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  • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

• [n] represents the nth reference cited in the reference list.
• [1] represents the first reference cited in the reference list, namely, Manthiram, A.; Fu, Y.; Chung, S.-H.; Zu, C; Su, Y.-S. Rechargeable Lithium- Sulfur Batteries. Chem. Rev. 2014, 114, 11751-11787. FIELD OF THE INVENTION
• the present invention relates generally to fabrication of a regenerative polysulfide- scavenging layer, and more particularly to methods and systems for fabricating a regenerative poly sulfide- scavenging layer enabling lithium- sulfur (Li-S) batteries with high energy density and prolonged cycling life.
• Li-S lithium- sulfur
• Li-S batteries Lithium- sulfur (Li-S) batteries, notable for high theoretical energy capacity, Li-S batteries, Li-S batteries, notable for high theoretical energy capacity, Li-S batteries, Li-S batteries, notable for high theoretical energy capacity, Li-S batteries, Li-S batteries, notable for high theoretical energy capacity, Li-S batteries, Li-S batteries, notable for high theoretical energy capacity, Li-S batteries, Li-S batteries, notable for high theoretical energy capacity, Li-S batteries, notable for high theoretical energy capacity
• polysulfides anions through electrostatic repulsion High loading of high-cost Nafion ® , however, is required to achieve sufficient blocking effect (e.g., 0.7 mg cm "2 loading of Nafion ® for cathodes with 0.53 mg cm " loading of sulfur) [12] .
• the metal-oxide layers, represented by V 2 O 5 layers, allow effective transport of Li + ions while block the diffusion of polysulfides [14] .
• Such inorganic coatings are generally achieved by sol-gel process, which are often brittle and defective.
• One of the objectives of this invention is to fabricate regenerative polysulfide- scavenging layers (RSL) enabling lithium- sulfur batteries with high energy density and prolonged cycling life.
• RSL regenerative polysulfide- scavenging layers
• this invention discloses an effective polysulfide-blocking strategy based on regenerative polysulfide- scavenging layers (RSL), which can dynamically block the diffusion of polysulfides and regenerate themselves during cycling.
• RSL regenerative polysulfide- scavenging layers
• the invention relates to a method for fabricating a RSL.
• the method includes embedding nanowires or nanocrystals of metal oxides with a membrane of carbon nanotubes (CNTs); and forming the RSL with the embedded nanowires or nanocrystals of the metal oxides and the membrane, so as to enable lithium- sulfur batteries with high energy density and prolonged cycling life.
• CNTs carbon nanotubes
• the nanowires or nanocrystals of the metal oxides and the membrane form a continuous fibrous structure.
• the nanowires or nanocrystals of the metal oxides are porous with a diameter of about 30 nanometers.
• the metal oxides include V 2 O 5 .
• the nanowires or nanocrystals of the metal oxides are layered crystalline structures.
• the weight percentage of the CNTs in the RSL is about 9.8%.
• the membrane is flexible and conductive.
• the RSL has a thickness of 15 ⁇ and includes porous CNTs layers sandwiched with a V 2 0 5 -rich layer in the center.
• the method further includes disposing the RSL between a layer of sulfur cathode and a separator.
• the location of the embedding nanowires or nanocrystals of metal oxides with a membrane is in the center of the membrane.
• as-generated polysulfides are adsorbed by or reacted with the RSL and are immobilized onto the RSL denoted as Polysulfides-RSL complexes; and a subsequent charging process strips away the immobilized Polysulfides-RSL complexes and regenerates the RSL, enabling dynamic blocking of the as-generated
• the invention in another aspect, relates to a method of fabricating a regenerative poly sulfide- scavenging layer (RSL) that includes synthesizing composites of metal oxide nanowires intertwined with carbon nanotubes (CNTs) using hydrothermal reaction; dispersing the CNTs and CNTs/metal oxide composites in ethanol by sonication; filtrating dispersion of the CNTs, the CNTs/metal oxide composites and the CNTs through a polypropylene membrane; and forming a flexible triple-layered membrane to fabricate the RSL.
• RSL regenerative poly sulfide- scavenging layer
• the metal oxide nanowires are made from V 2 O 5 .
• the method further includes entangling the CNTs from the dispersing to form CNTs networks for effective electron conduction, thereby allowing performing effecting redox reaction with CNTs/ V 2 O 5 RSL.
• the method further includes the following steps after the dispersing: forming a CNT suspension having a first concentration and a CNTS/V 2 O 5 suspension having a second concentration, respectively; selecting a first volume of the CNT suspension and a second volume of the CNTS/V 2 O 5 suspension; and filtering the first volume of the CNT suspension and the second volume of the CNTs/metal oxide suspension through the polypropylene membrane.
• the method further includes drying the flexible triple-layered membrane at a first temperature for a first predetermined time; and punching the flexible triple- layered membrane into a round shape with a diameter.
• the synthesizing includes: dispersing a first mass of ammonium metavanadate and a second mass of P123 (EO20PO70EO20) in a first volume of deionized (DI) water with a second volume of 2 molar (M) HC1 to form a mixture; adding activated CNTS to the mixture and sonicating for a first time; stirring the mixture at a room temperature for a second time; transferring the mixture to an autoclave; heating the mixture at a second temperature for a second predetermined time; rinsing the mixture with the DI water and ethanol for three times; and drying the mixture at a third temperature in vacuum.
• the weight of the RSL on each separator is around 0.4-0.6 mg cm " .
• the invention in yet another aspect, relates to a lithium- sulfur battery that includes an anode using lithium metal; a polypropylene separator adjacent to the anode; a sulfur anode opposed to the anode; and a regenerative polysulfide-scavenging layer (RSL) disposed between the polypropylene separator and the sulfur anode.
• the RSL is fabricated by embedding nanowires or nanocrystals of metal oxides with a membrane of carbon nanotubes (CNTs).
• the sulfur anode is prepared using a slurry casting method.
• sulfur, carbon black and polyvinylidene fluoride (PVDF) are mixed with a weight ratio of 5:4: 1 to form a homogenous slurry with N-methyl-2-pyrrolidone, and then are casted onto a carbon-coated aluminum foil with a doctor blade; and for electrodes with higher sulfur loading with a density up to 6 mg cm " , carbon/sulfur composites, carbon nanofiber, carbon black and PVDF are mixed with a weight ratio of 88:4: 1 :7 to form a slurry.
• porous carbon particles are fabricated using Kejent black, and carbon and sulfur composites are prepared using liquid infiltration method at 159 °C with a weight ratio of 1 :4.
• the electrodes are dried at 70 °C in vacuum for four hours and then cut into pieces with a diameter of 16 mm.
• Lithium- sulfur batteries notable for high theoretical energy density, environmental benignity and low cost, hold great potentials for next-generation energy storage. Polysulfides, the intermediates generated during cycling, may shuttle between electrodes, compromising the energy density and cycling life.
• a class of regenerative polysulfide- scavenging layers is reported, which effectively immobilize and regenerate polysulfides, especially for electrodes with high sulfur loadings (e.g., 6 mg cm " ).
• the resulted cells exhibit high gravimetric energy density of 365 Wh. kg “1 , initial areal capacity of 7.94 rnAh cm “2 , a low self-discharge rate of 2.45% after resting for 3 days and dramatically prolonged cycling life.
• FIG. 1 shows a schematic presentation of a Li-S cell with a regenerative polysulfide- scavenging layer (RSL), according to one embodiment of the invention.
• the RSL is made from a CNTs membrane of which the center is embedded with interpenetrating nanowires or nanocrystals of metal oxides, (i) During discharging, as-generated polysulfides are adsorbed by or reacted with the RSL, immobilized onto the RSL denoted as [Polysulfides-RSL] complexes, (ii) Subsequent charging process strips away the immobilized species and regenerates the RSL, enabling dynamic blocking of the polysulfides.
• FIG. 2 shows a structure of the CNTS/V 2 O 5 composites and CNTS/V 2 O 5 RSL, according to one embodiment of the invention,
• (a) SEM image of a CNTS/V 2 O 5 composite with a fibrous structure made from interpenetrative V 2 O 5 nanowires and CNTs.
• (b) TEM images of the CNTS/V 2 O 5 composites, showing a continuous and porous structure with average nanowire diameter of -30 nm.
• (c) High-resolution TEM image and its corresponding selected area FFT image (inset) of the CNTS/V 2 O 5 composites
• (d) X-ray diffraction profile of the CNTS/V 2 O 5 composites, e.
• FIGS. 3(a)-3(h) show electrochemical performance of Li-S cells with Celgard
• FIG. 3(a) shows cyclic voltammetries obtained at a scanning rate of 0.2 mV s "1 .
• FIG. 3(b) shows Nyquist plots showing a reduced charge-transfer resistance with the RSL.
• FIG. 3(c) shows rate performance at 0.3 C, 0.5 C, 1 C, 2 C, 4 C and 0.3 C rate (sulfur loading 2 mg cm " ).
• FIGS. 3(d)-3(f) show galvanostatic cycling performance at 1 C rate, 0.1 C rate and 0.2 C rate, respectively.
• the empty bullets (o) represent the discharge capacity and circle bullets ( ⁇ ) represent the Coulombic efficiency.
• Cells in FIG. 3(e) were activated at 0.05 C rate while cells in FIG. 3(f). were activated at 0.1 C rate.
• FIGS. 3(g) and 3(h) shows self-discharge tests. The cells were cycled at 0.2 C for 9 cycles, stopped at 2.1 V during 10 th discharge and rested for 3 days before the discharging process was resumed. Voltage-capacity profiles of the cells were recorded, suggesting the cell with CNTS/V 2 O 5 RSL exhibit a dramatically reduced self- discharge rate.
• FIG. 4 shows SEM images and element-mapping of CNTS/V 2 O 5 RSL at discharged and charged stages. Li-S cells were cycled at 0.3 C between 1.7 and 2.8 V, according to one embodiment of the invention.
• FIG. 4(a) was interrupted at 2.05 V during the discharging and
• FIG. 4(b) was interrupted at 2.60 V during the charging.
• the arrows from circles show the direction of the line scan, while the circles represent the starting and ending points.
• a purple line represents sulfur and an orange line represents vanadium.
• Scale bars are 20 ⁇ for FIG. 4(a) and 25 ⁇ for FIG. 4(b).
• FIG. 5 shows reactions between V 2 0 5 and polysulfides probed by x-ray photoelectron spectroscopy (XPS), according to one embodiment of the invention,
• Sulfur 2p core spectra of Li 2 S6 showing the terminal (S T "1 ) and bridging (S B °) sulfur atoms with an expected ratio of 1:2.
• (d) Sulfur 2p core spectra of the V 2 0 5 /sulfide compound.
• the formation of polythionate groups indicates redox reactions between Li 2 S6 and V 2 0 5 .
• FIG. 6 shows correlations between cell performance, work function of oxide moieties and bond energy between the oxides and polysulfides, according to one embodiment of the invention,
• a comparison of the bond energies between the metal oxides and polysulfides (Light green) with the specific capacity of the corresponding Li-S cells after 100 cycles at 1 C (Green). These cells were made using RSL containing these metal oxides, respectively.
• the bond energies were calculated with Flore's equation based on dissociation energy, electronegativity and chemical hardness of metal oxides and polysulfides.
• FIG. 7 shows the RSL with scavenging capability for polysulfides and regenerative ability, according to one embodiment of the invention.
• FIG. 8 shows direct-current polarization profile of a CNTS/V2O5 RSL and a V 2 0 5 composite layer (with same area) at 0.2 V, showing that the conductivity of the V 2 O 5 composite layer (without extra CNTs layers) is 4 magnitudes lower than that of the CNTS/V 2 O 5 RSL, according to one embodiment of the invention.
• Such significant difference in conductivity is due to their fabrication method.
• To prepare the CNTS/V 2 O 5 RSL dispersions of CNTs, V 2 O 5 composite, and CNTs were sequentially filtrated onto a porous separator, during which the CNTs and the CNTs from the V 2 O 5 composite can effectively entangled forming effectively conductive networks.
• V 2 O 5 composite dispersion was filtrated to form the V 2 O 5 composite layer, resulting in significantly lower electronic conductivity because of less amount of CNTs present and less effective conductive network.
• FIG. 9 shows thermogravimetric analysis (TGA) plot of the CNT/V 2 O 5 composites indicating that the composites contain about 9.8% of CNT, according to one embodiment of the invention.
• FIG. 10 shows digital photographs of CNTS/V 2 O 5 RSL: (A) as-prepared, (B) folded; and (C) recovered states, according to one embodiment of the invention.
• FIG. 11 shows galvanostatic cycling performance of the cells with Celgard PP separator, with CNTs (2) RSL or CNTs/V 2 0 5 RSL. All cells were cycled at 0.3 C rate for 3 cycles and then 1 C rate for 250 cycles, according to one embodiment of the invention.
• FIG. 12 shows capacity retention of the cells made with CNTS/V 2 O 5 RSL prepared with different CNTS/V 2 O 5 mass ratios but a fixed composite mass, according to one embodiment of the invention.
• FIG. 13 shows galvanostatic cycling performance of Li-S batteries with Celgard PP separator, CNTs RSL, CNTS/V 2 O 5 RSL, and V 2 O 5 composite layer at 0.3 C rate, according to one embodiment of the invention.
• the CNTS/V 2 O 5 RSL shows better capacity performance than CNTs RSL due to the regenerative scavengers
• CNTs RSL shows better performance than the Celgard PP separator due to its ability to adsorb and desorb polysulfides.
• the V 2 O 5 composite layer can effectively scavenge the polysulfides, however, because of its poor conductivity, as- scavenged polysulfides could not be released back during the charging process. Such an un-regenerative process trapped polysulfides continuously within the composite, resulting in continuous decay of the capacity.
• FIG. 14 shows energy densities of Li-S cells with different sulfur loadings, ratios between electrolyte and sulfur (E/S) and specific capacities of active materials.
• the specific capacity and average working potential of Li-S cells are based on the electrochemical performance of coin cells, while the mass is based on the whole cell, which includes the weights of separator, RSL, sulfur cathode, lithium anode and liquid electrolytes, according to one embodiment of the invention.
• FIG. 15(A) shows scavenging capacitance of the CNTs and CNTS/V 2 O 5 RSL at different Li 2 S 6 concentrations, according to one embodiment of the invention.
• FIGS. 15 (B) and 15(C) show cyclic voltammetries of the equilibrated CNTs RSL and the equilibrated CNTs/V 2 0 5 RSL cathodes at a scanning rate of 0.05 mV s "1 , according to one embodiment of the invention.
• CNTs and CNTS/V 2 O 5 RSL were equilibrated in L1 2 S 6 solutions with various concentrations, respectively. After wiping off the residual solution on the surface, cells were assembled using the equilibrated RSL as the cathode and lithium metal as the anode. Both cells exhibit an open circuit voltage (OCV) of about 2.36 V, which is the same as the redox potential of the polysulfides.
• OCV open circuit voltage
• the cells were then hold at 2.8 V till the current reached 1 ⁇ , during which the Li + from the adsorbed polysulfides were stripped off from the RSL.
• the total charges were measured and converted to the amount of L1 2 S 6 adsorbed on the RSL.
• FIG. 16 shows SEM images of a lithium surface after cycling at 1 C for 50 cycles (A) and (B) Li-S cell with CNTs (1) RSL, and (C) and (D) Li-S cell with CNTs/V 2 0 5 RSL, according to one embodiment of the invention.
• FIG. 17 shows cross-sectional SEM images and elemental mappings of lithium anodes after cycling with sulfur cathodes with and without the RSL.
• Li-S cells were assembled with CNTs (1) RSL for (A) and (B) and CNTs/V 2 0 5 RSL for C and D. Yellow color represents the distribution of sulfur species, according to one embodiment of the invention.
• FIG. 18 shows electron transfer directions regarding the relative positions of conduction bands and valence bands of the oxides and redox potential of the molecule, according to one embodiment of the invention.
• FIG. 19 shows the interaction in physisorption and chemisorption, according to one embodiment of the invention.
• FIG. 20 shows X-ray diffraction (XRD) plots of CNTs/oxide composites used in the RSL, according to one embodiment of the invention.
• FIG. 21 shows a self-discharge rate of Li-S cells with different CNTs/oxide RSL, according to one embodiment of the invention.
• Combinations such as "at least one of A, B, or C", “one or more of A, B, or C", “at least one of A, B, and C", “one or more of A, B, and C", and "A, B, C, or any combination thereof include any combination of A, B, and/or C, and may include multiples of A, multiples of B, or multiples of C.
• combinations such as “at least one of A, B, or C", “one or more of A, B, or C”, “at least one of A, B, and C", “one or more of A, B, and C", and "A, B, C, or any combination thereof may be A only, B only, C only, A and B, A and C, B and C, or A and B and C, where any such combinations may contain one or more member or members of A, B, or C.
• first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the invention.
• orientations of the device in addition to the orientation depicted in the Figures. For example, if the device in one of the figures is turned over, elements described as being on the "lower” side of other elements would then be oriented on “upper” sides of the other elements.
• the exemplary term “lower” can, therefore, encompass both an orientation of “lower” and “upper”, depending on the particular orientation of the figure.
• the device in one of the figures is turned over, elements described as being on the "lower” side of other elements would then be oriented on “upper” sides of the other elements.
• the exemplary term “lower” can, therefore, encompass both an orientation of “lower” and “upper”, depending on the particular orientation of the figure.
• the device in one of the figures is turned over, elements described as being on the "lower” side of other elements would then be oriented on “upper” sides of the other elements.
• the exemplary term “lower” can, therefore, encompass both an orientation of “lower” and “upper”, depending on the particular orientation of the figure
• this invention relates to methods and systems for fabricating a regenerative polysulfide- scavenging layer enabling lithium- sulfur (Li-S) batteries with high energy density and prolonged cycling life.
• this invention discloses an effective polysulfide-blocking strategy based on regenerative poly sulfide- scavenging layers (RSL), which can dynamically block the diffusion of polysulfides and regenerate themselves during cycling.
• RSL regenerative poly sulfide- scavenging layers
• Lithium- sulfur batteries notable for high theoretical energy density, environmental benignity and low cost, hold great potentials for next-generation energy storage. Polysulfides, the intermediates generated during cycling, may shuttle between electrodes, compromising the energy density and cycling life.
• a class of regenerative polysulfide- scavenging layers is reported, which effectively immobilize and regenerate polysulfides, especially for electrodes with high sulfur loadings (e.g., 6 mg cm " ).
• the resulted cells exhibit high gravimetric energy density of 365 Wh kg " 1 , initial areal capacity of 7.94 mAh cm “ 2 , low self-discharge rate of 2.45% after resting for 3 days and dramatically prolonged cycling life.
• the RSL are made from flexible and conductive membranes of carbon nanotubes (CNTs), in which the center layers are embedded with nanowires or nanocrystals of metal oxides.
• CNTs carbon nanotubes
• the outward diffused polysulfides are adsorbed by or reacted with the RSL, forming [Polysulfides-RSL] complexes and being immobilized within the RSL.
• V2O5 nanowires were selected as a model oxide, which has been extensively explored for electrochemical energy storage with high capacity (294 mAh g "1 with 2 Li + insertion/extraction per unit), fast Li + intercalation kinetics, and long cycling life (> 500 cycles) [35] . Besides, it exhibits a redox window from 1.8 to 4.0 V (vs. Li + /Li), matching well with the redox window of sulfur (1.7 to 2.8 V vs. Li + /Li).
• the composites of V 2 0 5 nanowires intertwined with CNTs were synthesized using hydrothermal reaction [36, 37] . Based on such composites, CNTS/V 2 O 5 RSL was fabricated by sequentially filtration of the dispersion of CNTs,
• CNTs from the dispersions can be entangled forming CNTs networks for effective electron conduction, allowing effective redox reactions within the CNTS/V 2 O 5 RSL.
• sufficient conductivity is essential to endow the RSL with scavenging capability for polysulfides and regenerative ability, as shown in FIG. 7.
• FIG. 1 shows a schematic presentation of a Li-S cell with a regenerative polysulfide- scavenging layer (RSL).
• the RSL is made from a CNTs membrane of which the center is embedded with interpenetrating nanowires or nanocrystals of metal oxides: (i) during discharging, as-generated polysulfides are adsorbed by or reacted with the RSL, immobilized onto the RSL denoted as [Polysulfides-RSL] complexes; and (ii) a subsequent charging process strips away the immobilized species and regenerates the RSL, enabling dynamic blocking of the polysulfides.
• the RSL are made from flexible and conductive membranes of carbon nanotubes (CNTs), in which the center layers are embedded with nanowires or nanocrystals of metal oxides.
• CNTs carbon nanotubes
• the outward diffused polysulfides are adsorbed by or reacted with the RSL, forming [Polysulfides-RSL] complexes and being immobilized within the RSL.
• a subsequent charging process stripes away these polysulfides and regenerates the RSL.
• This combination of a large amount of polysulfides scavenged and the regenerative capability affords highly effective and dynamic scavenging of polysulfides, leading to dramatically reduced lithium corrosion and prolonged cycling life, especially for electrodes with high sulfur loadings.
• the RSL are electronically conductive and mechanically robust, thus further enhance the performance of the cells.
• the scavenging effects which are originated from the physisorption and chemical reaction with polysulfides, have been thoroughly investigated and correlated with electrochemical performance of the cells.
• FIG. 2 shows a structure of the CNTs/V 2 0 5 composites and CNTs/V 2 0 5 RSL.
• FIG. 2(a) shows SEM images of a CNTS/V 2 O 5 composite with a fibrous structure made from interpenetrative V 2 0 5 nanowires and CNTs.
• FIG. 2(b) shows transmission electron microscopy (TEM) images of the CNTS/V 2 O 5 composites, showing a continuous and porous structure with average nanowire diameter of about 30 nm.
• FIG. 2(c) shows a high-resolution TEM image and its corresponding selected area Fast Fourier Transformation (FFT) image (inset) of the CNTS/V 2 O 5 composites.
• FIG. 2(d) shows an X-ray diffraction profile of the CNTS/V 2 O 5 composites.
• FIG. 2(e) shows a cross-section SEM image of a CNTS/V 2 O 5 RSL are made from two CNTs layers and a sandwiched CNTS/V 2 O 5 layer.
• Scale bars are 500 nm for FIG. 2(a); 50 nm, 10 nm (inset) for FIG. 2(b), 5 nm, 1 nm (inset) for FIG. 2(c); and 5 ⁇ for FIG. 2(e).
• FIGS. 2(a) and 2(b) present the scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the CNTS/V 2 O 5 composites, respectively, demonstrating a continuously fibrous structure made from interpenetrative V 2 O 5 nanowires and CNTs.
• the nanowires are porous (see inset) with diameters of about 30 nm.
• FIG. 2(c) exhibits a high-resolution TEM (HRTEM) image and selected area Fast Fourier Transformation (FFT) of the V 2 O 5 nanowires, confirming its layered crystalline structure.
• the HRTEM image displays a ⁇ i-spacing of 0.211 nm, which is consistent with the (020) lattice plane of V 2 O 5 .
• X- ray diffraction (XRD) analysis in FIG. 2(d) reveals the characteristic peaks at 9.2, 13.2, 26.4, 29.1 and 41.8 ° , corresponding to the (001), (002), (111), (200) and (020) planes of V 2 0 5 with a layered structure, respectively [38, 39].
• TGA thermogravimetric analysis
• the weight percent of CNTs in the composites is about 9.8 %, as shown in FIG. 9.
• FIG. 2(e) shows a cross-sectional image of RSL with about 15 ⁇ in thickness, which contains porous CNTs layers sandwiched with a X ⁇ rich layer in the center.
• Such RSL is also flexible with good mechanical strength, as shown in the digital photographs of FIG. 10.
• FIG. 3 shows electrochemical performance of Li-S cells with Celgard PP separator, CNTs RSL or CNTs/V 2 0 5 RSL. Specifically, in FIG. 3(a), cyclic voltammetries are obtained at a scanning rate of 0.2 mV s "1 . In FIG. 3(b), Nyquist plots show a reduced charge-transfer resistance with the RSL. FIG. 3(c) shows rate performance at 0.3 C, 0.5 C, 1 C, 2 C, 4 C and 0.3 C rate (sulfur loading 2 mg cm "2 ). FIGS. 3(d), 3(e) and 3(f) show galvanostatic cycling performance at 1 C rate, 0.1 C rate and 0.2 C rate, respectively.
• the empty bullets (o) represent the discharge capacity and circle bullets ( ⁇ ) represent the Coulombic efficiency.
• Cells in FIG. 3(e) were activated at 0.05 C rate while cells in FIG. 3(f) were activated at 0.1 C rate.
• FIGS. 3(g) and 3(h) show self-discharge tests. The cells were cycled at 0.2 C for 9 cycles, stopped at 2.1 V during 10 th discharge and rested for 3 days before the discharging process was resumed. Voltage-capacity profiles of the cells were recorded, suggesting the cell with CNTS/V 2 O 5 RSL exhibit a dramatically reduced self-discharge rate.
• the cathode exhibits well-defined redox peaks with less polarization.
• Table 1 Composition and thickness of regenerative polysulfide scavenging layers (RSL) made with different metal oxides and/or CNTs.
• the CNTs RSL has the same mass of the CNTs in the CNTs/oxide RSL, while the CNTs (2) has the same mass of the CNTs/oxide RSL.
• FIG. 3(b) further compares the electrochemical impedance spectroscopy (EIS) of the electrodes, indicating a charge transfer resistance of 160, 70 or 55 ohms with Celgard PP separator, CNTS/V 2 O 5 RSL or CNTs RSL, respectively.
• EIS electrochemical impedance spectroscopy
• the sulfur electrode with CNTs RSL presents an initial capacity of 1396 mAh g "1 at 0.3 C rate and reversible capacities of 901, 768, 694 and 614 mAh g "1 at 0.5, 1, 2 and 4 C rates, respectively.
• the sulfur electrode with the CNTS/V 2 O 5 RSL delivers a much higher initial capacity of 1513 mAh g "1 at 0.3 C rate and reversible capacities of 1170, 1063, 954 and 858 mAh g "1 at 0.5, 1, 2 and 4 C rates, respectively.
• FIG. 3(d) compares the cycling stability of Li-S cells with Celgard PP separator or RSL at 1 C rate.
• a low initial capacity of 315 mAh g "1 is observed, which decreases to 124 mAh g "1 after 250 cycles, while the cell with the CNTS/V 2 O 5 RSL delivers a much higher initial capacity of 1068 mAh g "1 and a reversible capacity of 939 mAh g "1 after 250 cycles.
• the cell with the CNTs RSL exhibits a reversible capacity of 498 mAh g "1 after 250 cycles, which is significantly lower than that with the CNTs/V 2 0 5 RSL.
• CNTs (2) RSL thicker CNTs RSL (denoted as CNTs (2) RSL) were also fabricated, which contain about 2.5 folds of the CNTs. It was found that the cell with CNTs (2) RSL only maintains a reversible capacity of 730 mAh g "1 after 250 cycles, as shown in FIG. 11.
• CNTS/V 2 O 5 RSL with fixed total mass but different mass ratios between V 2 O 5 and CNTs were also fabricated.
• CNTs layers facilitating charge transfer and regeneration of polysulfides
• CNTS/V 2 O 5 RSL with higher percentage of V 2 O 5 enables better cycling stability of lithium- sulfur batteries, as shown in FIG. 12.
• V 2 O 5 moieties continuously adsorb polysulfides during discharging and charging, such un- regenerative process results in rapid capacity decay of lithium-sulfur batteries, as shown in FIG. 13.
• FIGS. 3(e) and 3(f) compare the electrochemical performance of the Li-S cells with sulfur loading of 6 mg cm " at 0.1 C rate and 0.2 C rate, respectively.
• the cells with CNTs RSL and Celgard PP separator deliver similar capacity and cycling stability, while the cell with the CNTS/V 2 O 5 RSL exhibits significantly higher initial capacity at 0.05 C rate (1309 rnAh g "1 vs. about 1105 mAh g "1 ) and capacity retained after 50 cycles at 0.1 C rate (1037 mAh g "1 vs. about 613 mAh g "1 ).
• the initial specific capacities and sulfur contents of the lithium- sulfur cells are further calculated based on the total weight of the cathodes, which includes the weights of the carbon/sulfur composite, conductive agent, binder and RSL. As shown in Table 2, consideration of the weight contribution from the CNTS/V 2 O 5 RSL only slightly reduces the sulfur content from 70.4% to 66.6%.
• the specific capacity of the cathode with CNTs/V 2 0 5 RSL is still much higher than those with CNTs RSL or Celgard PP separator (814 mAh g "1 vs. about 700 mAh g "1 ).
• the enhancement in electrochemical performance becomes more pronounced at 0.2 C rate.
• the cell with the CNTS/V 2 O 5 RSL delivers a capacity of 1323 mAh g "1 and an area capacity of 7.94 mAh cm “2 after the 1 st activation cycle, and maintains about 100% Coulombic efficiency for 100 cycles.
• the cell with the CNTs RSL exhibits a lower capacity of 890 mAh g "1 during the 2 nd cycle and failed after 12 cycles due to the shuttling effect. Owing to the high charge transfer resistance, the cell with Celgard PP separator lost most of its capacity after 6 cycles. This observation further confirms that CNTs reduce the charge transfer resistance of the electrodes; while V 2 O 5 does endow the RSL with better blocking capability for polysulfides, alleviate lithium corrosion and dramatically extend the cycling life (>100 cycles vs. 12 cycles) of the cells.
• Table 2 Comparison of sulfur contents and initial specific capacity of lithium- sulfur cells with different inter layers.
• the energy density of Li-S cells increases with higher sulfur loadings, lower ratio between the volume of electrolyte and the mass loading of sulfur (E/S, ⁇ / ⁇ 3 ⁇ 4), as well as higher specific capacities of active materials.
• E/S the ratio between the volume of electrolyte and the mass loading of sulfur
• the specific capacity of sulfur may achieve 1500 mAh g "1 and the energy density of the cell can possibly reach up to 560 Wh. kg "1 , which could bring Li-S batteries to practical applications.
• Table 3 Comparison of interlayer materials, interaction with S n " and maximum sulfur loadings in the literatures and current work [1-22]. The sulfur loadings in Refs. [l]-[4] are not mentioned in the papers and labeled as N/A in the table.
• the polysulfides scavenged by the RSL could be released and recaptured reversibly upon cycling between 2.8 V to 1.7 V, as shown in FIGS. 8(B) and 8(C).
• the CNTs RSL exhibited significantly capacity decay whereas the
• CNTs/V 2 0 5 RSL retains the initial capacity (the amount of polysulfide scavenged), clearly indicating the outstanding scavenging and regenerative capability of the CNTs/V 2 0 5 RSL.
• Table 4 Table of capacity contributions from CNTs RSL and CNTs/V 2 0 5 RSL in Li-S cells with various sulfur loadings at 1 C rate. The capacity of the RSL were determined electrochemically using the RSL as the cathodes and lithium as the anode under the similar testing condition.
• FIGS. 3(g) and 3(h) display the charge-discharge voltage vs. capacity for cells without and with the CNTS/V2O5 RSL before and after the resting.
• the cell without the RSL exhibits a discharge capacity of 674 mAh g "1 in the 9 th cycle (denoted as Ccith), which decreases to 539 mAh g "1 after the resting (denoted as Cioth)-
• the cell with the RSL delivers a much higher capacity of 1174 mAh g "1 in the 9 th cycle (Ccith), and still maintains the high capacity after the resting (1145 mAh g "1 , Cioth)-
• the self- discharge rate of the cells can be estimated by (Ccith-Cioth) Ccith ⁇ 100%.
• the self-discharge rate of the cell was decreased from 26.7% to 2.5%, suggesting the significant role of the CNTS/V 2 O 5 RSL in blocking diffusion of polysulfides and minimizing the self-discharge rate, which is essential for practical utilization of lithium- sulfur batteries.
• SEM study of the scavenging and regeneration process to further understand the scavenging and regenerating process, distribution of sulfur moieties within the CNTS/V 2 O 5 RSL at different electrochemical stages was analyzed with SEM and energy dispersive x-ray (EDX) spectroscopy.
• Li-S cells with CNTS/V 2 O 5 RSL were cycled at 0.3 C and interrupted at 2.05 V during discharging or 2.60 V during charging, respectively.
• the CNTS/V 2 O 5 RSL were then disassembled from the cells and dried in an argon-filled glove box for SEM and EDX studies.
• FIG. 4 shows SEM images and element-mapping of CNTS/V 2 O 5 RSL at discharged and charged stages.
• Li-S cells were cycled at 0.3 C between 1.7 and 2.8 V.
• FIG. 4(a) was interrupted at 2.05 V during the discharging and
• FIG. 4(b) was interrupted at 2.60 V during the charging.
• the arrows 401 and 413 from circles 405 to 403 and from the circles 409 to 415 show the direction of the line scan, respectively, while the circles 403, 405, 409 and 415 represent the starting and ending points.
• Scale bars are 20 ⁇ for FIG. 4(a) and 25 ⁇ for FIG. 4(b).
• FIG. 4 (a) displays a cross-section SEM image and the corresponding EDX analysis of the RSL interrupted at 2.05 V.
• sulfur is mainly converted to polysulfides located within the electrode and in the electrolyte.
• EDX analysis shows two peaks associated with sulfur and vanadium co-localized in the center, indicating that the sulfur moieties are distributed dominantly within the V 2 O 5 layer (less amount of sulfur in the CNTs region). This observation is consistent with the critical role of V 2 O 5 in scavenging the polysulfides.
• FIG. 4 (b) presents a cross-section SEM image and the corresponding EDX analysis of the RSL interrupted at 2.60 V.
• the scavenging ability of CNTS/V 2 O 5 RSL also alleviates the corrosion of lithium anodes during cycling.
• the lithium anode from the cell with CNTs RSL exhibits a rough surface with a thick sulfur-containing passivation film (about 300 ⁇ ).
• the lithium anode from the cell with CNTS/V 2 O 5 RSL maintains a smooth surface with a significantly thinner penetration of polysulfides (about 80 ⁇ depth), indicating 73.3% less lithium corrosion.
• FIG. 5 shows reactions between V 2 0 5 and polysulfides probed by XPS.
• FIG. 5(a) shows
• FIG. 5(b) shows V 2 0 5 /sulfide compound formed by reacting V2O5 with Li 2 S6, indicating the formation of V 4+ in the presence of Li 2 S6-
• FIG. 5(c) shows sulfur 2p core spectra of Li 2 S6 showing the terminal (S T _1 ) and bridging (S B °) sulfur atoms with an expected ratio of 1:2.
• FIG. 5(d) shows sulfur 2p core spectra of the V 2 0 5 /sulfide compound. The formation of polythionate groups indicates redox reactions between Li 2 S6 and V 2 0 5 .
• FIGS. 5(a) and 5(b) XPS spectra of V 2 0 5 before and after mixing with Li 2 S6 are presented in FIGS. 5(a) and 5(b), respectively.
• V 2 0 5 displays a typical 2p 3/2 spectrum for the V 5+ state at 517.5 eV. After the mixing, the 2p 3/2 peak splits into two peaks centered at 517.5 eV and 516.0 eV, which are originated from the V 5+ and V 4+ states, respectively [42].
• FIGS. 5 (c) and 5(d) further compare the sulfur 2p core spectra of Li 2 S6 and oxide/sulfur solid.
• Li 2 S6 exhibits two sulfur states at 163.0 eV and 161.7 eV, which can be assigned to bridging (S B °) and terminal (S T "1 ) sulfur atoms in polysulfide anions, respectively [6, 43, 44].
• the ratio between S B ° and S T 1 is around 2: 1, which is in accordance with the composition of Li 2 S6-
• the S 2p spectrum of the oxide/sulfide solid illustrates two sulfur states, which can be attributed to S B ° at 163.2 eV and polythionate complex at 167.9 eV, respectively [43].
• V 4+ and the polythionate complex suggests the occurrence of redox reactions between Li 2 S6 and V 2 0 5 , forming Li-V-O-S complexes. Meanwhile, the terminal sulfur atoms (S T _1 ) were not detected in the oxide/sulfide solid, suggesting that the Li + ions, which were paired with the polysulfides, are intercalated or inserted into V 2 0 5 .
• FIG. 6 shows correlations between cell performance, work function of oxide moieties and bond energy between the oxides and polysulfides.
• FIG. 6(a) shows photographs of Li 2 S6 solutions mixed with metal oxides after centrifugation.
• FIG. 6(b) shows absolute potentials of the conduction bands 601 and valence bands 603 of various metal oxides, as well as the oxidation potential of polysulfides (2.2 to 2.5 V vs. Li + /Li, labeled in purple).
• FIG. 6(c) shows work functions of series metal oxides.
• 6(d) shows a comparison of the bond energies between the metal oxides and polysulfides (Light green 605 on the left) with the specific capacity of the corresponding Li-S cells after 100 cycles at 1 C (Green 607 on the right). These cells were made using RSL containing these metal oxides, respectively.
• the bond energies were calculated with Flore' s equation based on dissociation energy, electronegativity and chemical hardness of metal oxides and polysulfides.
• such oxides can be categorized into two groups: one group that can physically adsorb polysulfides without electron transfer (physisorption) including MgO, A1 2 0 3 , Si0 2 , Li 2 0, Ce0 2 , PbO, NiO and ZnO; the other group that can react with polysulfides (chemisorption) including Sn0 2 , CoO, Ti0 2 , Fe 2 0 3 , CuO, Mn0 2 , Mo0 3 , V 2 0 5 , W0 3 , and Cr0 3 .
• the adsorption is mainly governed by work function (or surface energy, which is proportional to surface potential) of the oxides.
• work function or surface energy, which is proportional to surface potential
• An oxide with higher surface potential my build up a stronger electric field within its Debye length, resulting in a stronger adsorption of the adsorbates [49], as shown in FIG. 19.
• FIG. 6(c) displays the work functions of a series of oxides [47], which can be used as an indicator for adsorption ability or polysulfide- scavenging capability.
• A1 2 0 3 Comparing with MgO, Ce0 2 and ZnO, A1 2 0 3 has the highest work function and the best poly sulfide- scavenging performance as observed in the visual experiment, in which the Li 2 S 6 solutions with MgO, Ce0 2 or ZnO remain brownish while that with A1 2 0 3 shows light yellow color.
• FIG. 6(d) presents their bond energies with polysulfides, as well as capacities of Li- S cells with such RSL after 100 cycles at 1 C.
• FIG. 20 there is a significant correlation between bond energy and cycling stability: stronger bond energies between the oxides and polysulfides lead to higher capacity retentions and lower self-discharge rate, as shown in FIG. 20.
• W0 3 and polysulfides exhibit high bond energy of 13.62 eV, leading to cells with a high capacity of 1075 rnAh g "1 and a near- zero self-discharge rate.
• CuO and polysulfides show lower bond energy of 9.83 eV, as expected, resulting in lower capacity retention of 572.9 rnAh g "1 and about 9.0% of self-discharge rate. This observation suggests that it is possible to use the bond energy between the scavenging materials and polysulfides to evaluate or predict their polysulfide-scavenging capability, providing a quantified guidance for Li-S batteries.
• FIG. 6(d) presents their bond energies with polysulfides, as well as capacities of Li-S cells with such RSL after 100 cycles at 1 C.
• FIG. 20 there is a significant correlation between bond energy and cycling stability: stronger bond energies between the oxides and polysulfides tend to result in higher capacity retentions and lower self-discharge rate, as shown in FIG. 20.
• W0 3 and polysulfides exhibit high bond energy of 13.62 eV, leading to cells with a high capacity of 1075 rnAh g "1 and a near- zero self-discharge rate.
• CuO and polysulfides show lower bond energy of 9.83 eV, as expected, resulting in lower capacity retention of 572.9 rnAh g "1 and about 9.0% of self-discharge rate. This observation suggests that it is possible to use the bond energy between the scavenging materials and polysulfides to evaluate or predict their poly sulfide- scavenging capability, providing a quantified guidance for Li-S batteries.
• FIG. 7 shows the RSL with scavenging capability for polysulfides and regenerative ability.
• the RSL are made from two CNTs layers and a sandwiched CNTs/metal oxides layer.
• Li-S batteries with CNTS/V 2 O 5 RSL exhibit high areal capacity of >6 mAh cm " for 60 cycles, dramatically extended cycling life (>100 cycles vs. 12 cycles), low self-discharge rate of 2.45% after resting for 3 days and about 73.3% less lithium corrosion.
• CNTs/oxide composites CNTS/V 2 O 5 composites were synthesized with activated CNTs according to the previously reported procedure [37]. Briefly, 0.6 g of ammonium metavanadate (Sigma- Aldrich) and 1 g of P123 (EO 20 PO- 70 EO 20 ) (Sigma- Aldrich) were dispersed in 60 mL deionized water with 3 mL 2 M HC1. 20 mg activated CNTs was added to the mixture and sonicated for 30 min. The mixture was stirred at room temperature for 12 hours and then transferred to an autoclave and heated at 120 °C for 24 h. The resulted composites were rinsed with deionized (DI) water and ethanol for 3 times, and dried at 80 °C overnight in vacuum. Other CNTs composites containing different metal oxides were synthesized using similar hydrothermal methods.
• DI deionized
• RSL Fabrication of RSL: the RSL were prepared using a vacuum-filtration method. CNTs and CNTs/metal oxides composites were dispersed in ethanol by sonication and formed 0.1 mg mL "1 and 1 mg mL "1 suspensions, separately. Subsequently, 20 mL CNT suspension, 6 mL suspension of CNTs/metal oxides composites and 20 mL CNT suspension were vacuum filtered through a polypropylene membrane (Celgard 2500, diameter: 47 mm) and form a flexible triple layer membrane. The membranes were dried at 70 °C overnight and then punched into a round shape with a diameter of 18 mm. The weight of the RSL on each separator is around 0.4-0.6 mg cm " in Table. For CNTs RSL, 100 mL CNT suspension was filtrated.
• sulfur cathodes were prepared using a slurry casting method.
• sulfur, carbon black and polyvinylidene fluoride (PVDF) were mixed with weight ratio of 5:4: 1 to form a homogenous slurry with N-methyl-2-pyrrolidone, then casted onto a carbon-coated aluminum foil with a doctor blade.
• PVDF polyvinylidene fluoride
• carbon/sulfur composites, carbon nanofiber, carbon black and PVDF were mixed with weight ratio of 88:4: 1:7 to form a slurry.
• Li 2 S6 solution was prepared by mixing stoichiometric amounts of elemental sulfur (Sigma Aldrich) and Li 2 S (Alfa Aesar) in DOL: DME (volume ratio 1: 1). A homogenous dark-red solution of Li 2 S 6 was obtained after stirring for 24 hours at 130 °C.
• Electrochemical measurements to evaluate the electrochemical performance, 2032-type coin cells (MTI Corporation) were assembled using lithium metal as the anodes. RSL were placed between polypropylene separator and sulfur cathode. 0.5 M LiTFSI and 2 wt-% L1NO 3 in DOL/DME was used as electrolyte. CV measurements were performed on a Bio-Logic VMP3 electrochemical workstation. Galvano static charge-discharge measurements were carried out using Land CT2000 battery tester in a voltage range of 1.7-2.8 V for all rates. Specific capacities were calculated with respect to the mass of sulfur. EIS tests were carried out on a Solartron 1860/1287 electrochemical interface.
• Instrument SDT Q600 employing a heating rate of 5 °C min "1 from 40 °C to 600 °C under airflow.
• SEM studies were conducted on a JEOL JSM-6700 FE-SEM and TEM studies were carried out on a FEI T12 operating at 120 kV.
• XPS studies the samples were sealed in a transporter in the glove box before being quickly transferred to the high- vacuum chamber of XPS (AXIS Ultra DLD) for analysis. All the spectra were fitted to Gaussian-Lorentzian functions and a Shirley-type background using CasaXPS software. The binding energy values were all calibrated using C Is peak at 285.0 eV.
• CNTs composites with different metal oxides are synthesized as follows.
• CNTs/CuO composite 0.94 g copper nitrate (Sigma- Aldrich), 1 g of P123 and 20 mg active CNTs were dispersed in 40 mL DI water by sonication. Then, 10 mL of ammonium hydroxide solution (27-30 wt %) was added. The mixture was then stirred for Ih at room temperature, and then underwent hydrothermal reaction at 110 °C for 4 h. As-formed product was rinsed with deionized water and ethanol for 3 times and dried at 80 °C overnight in vacuum. The product was further calcinated in nitrogen at 300 °C for 2 h.
• CNTs/Mn0 2 composite 0.72 g KMn0 4 and 20 mg active CNTs were dispersed in 60 mL of deionized water by sonication. The mixture underwent hydrothermal reaction at 100 °C for 24 h. The products were rinsed with deionized water for 3 times and then dried at 80 °C overnight in vacuum.
• CNTS/M0O 3 composite 1 gram (g) ammonium heptamolybdate tetrahydrate and 1 gram of P123 and 40 mg active CNTs were dispersed in 33 mL of deionized water by sonication. Then, 6 mL HNO 3 was added and allowed to react at 180 °C for 24 h. The products were rinsed with deionized water for 3 times and then dried at 80 °C overnight in vacuum.
• CNTS/WO 3 composite 0.5 g of sodium tungstate, 0.25 g of ammonium sulfate (Sigma- Aldrich) and 20 mg activated CNTs were dispersed in 10 mL of deionized water by sonication. The pH of the solution was then adjusted to 1 by adding 3 M H 2 S0 4 aqueous solution. The mixture underwent hydrothermal reaction at 100 °C for 12 h. The products were rinsed with deionized water for 3 times and then dried at 80 °C overnight in vacuum.
• FIG. 8 shows direct-current polarization profile of a CNTs/V 2 0 5 RSL and a V 2 0 5 composite layer (with same area) at 0.2 V.
• the direct-current polarization profile shows that the conductivity of the V 2 0 5 composite layer (without extra CNTs layers) is 4 magnitudes lower than that of the CNTs/V 2 0 5 RSL.
• Such significant difference in conductivity is due to their fabrication method.
• To prepare the CNTs/V 2 0 5 RSL dispersions of CNTs, V 2 0 5 composite, and CNTs were sequentially filtrated onto a porous separator, during which the CNTs and the CNTs from the V 2 0 5 composite can effectively entangled forming effectively conductive networks.
• V 2 0 5 composite dispersion was filtrated to form the V 2 0 5 composite layer, resulting in significantly lower electronic conductivity because of less amount of CNTs present and less effective conductive network.
• FIG. 9 shows TGA plot of the CNT/V 2 0 5 composites indicating that the composites contain about 9.8% of CNT.
• FIG. 10 shows digital photographs of CNTs/V 2 0 5 RSL: (A) as-prepared, (B) folded and (C) recovered states.
• FIG. 11 shows galvanostatic cycling performance of the cells with Celgard PP separator, with CNTs (2) RSL or CNTs/V 2 0 5 RSL. All cells were cycled at 0.3 C rate for 3 cycles and then 1 C rate for 250 cycles.
• specific capacity decreases as the cycle number increases.
• specific capacity for cell with CNTs (2) RSL is lower than that of cells with CNTS/V 2 O 5 RSL, but higher than that of the cells with Celgard PP separator.
• FIG. 12 shows capacity retention of the cells made with CNTS/V 2 O 5 RSL prepared with different CNTS/V 2 O 5 mass ratios but a fixed composite mass.
• specific capacity decreases as the cycle number increases.
• specific capacity for the cells with a ratio 1: 1.17 of CNTS/V 2 O 5 RSL is lower than that of cells with a ratio of 1: 1.33 of CNTS/V 2 O 5 , but higher than that of the cells with a ratio of 1:0.92 of CNTs/V 2 0 5 .
• FIG. 13 shows galvanostatic cycling performance of Li-S batteries with Celgard PP separator, CNTs RSL, CNTs/V 2 0 5 RSL, and V 2 0 5 composite layer at 0.3 C rate.
• CNTS/V 2 O 5 RSL shows better capacity performance than CNTs RSL due to the regenerative scavengers
• CNTs RSL shows better performance than the Celgard PP separator due to its ability to adsorb and desorb poly sulfides.
• the V 2 O 5 composite layer can effectively scavenge the polysulfides, however, because of its poor conductivity, as-scavenged polysulfides could not be released back during the charging process. Such an un-regenerative process trapped polysulfides continuously within the composite, resulting in continuous decay of the capacity.
• FIG. 14 shows energy densities of Li-S cells with different sulfur loadings, ratios between electrolyte and sulfur (E/S) and specific capacities of active materials.
• the specific capacity and average working potential of Li-S cells are based on the electrochemical performance of coin cells, while the mass is based on the whole cell, which includes the weights of separator, RSL, sulfur cathode, lithium anode and liquid electrolytes.
• FIG. 15(A) shows scavenging capacitance of the CNTs and CNTS/V 2 O 5 RSL at different L1 2 S 6 concentrations.
• FIGS. 15(B) and 15(C) show cyclic voltammetries of the equilibrated CNTs RSL and CNTS/V 2 O 5 RSL cathodes at a scanning rate of 0.05 mV s "1 , respectively.
• CNTs and CNTs/V 2 0 5 RSL were equilibrated in Li 2 S 6 solutions with various concentrations, respectively.
• cells were assembled using the equilibrated RSL as the cathode and lithium metal as the anode. Both cells exhibit an open circuit voltage (OCV) of about 2.36 V, which is the same as the redox potential of the polysulfides.
• OCV open circuit voltage
• the cells were then hold at 2.8 V till the current reached 1 ⁇ , during which the Li + from the adsorbed polysulfides were stripped off from the RSL.
• the total charges were measured and converted to the amount of Li 2 S6 adsorbed on the RSL.
• FIG. 16 shows SEM images of a lithium surface after cycling at 1 C for 50 cycles (A) and (B) Li-S cell with CNTs (1) RSL, (C) and (D) Li-S cell with CNTs/V 2 0 5 RSL.
• FIG. 17 shows cross-sectional SEM images and elemental mappings of lithium anodes after cycling with sulfur cathodes with and without the RSL.
• Li-S cells were assembled with (A-B) CNTs (1) RSL and (C-D) CNTs/V 2 0 5 RSL.
• Yellow color 1701 represents the distribution of sulfur species.
• FIG. 18 shows electron transfer directions regarding the relative positions of conduction bands and valence bands of the oxides and redox potential of the molecule.
• FIG. 19 shows the interaction in physisorption and chemisorption.
• the electric field generated by an oxide is proportional to its surface potential [49];
• the bond energy between an oxide and adsorbed species is related to the dissociation energy, electronegativity and chemical hardness of the both, and can be calculated with Flore's equation [51].
• FIG. 20 shows XRD plots of CNTs/oxide composites used in the RSL.
• the composites include, but are not limited to, Fe 2 C>3, CuO, Mn0 2 , M0O 3 , V 2 0 5 and WO 3 .
• FIG. 21 shows a self-discharge rate of Li-S cells with different CNTs/oxide RSL.
• the self-discharge rate of the Li 2 S6 solution (control) is around 20%.
• the self- discharge rates of NiO, CuO, C0 3 O 4 , CNTs, ZnO, V 2 0 5 and WO 3 becomes smaller and smaller.
• the exact self-discharge rate (%) for each and every above-mentioned component can be scaled according to FIG. 21.
• Oxide Work Functions The Influence of Cation Oxidation State and Oxygen Vacancies. Adv. Funct. Mater. 2012, 22, 4557-4568.

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