance of lithium-sulfide batters.

[0004] The development of multifunctional separators may overcome many difficulties encountered with lithium- sulfur batteries. Carbon materials, owing to their high specific surface area and abundant pore structure, can adsorb polysulfides. Such carbon materials are widely used to modify separators in electrical storage devices. Studies in carbon modified separators, however, is mainly focused on physical adsorption. Chemical adsorption has a stronger ability for adsorbing polysulfides, however, there have been very few studies focused on chemical adsorption. Furthermore, the carbon materials, with high specific surface, abundant pore structure, high conductivity and rich oxygen-containing functional groups, can both physically and chemically adsorb polysulfides, reducing the shuttling effect of poly sulfides. Such multifunctional materials, however, are difficult to prepare and very little development has occurred in this area.

[0005] Accordingly, a need exists for improvements in separators for electrical storage devices to overcome these shortcomings. SUMMARY

[0006] A separator for an electrical energy storage device or battery comprises a separator body having a composite material coated thereon. The composite material comprises a reduced graphene oxide (rGO) having an oxygen content of from 5 wt.% to 40 wt.% by total weight of rGO. The composite material further comprises an electrically conductive carbon black and a binder.

In particular embodiments, the rGO of the composite material is present in an amount of from 15 wt.% to 85 wt.% based upon the total weight of the rGO and electrically conductive carbon black. The rGO may have an oxygen content of from 5 wt.% to 25 wt.% by total weight of rGO. The rGO may have a laminar thickness of from 3 to 10 layers and a sheet size of from 0.5 pm to 5 pm.

[0007] In certain applications the composite material may have at least one of a specific surface area (SA) of from 400 m²/g to 1300 m²/g, a pore volume of from 1.5 to 2.8 cm³/g, a pore size of from 1 nm to 10 nm, a conductivity of from 15 S/m to 80 S/m, and an oxygen content of from 5 wt.% to 15 wt.% by total weight of the composite material. The composite material coating may have a coating thickness of from 5 pm to 200 pm.

[0008] A binder of the composite material may comprise at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), carboxymethylcellulose sodium (CMC), and poly(ethyleneoxide) (PEO), styrene-butadiene rubber (SBR), acrylonitrile copolymer, sodium alginate (SA), gelatin, polyacrylic acid (PAA), and b-cyclodextrin. The binder may be present in the composite material in an amount of from 10 wt.% to 60 wt.% by total weight of the composite material.

[0009] The separator body may comprise at least one of polyethylene, polypropylene, polyimide, glass fiber, ceramic, polyvinylidene fluoride, and polyacrylonitrile. The separator body may have at least one of a porosity of from 30% to 60%, a thickness of from 10 pm to 30 pm, an average pore size of from 20 nm to 60 nm, and a puncture strength of from 300 g to

600 g.

[0010] The electrically conductive carbon black may have at least one of a conductivity of from 10 S/m to 500 S/m, a surface area of the carbon black may range from 50 m²/g to 2000 m²/g, a pore volume from 0.5 cm³/g to 5 cm³/g, a pore size of from 1 nm to 10 nm, and a particle size of from 10 nm to 100 nm.

[0011] The separator may be incorporated into an electrical energy storage device or battery. The electrical energy storage device may be a lithium-sulfur electrical energy storage device or battery. [0012] In a method of forming a separator for an electrical energy storage device or battery, a reduced graphene oxide (rGO) having an oxygen content of from 5 wt.% to 40 wt.% is combined with an electrically conductive carbon black and a binder to form a composite material. A separator body is coated with the composite material so that composite material is coated thereon.

[0013] In certain applications the rGO of the composite material is present in an amount of from 15 wt.% to 85 wt.% based upon the total weight of the rGO and electrically conductive carbon black. The rGO may have an oxygen content of from 5 wt.% to 25 wt.% by total weight of rGO.

[0014] In the method, the composite material may have at least one of a specific surface area (SA) of from 400 m²/g to 1300 m²/g, a pore volume of from 1.5 cm³/g to 2.8 cm³/g, a pore size of from 1 nm to 10 nm, a conductivity of from 15 S/m to 80 S/m, and an oxygen content of from 5 wt.% to 15 wt.% by total weight of the composite material. The composite material coating may have a coating thickness of from 5 pm to 200 pm. The rGO may have a laminar thickness of from 3 to 10 layers and a sheet size of from 0.5 pm to 5 pm. The separator may be incorporated into an electrical energy storage device or battery.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a more complete understanding of the embodiments described herein, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying figures, in which:

[0016] FIG. 1 is schematic diagram of a process for forming a separator for an electrical energy storage device or battery in accordance with particular embodiments of the invention;

[0017] FIG. 2 is a top view scanning electron microscope (SEM) image at 10,000X magnification of the modified separator of Embodiment Sample 1 of Example 2;

[0018] FIG. 3 is a cross-section SEM image at 1000X magnification of the modified separator of Embodiment Sample 1 from Example 2;

[0019] FIG. 4 is a plot of cycle performance of Comparative Samples 1-3 and Embodiment Sample 1 from Example 3 at a charge/discharge current density of 1 C over 1000 cycles; and

[0020] FIG. 5 is a plot of the rate performance of Comparative Samples 1-3 and Embodiment Sample 1 from Example 4 at different charge/discharge current densities (0.1 C, 0.2 C, 0.5 C, 1 C, 2C, and 0.1 C) and cycles. DETAILED DESCRIPTION

[0021] In lithium-sulfur (Li-S) batteries, during discharge of the battery, lithium metal plated on the anode is oxidized to lithium ions and electrons, the lithium ions pass through the electrolyte of the battery to the sulfur-containing cathode where lithium ions react with the sulfur to form lithium polysulfide, where two lithium atoms are bonded to the polysulfide molecule. Where the polysulfide is Ss, for example, this may be represented by reaction (1) below:

Sg + 2Li Li₂Sg (1)

[0022] The reaction may continue with the Li₂Sx reacting further with additional lithium, as shown in reaction (2) below:

Li₂Ss +2Li Li₂S_8-x + Li₂S_x, where x = 2 to 7 (2)

[0023] With more lithium being drawn to the cathode during discharge, the length of the lithium polysulfide chains will decrease, ultimately being reduced to Li₂S, as shown in the exemplary reaction (3) below:

Li₂S₂ +2Li 2Li₂S (3)

[0024] Charging of the battery reverses this process so that lithium atoms from the lithium sulfide or polysulfides are plated back on the anode as metal, as represented by the exemplary reactions (4) and (5) below:

Li₂S_x + Li₂S Li₂Si_+y + 2Li, where y = 1 to 7 (4)

Li₂Sn S_n + 2Li, where n = 1 to 8 (5)

[0025] One of the degrading mechanisms of Li-S batteries during charge-discharge cycles is the dissolution of polysulfide ions from the cathode to the anode. In Li-S batteries, the polysulfide ions are predominantly S4^m -S8^m (with m usually equal to 2). The polysulfide ions (S_n) where n>4 are easily moved around when subjected to an electric field and are prone to dissolve in the organic electrolyte and diffuse from the cathode to the anode where the polysulfide deposits on the anode. The ionic organic electrolyte may be designed to be less soluble for smaller polysulfides (up to S4) and therefore preventing migration of these ions is of less concern. The loss of sulfur from the cathode of the battery is permanent, so power density and charge capacity drop with increased number of cycles as the sulfur is gradually lost.

[0026] Separators are permeable membranes used in electrical storage devices to separate the anode and cathode to keep them apart so that they do not short circuit. While the separator keeps the anode and cathode apart, the permeability of the separator allows the transmission of ions between the anode and cathode during discharge and recharging of the battery. Those separator membranes typically used in electronic storage devices, however, do little to stop the passage or transmission of the polysulfide ions dissolved in the electrolyte through the membrane in Li-S batteries so that they are permanently deposited on the anode, thus degrading the battery.

[0027] By modifying the separator to prevent the migration or passage of the polysulfide ions through the separator to the anode, the capacity of the energy storage device can be prevented from dropping. The use of graphene oxide (GO) as a coating material in Li-S batteries has been investigated for modifying such membranes since the rich functional groups in the carbon framework could effectively trap the polysulfide ions. Because the electrical conductivity for GO is rather low, however, the trapped polysulfides lose electrical contact with the electrode. The capacity for the Li-S battery therefore decreases with the use of GO-modified separators.

[0028] If the coated separator can possess good conductivity on cathode side, the trapped polysulfides can still be charged and discharged. Thus, the coated or modified separator can be taken as an extension of the cathode. All of the sulfur can be effectively used and the capacity of the energy storage device will not drop. If the surface chemistry of a GO coating could be optimized for obtaining both negatively charged oxygen and good conductivity simultaneously then it would be useful as an effective coating for a separator in Li-S battery.

[0029] Reduced graphene oxide (rGO) can be prepared by the reduction of GO and it is a promising material for separators in electrical storage devices. Depending upon the degree of reduction, rGO can possess varying concentrations of oxygen and conductivity. Therefore, rGO with moderate reduction is more effective in retarding polysulfide shuttling in Li-S batteries due to its high conductivity and abundant functional groups. The reduction degree of GO plays an important role in determining the final properties of the rGO. If the reduction degree of GO is too high, although the conductivity is very high, the number of functional groups will decrease greatly, which reduces its ability to trap polysulfides. Thus, the degree of reduction of GO should be optimal for both high conductivity and rich functional groups. [0030] The GO that is reduced for preparing suitable rGO may be that with an oxygen content of from 40 wt. to 47 wt.% based upon total weight of GO, an interplanar spacing doo2 of from 0.7 nm to 1.0 nm, a water content of from 5 wt.% or less by total weight of GO, and a sheet size of from 3 pm to 10 pm.

[0031] It should be noted in the description, if a numerical value, concentration or range is presented, each numerical value should be read once as modified by the term "about" (unless already expressly so modified), and then read again as not so modified unless otherwise indicated in context. Also, in the description, it should be understood that an amount range listed or described as being useful, suitable, or the like, is intended that any and every value within the range, including the end points, is to be considered as having been stated. For example,“a range of from 1 to 10” is to be read as indicating each and every possible number along the continuum between about 1 and about 10. Thus, even if specific points within the range, or even no point within the range, are explicitly identified or referred to, it is to be understood that the inventor appreciates and understands that any and all points within the range are to be considered to have been specified, and that inventor possesses the entire range and all points within the range.

[0032] The GO may be reduced thermally to provide the desired conductivity and functional groups. The GO powder may be placed in a furnace and irradiated by infrared radiation under vacuum at 10 Pa to 500 Pa and heated at a rate of from 50 °C/s to 120 °C/s to from 500 to 1000 °C for 10-200 min and then allowed to cool naturally to room temperature.

[0033] The final rGO materials used after reduction of the GO material may have an oxygen content of from 5 wt.% to 40 wt.% by total weight of rGO, with rGO materials having an oxygen content of from 5 wt.% to 25 wt.% being particularly useful in particular embodiments. In certain instances, the rGO used may have an oxygen content of from at least, equal to, and/or between any two of 5 wt.%, 6 wt.%, 7 wt.%, 8 wt.%, 9 wt.%, 10 wt.%, 11 wt.%, 12 wt.%, 13 wt.%, 14 wt.%, 15 wt.%, 16 wt.%, 17 wt.%, 18 wt.%, 19 wt.%, 20 wt.%, 21 wt.%, 22 wt.%, 23 wt.%, 24 wt.%, 25 wt.%, 26 wt.%, 27 wt.%, 28 wt.%, 29 wt.%, 30 wt.%, 31 wt.%, 32 wt.%, 33 wt.%, 34 wt.%, 35 wt.%, 36 wt.%, 37 wt.%, 38 wt.%, 39 wt.%, and 40 wt.%.

[0034] Furthermore, the rGO particles may have a laminar thickness of from 3 to 10 layers, as determined by atomic force microscopy (AFM) and a sheet size of from 0.5 pm to 5 pm as determined by SEM.

[0035] Additionally, the rGO may have a surface area of from 100 m²/g to 1000 m²/g, a pore volume of from 0.5 cm³/g to 5 cm³/g, and a pore size of from 0.5 nm to 10 nm. [0036] By utilizing rGO with optimal reduction levels it can be used as a coating for separators used in energy storage devices, such as Li-S batteries. At the same time, in order to further improve the physical adsorption of the rGO coating for polysulfides, the use of an electrically conductive carbon black. The electrical conductivity of the carbon black material is higher than that of the rGO. Such carbon black materials may have an electrical conductivity that ranges from 10 S/m to 500 S/m, as measured with a resistivity tester (e.g., Guance, GEST-125, China) with a lg mass sample and a test pressure of 2000N. In certain instances, the electrical conductivity may range from at least, equal to, and/or between any two of 10 S/m, 20 S/m, 30 S/m, 40 S/m, 50 S/m, 60 S/m, 70 S/m, 80 S/m, 90 S/m, 100 S/m, 110 S/m, 120 S/m, 130 S/m, 140 S/m, 150 S/m , 160 S/m, 170 S/m , 180 S/m , 190 S/m, 200 S/m, 210 S/m, 220 S/m, 230 S/m, 240 S/m, 250 S/m, 260 S/m, 270 S/m, 280 S/m, 290 S/m, 300 S/m, 310 S/m, 320 S/m, 330 S/m, 340 S/m, 350 S/m, 360 S/m, 370 S/m, 380 S/m, 390 S/m, 400 S/m, 410 S/m, 420 S/m, 430 S/m, 440 S/m, 450 S/m, 460 S/m, 470 S/m, 480 S/m, 490 S/m, and 500 S/m.

[0037] The surface area of the carbon black may range from 50 m²/g to 2000 m²/g. In particular embodiments, the surface area may range from at least, equal to, and/or between any two of 50 m²/g, 60 m²/g, 70 m²/g, 80 m²/g, 90 m²/g, 100 m²/g, 150 m²/g, 200 m²/g, 250 m²/g, 300 m²/g, 350 m²/g, 400 m²/g, 450 m²/g, 500 m²/g, 550 m²/g, 600 m²/g, 650 m²/g, 700 m²/g, 750 m²/g, 800 m²/g, 850 m²/g, 900 m²/g, 950 m²/g, 1000 m²/g, 1050 m²/g, 1100 m²/g, 1150 m²/g, 1200 m²/g, 1250 m²/g, 1300 m²/g, 1350 m²/g, 1400 m²/g, 1450 m²/g, 1500 m²/g, 1550 m²/g, 1600 m²/g, 1650 m²/g, 1700 m²/g, 1750 m²/g, 1800 m²/g, 1850 m²/g, 1900 m²/g, 1950 m²/g, 2000 m²/g.

[0038] The pore volume of the carbon black may range from 0.5 cm³/g to 5 cm³/g. In certain instances, the pore volume may range from at least, equal to, and/or between any two of 0.5 cm³/g, 0.6 cm³/g, 0.7 cm³/g, 0.8 cm³/g, 0.9 cm³/g, 1.0 cm³/g, 1.1 cm³/g, 1.2 cm³/g, 1.3 cm³/g,

1.4 cm³/g, 1.5 cm³/g, 1.6 cm³/g, 1.7 cm³/g, 1.8 cm³/g, 1.9 cm³/g, 2.0 cm³/g, 2.1 cm³/g, 2.2 cm³/g, 2.3 cm³/g, 2.4 cm³/g, 2.5 cm³/g, 2.6 cm³/g, 2.7 cm³/g, 2.8 cm³/g, 2.9 cm³/g, 3.0 cm³/g, 3.1 cm³/g, 3.2 cm³/g, 3.3 cm³/g, 3.4 cm³/g, 3.5 cm³/g, 3.6 cm³/g, 3.7 cm³/g, 3.8 cm³/g, 3.9 cm³/g, 4.0 cm³/g, 4.1 cm³/g, 4.2 cm³/g, 4.3 cm³/g, 4.4 cm³/g, 4.5 cm³/g, 4.6 cm³/g, 4.7 cm³/g, 4.8 cm³/g, 4.9 cm³/g, and 5.0 cm³/g.

[0039] The carbon black may also have an average pore size of from 1 nm to 10 nm. In particular embodiments, the average pore size may range from at least, equal to, and/or between any two of 1.0 nm, 1.5 nm, 2.0 nm, 2.5 nm, 3.0 nm, 3.5 nm, 4.0 nm, 5.0 nm, 5.5 nm, 6.0 nm,

6.5 nm, 7.0 nm, 7.5 nm, 8.0 nm, 8.5 nm, 9.0 nm, 9.5 nm, and 10.0 nm. [0040] The particle size of the carbon black material may range from 10 nm to 100 nm. In certain instances, the particle size may range from at least, equal to, and/or between any two of 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm, 20 nm, 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 25 nm, 26 nm, 27 nm, 28 nm, 29 nm, 30 nm, 31 nm, 32 nm, 33 nm, 34 nm, 35 nm, 36 nm, 37 nm, 38 nm, 39 nm, 40 nm, 41 nm, 42 nm, 43 nm, 44 nm, 45 nm, 46 nm, 47 nm, 48 nm, 49 nm, 50 nm, 51 nm, 52 nm, 53 nm, 54 nm, 55 nm, 56 nm, 57 nm, 58 nm, 59 nm, 60 nm, 61 nm, 62 nm, 63 nm, 64 nm, 65 nm, 66 nm, 67 nm, 68 nm, 69 nm, 70 nm, 71 nm, 72 nm, 73 nm, 74 nm, 75 nm, 76 nm, 77 nm, 78 nm, 79 nm, 80 nm, 81 nm, 82 nm, 83 nm, 84 nm, 85 nm, 86 nm, 87 nm, 88 nm, 89 nm, 90 nm, 91 nm, 92 nm, 93 nm, 94 nm, 95 nm, 96 nm, 97 nm, 99 nm, 99 nm, and 100 nm.

[0041] Suitable commercially available carbon black materials that meet the above criteria include KETJENBLACK^® EC300J and KETJENBLACK^® EC600JD carbon black, both available from AkzoNobel N.V., Super P, Super S, ENSACO^® 150G, ENSACO^® 210G, ENSACO^® 250G, ENSACO^® 260G, ENSACO^® 350G carbon black, each available from Imerys Graphite & Carbon Switzerland SA, BP2000, available from Cabot Corporation, Boston, MA. Acetylene black and other electrically conductive carbon black materials may be used. In particular instances, KETJENBLACK^® EC600JD carbon black has been found to be particularly well suited for incorporation into the composite material used for the separators discussed herein. The KETJENBLACK^® EC600JD carbon black material is known as having a surface area of 1301 m²/g, a pore volume of 2.6 cm³/g, an ash content of 0.1 wt.%, and a primary particle size of 34 nm.

[0042] Such carbon black materials are inexpensive and highly conductive and have high specific surface area and pore volume. These materials are therefore highly suitable for confining large amounts of sulfur. Coating a separator with a combination of the electrically conductive carbon black and rGO thus results in modified separator having a high specific surface area, abundant pore structure, high conductivity, and rich oxygen-containing functional groups. These facilitate effective physical and chemical adsorption of poly sulfides with outstanding electrochemical performance.

[0043] The rGO material may be used in combination with the electrically conductive carbon black in the composite material in an amount of from 15 wt.% to 85 wt.% based upon the total weight of rGO and carbon black. In particular instances, the amount of rGO may range from at least, equal to, and/or between any two of 15 wt.%, 16 wt.%, 17 wt.%, 18 wt.%, 19 wt.%, 20 wt.%, 21 wt.%, 22 wt.%, 23 wt.%, 24 wt.%, 25 wt.%, 26 wt.%, 27 wt.%, 28 wt.%, 29 wt.%, 30 wt.%, 31 wt.%, 32 wt.%, 33 wt.%, 34 wt.%, 35 wt.%, 36 wt.%, 37 wt.%, 38 wt.%, 39 wt.%, 40 wt.%, 41 wt.%, 42 wt.%, 43 wt.%, 44 wt.%, 45 wt.%, 46 wt.%, 47 wt.%, 48 wt.%, 49 wt.%,

50 wt.%, 51 wt.%, 52 wt.%, 53 wt.%, 54 wt.%, 55 wt.%, 56 wt.%, 57 wt.%, 58 wt.%, 59 wt.%,

60 wt.%, 61 wt.%, 62 wt.%, 63 wt.%, 64 wt.%, 65 wt.%, 66 wt.%, 67 wt.%, 68 wt.%, 69 wt.%,

70 wt.%, 71 wt.%, 72 wt.%, 73 wt.%, 74 wt.%, 75 wt.%, 76 wt.%, 77 wt.%, 78 wt.%, 79 wt.%,

80 wt.%, 81 wt.%, 82 wt.%, 83 wt.%, 84 wt.%, and 85 wt.% based upon the total weight of rGO and carbon black.

[0044] In forming the composite rGO/carbon black material, the rGO and carbon black is typically combined with a binder material. The binder may be selected to have superior bonding with the rGO/carbon black materials, good chemical resistance and thermal stability, high mechanical strength and electrolyte infiltration. The binder is typically used in an amount of from 10 wt.% to 60 wt.% by total weight of the final rGO/carbon black composite material. In particular instances, the amount of binder is from at least, equal to, and/or between any two of 10 wt.%, 15 wt.%, 20 wt.%, 25 wt.%, 30 wt.%, 35 wt.%, 40 wt.%, 45 wt.%, 50 wt.%, 55 wt.%, and 60 wt.% by total weight of the final rGO/carbon black composite material. Non limiting examples of suitable binder materials include polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), carboxymethylcellulose sodium (CMC), poly(ethyleneoxide) (PEO), styrene-butadiene rubber (SBR), acrylonitrile copolymer (LA132, LA133), sodium alginate (SA), gelatin, polyacrylic acid (PAA), b -cyclodextrin, etc., and combinations of these.

[0045] The separator itself on which the composite rGO/carbon black material is coated may be those separator or membranes commonly used in electronic storage devices. These may include separators formed of polyethylene, polypropylene, polyimide, glass fiber, ceramic, polyvinylidene fluoride, polyacrylonitrile, etc. The separator may be a single layer or multi layer separator consisting of one or more than two kinds of materials, such as those described above. The separator body may have a porosity of from 30% to 60%, a thickness of from 10 pm to 30 pm, an average pore size of from 20 nm to 60 nm, and a puncture strength of from 300 g to 600 g.

[0046] The final rGO/carbon black composite coating may be deposited on the separator substrate in a thickness ranging from 5 pm to 200 pm. In certain instances, the final coating thickness may range from at least, equal to, and/or between any two of 5 pm, 6 pm, 7 pm, 8 pm, 9 pm, 10 pm, 11 pm, 12 pm, 13 pm, 14 pm, 15 pm, 16 pm, 17 pm, 18 pm, 19 pm, 20 pm, 25 pm, 30 pm, 35 pm, 40 pm, 45 pm, 50 pm, 55 pm, 60 pm, 65 pm, 70 pm, 80 pm, 90 pm, 100 mpi, 110 mpi, 120 mpi, 130 mpi, 140 mpi, 150 mpi, 160 mhi, 170 mhi, 180 mpi, 190 mhi, and 200 mpi.

[0047] The final rGO/carbon black composite material used for the separator may have a variety of properties. The specific surface area (SA) of the composite material may range from 400 m²/g to 1300 m²/g. In particular instances, the SA may range from at least, equal to, and/or between any two of 400 m²/g, 450 m²/g, 500 m²/g, 550 m²/g, 600 m²/g, 650 m²/g, 700 m²/g, 750 m²/g, 800 m²/g, 850 m²/g, 900 m²/g, 950 m²/g, 1000 m²/g, 1050 m²/g, 1100 m²/g, 1150 m²/g, 1200 m²/g, 1250 m²/g and 1300 m²/g.

[0048] The pore volume of the final rGO/carbon black composite material may range from 1.5 to 2.8 cm³/g. In certain embodiments, the pore volume may range from at least, equal to, and/or between any two of 1.5 cm³/g, 1.6 cm³/g, 1.7 cm³/g, 1.8 cm³/g, 1.9 cm³/g, 2.0 cm³/g, 2.1 cm³/g, 2.2 cm³/g, 2.3 cm³/g, 2.4 cm³/g, 2.5 cm³/g, 2.6 cm³/g, 2.7 cm³/g and 2.8 cm³/g.

[0049] The final rGO/carbon black composite material may have a pore size of from 1 nm to 10. The pore size may range from at least, equal to, and/or between any two of 1 nm, 1.5 nm, 2 nm, 2.5 nm, 3.0nm, 3.5 nm, 4.0 nm, 4.5 nm, 5.0 nm, 5.5 nm, 6.0 nm, 6.5 nm, 7.0 nm, 7.5 nm, 8.0 nm, 8.5 nm, 9.0 nm, 9.5 nm, and 10.0 nm.

[0050] The conductivity of the rGO/carbon black composite may range from 10 S/m to 80 S/m, as determined by a Guance GEST-125 conductor resistivity tester, using a lgram sample and a test pressure of 2000N. In certain instances, the conductivity of the composite material may range from at least, equal to, and/or between any two of 10 S/m, 11 S/m, 12 S/m, 13 S/m, 14 S/m, 15 S/m, 16 S/m, 17 S/m, 18 S/m, 19 S/m, 20 S/m, 21 S/m, 22 S/m, 23 S/m, 24 S/m, 25

S/m, 26 S/m, 27 S/m, 28 S/m, 29 S/m, 30 S/m, 31 S/m, 32 S/m, 33 S/m, 34 S/m, 35 S/m, 36

S/m, 37 S/m, 38 S/m, 39 S/m, 40 S/m, 41 S/m, 42 S/m, 43 S/m, 44 S/m, 45 S/m, 46 S/m, 47

S/m, 48 S/m, 49 S/m, and 50 S/m, 51 S/m, 52 S/m, 53 S/m, 54 S/m, 55 S/m, 56 S/m, 57 S/m,

58 S/m, 59 S/m, 60 S/m, 61 S/m, 62 S/m, 63 S/m, 64 S/m, 65 S/m, 66 S/m, 67 S/m, 68 S/m, 69 S/m, 70 S/m, 71 S/m, 72 S/m, 73 S/m, 74 S/m, 75 S/m, 76 S/m, 77 S/m, 78 S/m, 79 S/m, and 80 S/m.

[0051] Furthermore, the final rGO/carbon black composite material may be characterized by a total oxygen content of from 5 wt.% to 15 wt.% by total weight of the composite material. In particular cases, the total oxygen content may range from at least, equal to, and/or between any two of 5.0 wt.%, 5.1 wt.%, 5.2 wt.%, 5.3 wt.%, 5.4 wt.%, 5.5 wt.%, 5.6 wt.%, 5.7 wt.%, 5.8 wt.%, 5.9 wt.%, 6.0 wt.%, 6.1 wt.%, 6.2 wt.%, 6.3 wt.%, 6.4 wt.%, 6.5 wt.%, 6.6 wt.%, 6.7 wt.%, 6.8 wt.%, 6.9 wt.%, 7.0 wt.%, 7.1 wt.%, 7.2 wt.%, 7.3 wt.%, 7.4 wt.%, 7.5 wt.%, 7.6 wt.%, 7.7 wt.%, 7.8 wt.%, 7.9 wt.%, 8.0 wt.%, 8.1 wt.%, 8.2 wt.%, 8.3 wt.%, 8.4 wt.%, 8.5 wt.%, 8.6 wt.%, 8.7 wt.%, 8.8 wt.%, 8.9 wt.%, 9.0 wt.%, 9.1 wt.%, 9.2 wt.%, 9.3 wt.%, 9.4 wt.%, 9.5 wt.%, 9.6 wt.%, 9.7 wt.%, 9.8 wt.%, 9.9 wt.%, 10.0 wt.%, 10.1 wt.%, 10.2 wt.%, 10.3 wt.%, 10.4 wt.%, 10.5 wt.%, 10.6 wt.%, 10.7 wt.%, 10.8 wt.%, 10.9 wt.%, 11.1 wt.%,

11.2 wt.%, 11.3 wt.%, 11.4 wt.%, 11.5 wt.%, 11.6 wt.%, 11.7 wt.%, 11.8 wt.%, 11.9v, 12.0 wt.%, 12.1 wt.%, 12.2 wt.%, 12.3 wt.%, 12.4 wt.%, 12.5 wt.%, 12.6 wt.%, 12.7 wt.%, 12.8 wt.%, 12.9 wt.%, 13.0 wt.%, 13.1 wt.%, 13.2 wt.%, 13.3 wt.%, 13.4 wt.%, 13.5 wt.%, 13.6 wt.%, 13.7 wt.%, 13.8 wt.%, 13.9 wt.%, 14.0 wt.%, 14.1 wt.%, 14.2 wt.%, 14.3 wt.%, 14.4 wt.%, 14.5 wt.%, 14.6 wt.%, 14.7 wt.%, 14.8 wt.%, 14.9 wt.%, and 15.0 wt.% by total weight of the composite material.

[0052] Various coating methods may be used for coating the separator with the composite material. In particular embodiments, the composite material and coating are prepared by a slurry coating method, which is illustrated in FIG. 1. In such method, the rGO and electrically conductive carbon black are dispersed in a solvent, such as by ultrasonic means. The solvent may be any suitable solvent capable of dissolving and/or dispersing the materials described herein without detrimentally affecting the components of the composition. Suitable solvents may include N-methyl-2pyrrolidone (NMP), deionized water, ethanol, acetone, isopropanol, dimethylformamide (DMF), and combinations of these. The solvent is evaporated and the resulting material dried to form a well-distributed rGO/carbon black composite material.

[0053] A binder, such as those previously described (e.g., PVDF), which may be dissolved in a solvent. The binder and solvent are stirred together to obtain a homogenous solution. The concentration of binder in solution ranges from 0.1 wt.% to 5 wt.% by total weight of the solution. In particular cases, the binder content of the solution may range from at least, equal to, and/or between any two of 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%, 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 0.9 wt.%, 1.0 wt.%, 1.1 wt.%, 1.2 wt.%, 1.3 wt.%, 1.4 wt.%, 1.5 wt.%, 1.6 wt.%, 1.7 wt.%, 1.8 wt.%, 1.9 wt.%, 2.0 wt.%, 2.1 wt.%, 2.2 wt.%, 2.3 wt.%, 2.4 wt.%, 2.5 wt.%, 2.6 wt.%, 2.7 wt.%, 2.8 wt.%, 2.9 wt.%, 3.0 wt.%, 3.1 wt.%, 3.2 wt.%, 3.3 wt.%, 3.4 wt.%, 3.5 wt.%, 3.6 wt.%, 3.7 wt.%, 3.8 wt.%, 3.9 wt.%, 4.0 wt.%, 4.1 wt.%, 4.2 wt.%, 4.3 wt.%, 4.4 wt.%, 4.5 wt.%, 4.6 wt.%, 4.7 wt.%, 4.8 wt.%, 4.9 wt.%, and 5.0 wt.% by total weight of the solution.

[0054] The binder solution is then combined with the rGO/carbon black composite material and the materials are uniformly mixed to form a slurry. The slurry is then applied to a pristine separator, such as those previously described, so that a coating is applied thereon. The coated separator is then dried to form the final modified separator. Such modified separators have good strength and flexibility. The thickness of the coating layer is easy to control and is suitable for scaling to produce in commercial quantities.

[0055] The modified separator having the coating of rGO/carbon black can be used in a variety of energy storage applications or devices (e.g., fuel cells, batteries, supercapacitors, electrochemical capacitors, lithium-ion battery cells or any other battery cell, system or pack technology). The term“energy storage device” can refer to any device that is capable of at least temporarily storing energy provided to the device and subsequently delivering the energy to a load. Furthermore, an energy storage device may include one or more devices connected in parallel or series in various configurations to obtain a desired storage capacity, output voltage, and/or output current. Such a combination of one or more devices may include one or more forms of stored energy. By way of example a lithium-sulfur battery can include the previously described modified separator, with the separator being positioned between the anode and cathode of the device. In other instances the separator may be used on or incorporated in an anode electrode and/or a cathode electrode. In another example, the energy storage device can also, or alternatively, include other technologies for storing energy, such as devices that store energy through performing chemical reactions (e.g., fuel cells), trapping electrical charge, storing electric fields (e.g., capacitors, variable capacitors, ultracapacitors, and the like), and/or storing kinetic energy (e.g., rotational energy in flywheels).

[0056] In a typical lithium-sulfur battery, the modified separator is incorporated into the battery at a position between the anode and cathode. The use of such separator can thus be used to greatly increase the number of discharge/recharge cycles and the life of the battery. Because S4^m -S8^m polysulfide ions, which have a particle size of 0.7 nm or more, are greater in size than the pore size of the composite coating, they are physically adsorbed and prevented from migrating through electrolyte solution through the separator membrane to the anode so that they cannot be permanently deposited on the anode. Furthermore, the functional groups of the composite material further help to trap the polysulfides through chemical adsorption thus further preventing their migration. While the composite is abundant in functional groups to facilitate chemical adsorption of the poly sulfides, good conductivity of the composite material is still retained, due to the use of the rGO and electrically conductive carbon black.

[0057] The following examples serve to further illustrate various embodiments and applications EXAMPLES

EXAMPLE 1

[0058] The following is an exemplary example of preparing a modified separator according to particular embodiments of the invention.

[0059] 1. Graphene oxide (GO) powder is placed in a furnace tube and irradiated by infrared radiation at a vacuum of from 10 Pa to 500 Pa. The rGO material is obtained by heating the furnace tube at a heating rate of from 50 °C/s to 120 °C/s to from 500 °C to 1000 °C for from 10 to 200 min. The material is then allowed to cool naturally to room temperature.

[0060] 2. The resulting rGO is mixed with KETJENBLACK^® EC600JD carbon black in an ethanol solvent at 25 °C to 30 °C for 2 to 3 hours to obtain a homogeneous suspension. The mass ratio of rGO and carbon black is 0.2 to 5, with the concentration of rGO/carbon black in solution being from 1 mg/mL to 10 mg/mL. Ultrasonic mixing is used to mix the components at an ultrasonic mixing power of 500 W to 700 W.

[0061] 3. The as-obtained suspension is heated from 60°C to 90 °C for 12 hr to 24 hr at a heating rate of from 3 °C/min to 5 °C/min under a vacuum of from lOPa to 200 Pa. The rGO/ carbon black composite materials are obtained.

[0062] 4. The binder and a solvent are stirred at from 25 °C to 30 °C for 12 hr to 24 hr to obtain a homogenous solution. The rGO/carbon black composite materials are dissolved in the solution and stirred at from 25°C to 30 °C for 24 hr to 96 hr to obtain a homogenous slurry. The stirring speed is from 800 r/min to 1500 r/min, the concentration of binder solution is from 0.1 wt.% to 5 wt.%, and the mass ratio of rGO/carbon black composite to binder is from 1 to 8.

[0063] 5. The resulting slurry is coated onto a pristine separator by a coating machine. The coating speed is from 2 mm/s to 50 mm/s and the coating thickness is from 5 pm to 200 pm.

[0064] 6. The coated separator is dried at from 40 °C to 60 °C for 12 hr to 24 hr at a heating rate of from 2 °C/s to 20 °C/s and the rGO/carbon black composite modified separator is obtained.

EXAMPLE 2

Cathode and Cell Preparation

[0065] A cathode for the samples discussed below was synthesized using the following procedure. A sublimed sulfur (S, Sigma- Aldrich) and KETJENBLACK^® EC300J (mass content of sulfur was 70 wt.%) were mixed evenly. The mixed materials were sealed in a container filled with argon gas. The mixture was then heated at 155 °C for 8 hr in muffle furnace and cooled down to room temperature.

[0066] The cathode slurry was produced by mixing 70 wt.% cathode materials, 20 wt.% conductive carbon black (Super-P) and 10 wt.% PVDF binder in N-methyl-2-pyrrolidinone (NMP). After stirring, the uniform slurry was cast onto aluminum foils and then dried at 60 °C for 24 hr. The electrodes were cut into round electrodes with a diameter of 10 mm.

[0067] Coin-type (CR2032) cells with Li foil as the counter electrode were assembled in an argon-filled glovebox. The electrolyte used was 0.2M L1NO3 in 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)/(DOL:DME, 1:1 , V/V)). The separator used was pristine polypropylene single layer separator (Celgard-2400) having a thickness of 25pm, a porosity of 41%, an average pore size is 43 nm, and a puncture strength of 450g for the non- coated and coated separators. The separators were cut into a circular shape with a diameter of 19mm for use in the coin cells. The conductivity of the separators was tested using a four- probe tester (RTS-9, Guangzhou Four-Point Probe Technology Co.). The coin cells were also tested with a battery test system with a voltage range of 1.7-2.8 V. 1 C at 1675 mA/g.

Comparative Sample 1

[0068] The cathode prepared above was combined with a pristine separator (Celgard-2400 separator) and Li foil and assembled into a coin cell.

Comparative Sample 2

[0069] Graphene oxide (GO) powder at 10 g with a sheet size of 5 pm was placed in a furnace tube and irradiated by infrared radiation at a vacuum degree of from lOPa to 500 Pa. The rGO was obtained by heating the furnace tube at a heating rate of from 80 °C/s to 600 °C/s for 60 min and then cooled naturally to room temperature. The obtained rGO powder showed a lamellar thickness of 4-5 layers, a sheet size of 1pm, and an oxygen content is 17.6 wt.% by total weight of the rGO. The specific surface area was 411.29 m²/g.

[0070] 0.25 g polyvinylidene fluoride (PVDF) as a binder and 24.75 g N-methyl-2-pyrrolidone (NMP) were stirred at 25 °C for 24 hr to obtain a homogenous solution. The obtained 0.75 g rGO powder was added into the above PVDF solution and magnetically stirred at 25 °C for 72 hr at a speed of 1000 r/min. A homogenous rGO slurry was obtained.

[0071] The slurry was coated onto a pristine separator (Celgard-2400) by a coating machine. The coating speed was 10 mm/s and the coating thickness was 15 pm. The coated separator was dried at 50 °C for 24 hr with a heating rate of 5 °C/s to obtain an rGO modified separator. The conductivity of the separator was 0.0392 S/cm. The cathode, rGO modified separator and Li foil were assembled into a coin-cell.

Comparative Sample 3

[0072] 0.25g polyvinylidene fluoride (PVDF) and 24.75 g N-methyl-2-pyrrolidone (NMP) were stirred at 25 °C for 24 hr to obtain a homogenous solution. 1 g KETJENBLACK^® EC600JD carbon black powder (BET=1301.04 m²/g) was added into the above PVDF solution and magnetically stirred at 25 °C for 96 hr at a speed of 1000 r/min. The homogenous slurry was obtained.

[0073] The slurry was coated onto a pristine separator (Celgard-2400) by a coating machine. The coating speed was 10 mm/s and the coating thickness was 15 pm. The coated separator was dried at 50 °C for 24 hr with a heating rate of 5 °C/s and the carbon black modified separator was obtained. The conductivity was 0.0476 S/cm.

[0074] The cathode, carbon black modified separator and Li foil were assembled into a coin cell.

Embodiment Sample 1

[0075] rGO was prepared in the same way as described for Comparative Sample 1 above. 2 g of rGO, 1 g of KETJENBLACK^® EC600JD carbon black powder (BET= 1301.04 m²/g), and 600 mL of ethanol were mixed at 25 °C by ultrasonic mixing under a power of 500 W for 3 hr to obtain a homogenous suspension. The as-obtained suspension was heated to 80 °C at a heating rate of 5 °C/min at a vacuum degree of 90 Pa. A rGO/carbon black composite material having a rGO/carbon black mass ratio of 2: 1 was obtained. The oxygen content of the final composite was 12.1 wt.%, a specific surface area is 775.37 m²/g, and a pore size mainly distributed at from 3 nm to 4 nm.

[0076] 0.25 g polyvinylidene fluoride (PVDF) as a binder and 24.75 g N-methyl-2-pyrrolidone (NMP) were stirred at 25 °C for 24 hr to obtain a homogenous solution. 1 g of the 2: 1 rGO/carbon black composite powder was added into the above PVDF solution and magnetically stirred at 25 °C for 72 hr at a speed of 1000 r/min. A homogenous 2:1 rGO/carbon black composite slurry was obtained. The slurry was coated onto a pristine separator (Celgard- 2400) by a coating machine. The coating speed was 10 mm/s and the coating thickness was 15 pm. The slurry coated separator was dried at 50 °C for 24 hr with a heating rate of 5 °C/s and the 2: 1 rGO/carbon black composite modified separator was obtained. The conductivity was 0.0501 S/cm.

[0077] FIGS. 2 and 3 show scanning electron microscope images of the sample produced at a magnification of 10,000X and 1,000X, respectively. FIG. 2 shows the composite structure of 2: 1 rGO/carbon black composite materials. As can be seen, nanoparticles of carbon black and sheet structure of rGO are well combined together. FIG. 3 shows the layer by layer structure of the 2:1 rGO/carbon black composite modified separator. The rGO forms the skeleton while the carbon black is filled within the graphene sheets. The bifunctional structure of the composite facilitates the increased adsorption of polysulfide.

[0078] The cathode, 2: 1 rGO/carbon black composite modified separator and Li foil were assembled into a coin-cell.

Embodiment Sample 2

[0079] rGO was prepared in the same way as Comparative Sample 1. 1.5 g rGO, 1.5 g KETJENBLACK^® EC600JD carbon black powder (BET=1301.04 m²/g) and 600 mL ethanol were mixed at 25 °C by ultrasonic mixing under a power of 500 W for 3 hr to obtain a homogenous suspension. The as-obtained suspension was heated to 80 °C at a heating rate of 5 °C/min at a vacuum degree of 90 Pa. A rGO/carbon black composite material having a rGO/carbon black mass ratio of 1 : 1 was obtained. The oxygen content of final composite was 10.2 wt.%, with a specific surface area is 929.44 m²/g, and a pore size mainly distributed at from 3 nm to 4 nm.

[0080] 0.25 g polyvinylidene fluoride (PVDF) and 24.75 g N-methyl-2-pyrrolidone (NMP) were stirred at 25 °C for 24 hr to obtain a homogenous solution. 1 g of the 1 : 1 rGO/carbon black composite powder was added into the above PVDF solution and magnetically stirred at 25 °C for 72 hr at a speed of 1000 r/min. A homogenous 1 : 1 rGO/carbon black composite slurry was obtained. The slurry was coated onto the pristine separator (Celgard-2400) by a coating machine. The coating speed was 10 mm/s and the coating thickness was 15 pm. The slurry coated separator was dried at 50 °C with a heating rate of 5 °C/s and the 1 : 1 rGO/ carbon black composite modified separator was obtained. The conductivity was 0.8333 S/cm.

[0081] The cathode, 1 : 1 rGO/carbon black composite modified separator and Li foil were assembled into a coin-cell. Embodiment Sample 3

[0082] rGO was prepared in the same way as Comparative Sample 1. 1 g rGO, 2g KETJENBLACK^® EC600JD carbon black powder (BET=1301.04 m²/g) and 600mL ethanol were mixed at 25 °C by ultrasonic mixing under a power of 500 W for 3 hr to obtain a homogenous suspension. The as-obtained suspension was heated to 80 °C at a heating rate of 5 °C/min at a vacuum degree of 90 Pa. A rGO/carbon black composite material having a rGO/carbon black mass ratio of 1 :2 was obtained. The oxygen content of final composite was 8.8 wt.%, with a specific surface area is 1093.39 m²/g, and a pore size mainly distributed at from 3 nm to 4 nm.

[0083] 0.25 g polyvinylidene fluoride (PVDF) and 24.75 g N-methyl-2-pyrrolidone (NMP) were stirred at 25 °C for 24 hr to obtain a homogenous solution. 1 g of 1:2 rGO/carbon black composite powder was added into the above PVDF solution and magnetically stirred at 25 °C for 72 hr at a speed of 1000 r/min. A homogenous 1 :2 rGO/carbon black composite slurry was obtained. The slurry was coated onto a pristine separator (Celgard-2400) by a coating machine. The coating speed was 10 mm/s and the coating thickness was 15 pm. The slurry coated separator was dried at 50 °C with a heating rate of 5 °C/s and a 1 :2 rGO/carbon black composite modified separator was obtained. The conductivity was 0.9286 S/cm.

[0084] The cathode, 1 :2 rGO/carbon black composite modified separator and Li foil were assembled into coin-cell.

EXAMPLE 3

[0085] Each of Comparative Samples 1-3 and Embodiment Sample 1 from Example 2 were assembled into coin-cell and tested by battery test system. The charge/discharge current density is both 1C (1675 mA/g) and the coin-cell charge/ discharge for 1000 cycles.

[0086] FIG. 4 shows a plot of cycle performance of Comparative Samples 1-3 and Embodiment Sample 1 after 1000 battery discharge/recharge cycles. As can be seen in FIG. 4, compared with Comparative Sample 1 using a pristine separator, Comparative Sample 2 using an rGO modified separator and Comparative Sample 3 using a carbon black modified separator, the Embodiment Sample 1 with a 2: 1 rGO/carbon black modified separator had a better performance both for specific capacity and capacity retention. Even though the Comparative Sample 2 using the rGO modified separator had a high initial capacity, it decayed rapidly and showed relatively lower capacity retention compared with Embodiment Sample 1. Generally speaking, the Embodiment Sample 1 with a 2:1 rGO/carbon black modified separator exhibited the best performance, owing to the layer by layer structure of rGO/carbon black modified separator. Specifically, the Embodiment Sample 1 with a 2: 1 rGO/carbon black modified separator exhibited initial specific discharge capacity of 1929.9 mAh g ¹ at 0.1 C, 828.6 mAh g ¹ at 1C, and maintained the capacity retention of 80% after 500 cycles at 1 C. Even after 1000 cycles, the specific discharge capacity still achieved 520.4 mAh g ¹.

EXAMPLE 4

[0087] Each of Comparative Samples 1-3 and Embodiment Sample 1 from Example 2 were assembled into coin-cell and tested by battery test system. The charge/discharge current density was 0.1 C, 0.2 C, 0.5 C, 1 C, 2C, and 0.1 C (1C =1675 mA/g) and the coin-cell was charged/discharged for 10 cycles with each current density, respectively.

[0088] FIG. 5 shows a plot of the rate performance of Comparative Samples 1-3 and Embodiment Sample 1 at different charge/discharge current densities (0.1 C, 0.2 C, 0.5 C, 1 C, 2C, and 0.1 C). As can be seen in FIG. 5, Embodiment Sample 1 with a 2:1 rGO/carbon black modified separator had the best performance, especially at high current density. This can be ascribed to the synergism effect between the rGO and carbon black. It can be seen that the conductivity of rGO/carbon black modified separator is increased gradually with the increase of the conductive carbon black, and the 2: 1 rGO/carbon black modified separator showed the highest conductivity. This high conductivity is beneficial to the rate performance of the battery, and it explains the excellent performance of the 2: 1 rGO/carbon black modified separator at high current densities. The modified separator with high rate performance is useful for high power Li-S batteries.

[0089] While the invention has been shown in some of its forms, it should be apparent to those skilled in the art that it is not so limited but is susceptible to various changes and modifications without departing from the scope of the invention. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.

Claims

CLAIMS We claim:

1. A separator for an electrical energy storage device or battery, the separator comprising: a separator body having a composite material coated thereon, the composite material comprising:

a reduced graphene oxide (rGO) having an oxygen content of from 5 wt.% to 40 wt.% by total weight of rGO; an electrically conductive carbon black; and a binder.

2. The separator of claim 1, wherein: the rGO of the composite material is present in an amount of from 15 wt.% to 85 wt.% based upon the total weight of the rGO and electrically conductive carbon black.

3. The separator of any one of claims 1 and 2, wherein: the rGO has an oxygen content of from 5 wt.% to 25 wt.% by total weight of rGO.

4. The separator of any one of claims 1-3, wherein: composite material has at least one of a specific surface area (SA) of from 400 m²/g to 1300 m²/g, a pore volume of from 1.5 to 2.8 cm³/g, a pore size of from 1 nm to 10 nm, a conductivity of from 15 S/m to 80 S/m, and an oxygen content of from 5 wt.% to 15 wt.% by total weight of the composite material.

5. The separator of any one of claims 1-4, wherein: the composite material coating has a coating thickness of from 5 pm to 200 pm.

6. The separator of any one of claims 1-5, wherein: the rGO has a laminar thickness of from 3 to 10 layers and a sheet size of from 0.5 pm to 5 pm.

7. The separator of any one of claims 1-6, wherein: the binder comprises at least one of polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), carboxymethylcellulose sodium (CMC), and poly(ethyleneoxide) (PEO), styrene-butadiene rubber (SBR), acrylonitrile copolymer, sodium alginate (SA), gelatin, polyacrylic acid (PAA), and b-cyclodextrin.

8. The separator of any one of claims 1-7, wherein: the binder is present in the composite material in an amount of from 10 wt.% to 60 wt.% by total weight of the composite material.

9. The separator of any one of claims 1-8, wherein: the separator body comprises at least one of polyethylene, polypropylene, polyimide, glass fiber, ceramic, polyvinylidene fluoride, and polyacrylonitrile.

10. The separator of any one of claims 1-9, wherein: the separator body has at least one of a porosity of from 30% to 60%, a thickness of from 10 pm to 30 pm, an average pore size of from 20 nm to 60 nm, and a puncture strength of from 300 g to 600 g.

11. The separator of any one of claim 1-10, wherein: the electrically conductive carbon black has at least one of a conductivity of from 10 S/m to 500 S/m, a surface area of the carbon black may range from 50 m²/g to 2000 m²/g, a pore volume from 0.5 cm³/g to 5 cm³/g, a pore size of from 1 nm to 10 nm, and a particle size of from 10 nm to 100 n .

12. The separator of any one of clai s 1-11, wherein: the separator is incorporated into an electrical energy storage device or battery.

13. The separator of claim 12, wherein: the electrical energy storage device is a lithium-sulfur electrical energy storage device or battery.

14. A method of forming a separator for an electrical energy storage device or battery, the method comprising: combining a reduced graphene oxide (rGO) having an oxygen content of from 5 wt.% to 40 wt.%, an electrically conductive carbon black, and a binder to form a composite material; and coating a separator body with the composite material so that composite material is coated thereon.

15. The method of claim 14, wherein: the rGO of the composite material is present in an amount of from 15 wt.% to 85 wt.% based upon the total weight of the rGO and electrically conductive carbon black.

16. The method of any one of claims 14 and 15, wherein: the rGO has an oxygen content of from 5 wt.% to 25 wt.% by total weight of rGO.

17. The method of any one of claims 14-16, wherein: composite material has at least one of a specific surface area (SA) of from 400 m²/g to 1300 m²/g, a pore volume of from 1.5 cm³/g to 2.8 cm³/g, a pore size of from 1 nm to 10 nm, a conductivity of from 15 S/m to 80 S/m, and an oxygen content of from 5 wt.% to 15 wt.% by total weight of the composite material.

18. The method of any one of claims 14-17, wherein: the composite material coating has a coating thickness of from 5 pm to 200 pm.

19. The method of any one of claims 14-18, wherein: the rGO has a laminar thickness of from 3 to 10 layers and a sheet size of from 0.5 pm to 5 pm.

20. The method of any one of claims 14-19, further comprising: incorporating the separator into an electrical energy storage device or battery.