Nanocatalyst-containing cathode for lithium-sulfur batteries

Abstract

A graded lithium-sulfur battery cathode is provided that can include an active electrocatalyst and polysulfide capture system for improved sulfur utilization and capacity retention, as well as methods for its manufacture and use. The graded Li-S cathode can be fabricated using economical and scalable synthesis and coating methods.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims the benefit of U.S. Provisional Application No. 63 / 379,276, filed October 12, 2022, the disclosure of which is incorporated by reference in its entirety, including all figures, tables, and drawings. [Background technology]

[0002] Lithium-sulfur (Li-S) batteries are conversion electrochemical systems that promise high energy density for electric vehicles (EVs) due to the very high capacity of lithium metal (3860 milliampere-hours per gram (mAh / g)) and sulfur (1675 mAh / g), and in particular the active material sulfur (S8), which is naturally abundant, stable, and very inexpensive. The theoretical cell-level energy density of Li-S batteries is 2600 watt-hours per kilogram (Wh / kg) for a complete electrochemical reaction. However, Li-S batteries suffer from the volume expansion (80%) and dissolution of lithium polysulfides in the electrolyte, as well as the extremely poor electronic conductivity of sulfur (1x10 per centimeter). -30 The slow redox electrochemistry of sulfur is adversely affected by its low conductivity (Siemens / cm). The solubility of long-chain lithium polysulfides in liquid electrolytes leads to inefficient sulfur utilization, especially at high sulfur loadings. Due to the above issues, battery capacity declines rapidly during the initial charge-discharge cycles.

[0003] To improve the capacity of sulfur cathodes, research has been conducted to address the Li-S battery chemistry, including anode protection, new electrolyte formulations, separator designs, and sulfur cathode structures. The electrochemical reduction of sulfur in Li-S battery chemistry requires a catalyst with high electronic conductivity. While catalysts have shown improvements in the cycling stability of Li-S batteries, incorporating catalysts into conversion reaction cell systems often results in unintended and / or undesired side reactions. Summary of the Invention [Means for solving the problem]

[0004] In view of the above-mentioned problems, there is a need in the art for more effective implementation of electrocatalyst use in lithium-sulfur (Li-S) batteries (e.g., optimizing the spatial location of the catalyst in the cathode and designing the Li-S cathode structure to further improve sulfur utilization and cycling stability). Embodiments of the present invention provide novel and advantageous Li-S batteries having a graded structure as the positive electrode (i.e., cathode) that includes an active electrocatalyst and polysulfide scavenging system for improving sulfur utilization and capacity retention for Li-S battery applications, as well as methods for fabricating and using the same. The graded structure Li-S cathode can be fabricated using economical and scalable synthesis and coating methods. Electrochemical performance results show that the graded structure Li-S cathode exhibits improved sulfur utilization and cycling stability compared to Li-S batteries without the graded structure Li-S cathode. The cathode without the graded structure is referred to herein as the "baseline cathode" or "baseline structure." The graded Li-S cathode containing the nanocatalyst exhibits an average 20% increase in initial sulfur utilization and a more than 30% improvement in cell capacity retention over 200 cycles compared to the baseline cathode.

[0005] In one embodiment, a positive electrode for a Li-S battery can include a sulfur-rich layer including at least one first nanocatalyst and an electrocatalyst and polysulfide scavenging layer including at least one second nanocatalyst. The sulfur-rich layer can be fused and interlocked with the electrocatalyst and polysulfide scavenging layer in a graded structure. The at least one first nanocatalyst can include at least one nanocatalyst in common with the at least one second nanocatalyst. The at least one first nanocatalyst can comprise at least one metal (e.g., at least one transition metal such as at least one of nickel (Ni), cobalt (Co), platinum (Pt), palladium (Pd), ruthenium (Ru), iridium (Ir), rhodium (Rh), silver (Ag), and gold (Au)) and / or an oxide, sulfide, fluoride, and / or carbide of at least one metal (e.g., at least one transition metal such as at least one of Ni, Co, Pt, Pd, Ru, Ir, Rh, Ag, and Au). The at least one second nanocatalyst can include at least one metal (e.g., at least one transition metal such as at least one of Ni, Co, Pt, Pd, Ru, Ir, Rh, Ag, and Au) and / or at least one metal (e.g., at least one transition metal such as at least one of Ni, Co, Pt, Pd, Ru, Ir, Rh, Ag, and Au) oxide, sulfide, fluoride, and / or carbide. In the positive electrode, sulfur can be distributed uniformly (or substantially uniformly (i.e., at a concentration no greater than 5% different from any other point)) throughout the graded structure. Alternatively, sulfur can be distributed in a gradient across the graded structure in the positive electrode. The sulfur-rich layer can be composed of conductive carbon (e.g., carbon nanotubes (CNTs), graphene, fullerene, graphitized carbon, super P conductive carbon, acetylene black, carbon fiber, or carbon nanofiber). The electrocatalyst and polysulfide scavenging layer can be composed of conductive carbon (e.g., CNT, graphene, fullerene, graphitized carbon, Super P conductive carbon, acetylene black, carbon fiber, or carbon nanofiber).The positive electrode can optionally further comprise a binder (e.g., polyvinylidene fluoride (PVDF), polyacrylic acid (PAA)). Alternatively, the positive electrode can be binder-free (i.e., no binder is present in the positive electrode).

[0006] In another embodiment, a Li-S battery can include a current collector (e.g., aluminum (Al), Al foil, etc.), a positive electrode disclosed herein disposed on (optionally in direct physical contact with) the current collector, a separator disposed on (optionally in direct physical contact with) a first surface of the positive electrode, the first surface of the positive electrode being opposite a second surface of the positive electrode on which the current collector is disposed (optionally in direct physical contact with the current collector), and a lithium anode on (optionally in direct physical contact with) the separator (the anode can be on the opposite side of the separator from the positive electrode). The Li-S battery can further include a first spacer below the current collector, a spring below the first spacer and / or current collector, a bottom cap below the spring, first spacer, and / or current collector, a second spacer on the lithium anode, and / or a top cap on the second spacer and / or lithium anode.

[0007] In another embodiment, a method for fabricating a positive electrode includes preparing a composite mixture of a polymer binder (e.g., cellulose acetate, polyacrylic acid, polyvinylidene fluoride) and a carbon-containing material including conductive carbon (e.g., CNT, Super P conductive carbon, acetylene black, carbon fiber, or carbon nanofiber) and a metal nanocatalyst present in a predetermined weight percentage; adding a first organic solvent (e.g., acetone, tetrahydrofuran (THF)) to the composite mixture to form a first solution; performing vacuum filtration on the first solvent to form a graded porous film; and drying the graded porous film to form a positive electrode. The positive electrode can include a sulfur-rich layer fused and interlocked with an electrocatalyst and a polysulfide trapping layer in a graded structure. The metal nanocatalyst can include at least one metal (e.g., at least one transition metal such as at least one of Ni, Co, Pt, and Pd). The predetermined weight percentage can be in the range of 0.1 wt% to 10 wt% (e.g., 1 wt% to 5 wt%). The carbon-containing material can further include sulfur. Preparing the composite mixture can include dissolving the polymer binder in a second organic solvent (e.g., acetone, THF) to provide a second solution, gradually adding the second solution to a dry powder mixture of the carbon-containing material to provide a third solution, and grinding the third solution to evaporate the second organic solvent to provide the composite mixture. Drying the gradient porous film can include, for example, drying the gradient porous film at a temperature ranging from 40°C to 100°C (e.g., 40°C to 60°C) for a period ranging from 10 hours to 15 hours. The method can further include preparing the carbon-containing material prior to preparing the composite mixture.The carbon-containing material can be prepared by preparing a first mixture of conductive carbon and a salt (e.g., nitrate) of a metal nanocatalyst (e.g., a weight ratio of conductive carbon:pure metal nanocatalyst ranging from 99:1 to 95:5), adding water (e.g., deionized water) to the first mixture to give a second mixture, bath-sonicating the second mixture, drying the second mixture after bath-sonicating the second mixture, grinding the second mixture after drying the second mixture, and oxidizing the second mixture in a furnace to obtain the oxidized second mixture. The method can include providing a mixture, allowing the oxidized second mixture to cool (e.g., naturally to room temperature or about room temperature), reducing the oxidized second mixture by providing a gas flow containing hydrogen to provide a reduced second mixture, allowing the reduced second mixture to cool (e.g., naturally to room temperature or about room temperature), allowing the second mixture to cool, and then grinding the reduced second mixture to provide a dry powder mixture of carbon-containing material. Oxidizing the second mixture can include oxidizing the second mixture in a furnace at a temperature of, for example, 350°C or about 350°C for a period of, for example, 2 hours or about 2 hours. Reducing the second mixture can include providing a gas flow at a temperature of, for example, 400°C or about 400°C for a period of, for example, 2 hours or about 2 hours. The gas flow can further include argon (e.g., 5% hydrogen / argon). The positive electrode can have sulfur uniformly (or substantially uniformly) distributed throughout the graded structure. The sulfur-rich layer can include conductive carbon (e.g., CNT, Super P conductive carbon, acetylene black, carbon fiber, or carbon nanofiber). The electrocatalyst and polysulfide scavenging layer can include conductive carbon (e.g., CNT, Super P conductive carbon, acetylene black, carbon fiber, or carbon nanofiber). The positive electrode can further include a binder (e.g., PVDF, PAA). Alternatively, the positive electrode can be binder-free (i.e., no binder is present in the positive electrode).

[0008] In another embodiment, a method for manufacturing a Li-S battery can include fabricating a positive electrode using a method as disclosed herein, disposing the positive electrode on (and optionally in direct physical contact with) a current collector (e.g., Al, such as Al foil), disposing the positive electrode on (and optionally in direct physical contact with) a separator, and disposing a lithium anode on (and optionally in direct physical contact with) the separator. The separator can be on (and optionally in direct physical contact with) a first surface of the positive electrode opposite (and optionally in direct physical contact with) a second surface of the positive electrode on which (and optionally in direct physical contact with) the current collector is disposed. The anode can be on the opposite side of the separator from the positive electrode. The method can further include disposing a first spacer under the current collector, disposing a spring under the first spacer and / or current collector, disposing a bottom cap under the spring, first spacer, and / or current collector, disposing a second spacer on the lithium anode, and / or disposing a top cap on the second spacer and / or lithium anode. [Brief explanation of the drawings]

[0009] [Figure 1] Plots of heat flow (Watts per gram (W / g)) and weight (percentage (%)) versus temperature (°C) for a sulfur-carbon composite obtained by thermogravimetric analysis (TGA) (heat flow curve) and differential scanning calorimetry (DSC) (weight curve). The dashed line is the heat flow curve, and the solid line is the weight curve. [Figure 2] 1 shows a scanning electron microscope (SEM) image of a cross section of a platinum (Pt)-containing graded lithium-sulfur (Li-S) cathode (having a first configuration) coated on a polypropylene separator according to one embodiment of the present invention, with an elemental analysis line scan showing the sulfur content throughout the thickness of the cathode shown on the right side of the image. [Figure 3(a)] 1 shows a high magnification SEM surface image of a Pt-containing graded Li—S cathode according to one embodiment of the present invention, with the scale bar being 100 nanometers (nm). [Figure 3(b)]A high-magnification transmission electron microscope (TEM) image showing a carbon surface covered with catalyst particles is shown. The scale bar is 100 nm. [Figure 4] 1 is a plot of discharge capacity (milliamp-hours per gram (mAh / g)) versus cycle number showing the electrochemical performance of a graded-structure Li—S cathode (having a first configuration) according to an embodiment of the present invention with different metal catalysts compared to a baseline cathode (i.e., a cathode without a graded structure). The solid curve for the highest capacity at cycle 2 is for the graded-structure cathode with a Pt catalyst, the dashed curve for the second highest capacity at cycle 2 is for the graded-structure cathode with a nickel (Ni) catalyst, the dashed curve for the third highest capacity at cycle 2 is for the graded-structure cathode with a palladium (Pd) catalyst, the dotted curve for the fourth highest capacity at cycle 2 is for the graded-structure cathode with a cobalt (Co) catalyst, and the solid curve for the lowest capacity at cycle 2 is for the baseline cathode. [Figure 5(a)] Figure 1 shows a plot of discharge capacity (mAh / g) versus cycle number illustrating the electrochemical performance of a Pt-containing graded-structure Li-S cathode (having a first configuration) according to an embodiment of the present invention (labeled "new-structure cathode" in the figure) and a baseline cathode. The sulfur loading for both cathodes was 2.50 milligrams per square centimeter (mg / cm), and the test was conducted at a rate of +0.1 C / -0.2 C and 1.8 volts (V) - 3.0 V. The square data points with high discharge capacity values ​​are for the graded-structure cathode, and the circle data points with low discharge capacity values ​​are for the baseline cathode. [Figure 5(b)] Figure 1 shows a plot of discharge capacity (mAh / g) versus cycle number illustrating the electrochemical performance of a Pt-containing graded-structure Li-S cathode (having a first configuration) according to an embodiment of the present invention (labeled "new-structure cathode" in the figure) and a baseline cathode. Both cathodes had a sulfur loading of 5.0 mg / cm, and tests were conducted at a rate of +0.16 C / -0.2 C and 1.8 V to 3.0 V. The square data points with higher discharge capacity values ​​are for the graded-structure cathode, and the circle data points with lower discharge capacity values ​​are for the baseline cathode. [Figure 5(c)] Figure 1 shows a plot of discharge capacity (mAh / g) versus cycle number illustrating the electrochemical performance of a Pt-containing graded Li-S cathode (having a first configuration) according to an embodiment of the present invention (labeled "New Structure Cathode" in the figure) and a baseline cathode. Both cathodes had a sulfur loading of 3.4 mg / cm², and tests were conducted at a higher C rate (+0.33 C / -0.5 C rate) and from 1.8 V to 3.0 V. The generally higher discharge capacity values ​​represented by squares are for the graded structure cathode, while the generally lower discharge capacity values ​​represented by circles are for the baseline cathode. [Figure 6] Figure 1 shows a plot of discharge capacity (mAh / g) versus C-rate (milliamps per gram per square centimeter (mA / cm / gs)) illustrating the electrochemical performance of a Pt-containing graded-structure Li-S cathode (having a first configuration) according to an embodiment of the present invention (labeled "new-structure cathode" in the figure) and a baseline cathode. Both cathodes had a sulfur loading of 3.85 mg / cm, and tests were conducted between 1.8 V and 3.0 V. The square data points, which represent generally higher discharge capacity values, are for the new-structure cathode, and the circular data points, which represent generally lower discharge capacity values, are for the baseline cathode. [Figure 7] 1 shows a plot of voltage (V) versus capacity (mAh / g) illustrating the first cycle voltage profile of a Pt-containing graded-structure Li-S cathode (having a first configuration) according to an embodiment of the invention (labeled "new-structure cathode" in the figure) and a baseline cathode. The solid curve is for the graded-structure cathode, and the dashed curve is for the baseline cathode. [Figure 8] 1 shows a plot of current (in amperes (A)) versus voltage (in V, vs. Li / Li+) illustrating a cyclic voltammetry (CV) plot of a Pt-containing graded-structure Li-S cathode (having a first configuration) according to an embodiment of the invention (labeled "new-structure cathode" in the figure) and a baseline cathode. The solid curve is for the graded-structure cathode, and the dashed curve is for the baseline cathode. [Figure 9]Figure 1 shows plots of discharge capacity (mAh / g) versus cycle number illustrating the electrochemical performance of a graded Li-S cathode (having a second configuration) according to an embodiment of the present invention (labeled "New Structure Cathode" in the figure) with different metal catalysts compared to a baseline cathode. The solid curve showing the highest capacity at cycle 2 is for the graded Pt / Pt cathode, the dashed curve showing the second highest capacity at cycle 2 is for the graded Ni / Pt cathode, the dotted curve showing the third highest capacity at cycle 2 is for the graded Co / Pt cathode, the dashed curve showing the fourth highest capacity at cycle 2 is for the graded Pd / Pt cathode, and the solid curve showing the lowest capacity at cycle 2 is for the baseline cathode. [Figure 10(a)] SEM image of nanocatalyst loaded onto multi-walled carbon nanotubes (MWCNTs). Scale bar is 20 nm. [Figure 10(b)] 1 shows a plot of cell capacity (mAh / g of sulfur) versus cycle number for a nanocatalyst-containing cathode according to an embodiment of the invention compared to a baseline cathode (or "control cathode"). Data points with higher cell capacity values ​​are for the nanocatalyst-containing cathode, and data points with lower cell capacity values ​​are for the control cathode. [Figure 11] FIG. 1 shows a table illustrating the initial and cycle 10 discharge capacity (mAh / g) of a graded structure Li—S cathode (having a first configuration) according to an embodiment of the present invention (labeled “New Structure Cathode” in the figure) compared to a baseline cathode. [Figure 12] 1 shows a table illustrating the rate capability of a Pt-containing graded-structure Li—S cathode (having a first configuration) according to an embodiment of the present invention (labeled “new structure cathode” in the figure) and a baseline cathode. [Figure 13] FIG. 1 shows a table illustrating the initial and cycle 10 discharge capacity (mAh / g) of a graded structure Li—S cathode (having a second configuration) according to an embodiment of the present invention (labeled “New Structure Cathode” in the figure) compared to a baseline cathode. DETAILED DESCRIPTION OF THE INVENTION

[0010] Embodiments of the present invention provide a novel and advantageous lithium-sulfur (Li-S) battery with a graded structure as the positive electrode (i.e., cathode), including an active electrocatalyst and polysulfide scavenging system for improving sulfur utilization and capacity retention, as well as methods for fabricating and using the same. The graded-structure Li-S cathode can be fabricated using economical and scalable synthesis and coating methods. Electrochemical performance results show that the graded-structure Li-S cathode exhibits improved sulfur utilization and cycling stability compared to Li-S batteries without the graded-structure Li-S cathode. The cathode without the graded structure may be referred to herein as the "baseline cathode" or "baseline structure." The graded-structure Li-S cathode with nanocatalyst (also referred to herein as the "new-structure cathode" or "new-structure Li-S cathode") exhibits an average 20% increase in initial sulfur utilization and a greater than 30% improvement in cell capacity retention over 200 cycles compared to the baseline cathode.

[0011] The graded-structure Li-S cathode was developed to maximize sulfur utilization and improve the cycling stability of Li-S batteries through effective spatial distribution of metal catalysts. The new Li-S cathode structure includes a sulfur-rich, conductive carbon composite layer, a binder (e.g., as described in Example 1 or Example 3), and a buffer layer that actively forms an electrocatalyst and polysulfide trapping system. The layer thickness can be adjusted depending on the sulfur loading.

[0012] A graded Li-S cathode can be fabricated using a two-step process involving coating a sulfur-rich layer (e.g., using the structures fabricated in Example 1 or Example 3) followed by a buffer layer with both electronic conductivity and electrocatalytic activity. The layers can be applied sequentially, either with a binder or binder-free (i.e., without a binder). For example, a sulfur-rich layer can be coated on a current collector (e.g., on an Al foil) and dried, followed by the buffer layer. The same layer structure can also be achieved by coating directly on a separator in the reverse order: first the buffer layer, then the sulfur-rich layer. Other sequential coating approaches, using multiple stacks of two layers, can also be implemented. Upon drying, the final coating forms a mechanically interlocked, tailored / graded porous structure.

[0013] In embodiments of the present invention, interconnected electronic conduction pathways can be established throughout the cathode. The novel Li-S cathode maximizes sulfur utilization by confining higher-order polysulfide species (e.g., Li2S8 and Li2S6) within the cathode structure and effectively catalyzing them in a highly reversible process due to the spatial location of the catalyst. The cathode-current collector-separator configuration can include, for example, a current collector (e.g., a metal foil such as aluminum foil), a sulfur-rich layer disposed on the current collector, an electrocatalyst, and a polysulfide trapping layer (which can be fused and interlocked together to form the novel cathode), and a separator (e.g., a polypropylene separator) disposed on the sulfur-rich, electrocatalyst, and polysulfide trapping layer (i.e., disposed on the cathode). Sulfur can be distributed uniformly or substantially uniformly throughout the thickness of the cathode (e.g., throughout the sulfur-rich, electrocatalyst, and polysulfide trapping layers). Embodiments provide higher capacity retention, higher initial capacity, and sulfur utilization over cycling compared to related art Li-S batteries.

[0014] The cathode can include elemental sulfur, conductive carbon (e.g., carbon nanotubes, Super P conductive carbon, acetylene black, carbon fiber, or carbon nanofiber), and a binder (e.g., polyvinylidene fluoride, polyacrylic acid). A structure can be fabricated as described in Examples 1 and / or 3, and then a sulfur-rich layer can be coated onto the structure. A buffer layer with both electronic conductivity and electrocatalytic activity can then be coated onto the structure and / or sulfur-rich layer. For example, a sulfur-rich layer can be coated onto a current collector, dried, and then coated with a buffer layer. The same layer structure can also be achieved by coating directly onto a separator in the reverse order: first, the buffer layer, and then the sulfur-rich layer. Other sequential coating approaches, using multiple stacks of two layers, can also be implemented. Upon drying, the final coating forms the mechanically interlocked, tailored / graded porous structure used in the cathode.

[0015] In one embodiment, a sulfur-conductive carbon composite can be synthesized as follows: Elemental sulfur and conductive carbon (e.g., carbon nanotubes, such as multi-walled carbon nanotubes) can be combined (e.g., by grinding, e.g., with a mortar and pestle at room temperature) (e.g., in a weight ratio of 7:3 or any subrange therein, ranging from a weight ratio of 7:6 to a weight ratio of 7:1, including 2:1). The sulfur composite can optionally be further ball-milled (e.g., at 200-500 rpm for 4-8 hours in an inert (e.g., argon) atmosphere) (composite:ball = weight ratio of 1:10 or about 1:10, preferably, or a weight ratio of 1:1 to 1:20, including any subrange therein). The composite sample (after combining or optional ball-milling) can be placed in an inert atmosphere (e.g., in an argon glove box or a Teflon-lined stainless steel autoclave). An inert atmosphere (e.g., autoclave) can be heated (e.g., in an air oven) for melt diffusion of sulfur into the conductive carbon host (e.g., at 150-200 °C for 12-30 h). After natural cooling to room temperature, the sulfur composite can be optionally ground (e.g., with a mortar and pestle) and stored for several weeks for further use.

[0016] In embodiments, catalysts (e.g., nanocatalysts such as platinum group metal (PGM) nanocatalysts) can be implemented into Li-S cathodes using a process tailored to effectively improve catalyst dispersion and provide controlled catalyst-electrolyte contact. The nanocatalysts can be loaded onto conductive carbon (e.g., carbon nanotubes) at variable low loadings (e.g., 0.1 wt% to 5 wt%), allowing for sulfur loadings up to 70 wt% in cathodes. Figure 10(a) shows an SEM image of nanocatalysts loaded onto multi-walled carbon nanotubes (MWCNTs). This cathode improves the redox performance of Li-S batteries, improves sulfur utilization, and provides sulfur loadings of 4-5 mg / cm. 2The cell maintains stable capacity even at a high sulfur loading of 1000 sq. m. Comparing the performance of the cell containing the nanocatalyst (i.e., with the new structure cathode) with the cell with the baseline cathode (i.e., without the nanocatalyst), it can be seen in Figure 10(b) that the cell containing the nanocatalyst provides improved first-cycle capacity and stable capacity retention during the initial cycle life of the cell.

[0017] When ranges are used herein, combinations and subcombinations of ranges (e.g., subranges within the disclosed ranges), as well as specific embodiments therein, are expressly intended to be included. When the term "about" is used in conjunction with a numerical value herein, it is understood that the value can be within 95% to 105% of the value, i.e., the value can be + / - 5% of the stated value. For example, "about 1 kg" means 0.95 kg to 1.05 kg.

[0018] A better understanding of the embodiments of the present invention and their many advantages can be obtained from the following examples, given by way of illustration. The following examples illustrate some of the methods, uses, embodiments and variations of the present invention. Of course, they are not to be construed as limiting the invention. Numerous variations and modifications can be made to the embodiments of the present invention.

[0019] Materials and Methods The cathode contains elemental sulfur, conductive carbon (with or without a metal catalyst) (e.g., carbon nanotubes (CNTs), super P conductive carbon (SP), acetylene black (AB), or carbon fiber (CF)), and a binder (e.g., polyvinylidene fluoride (PVDF) or polyacrylic acid (PAA)). For example, sulfur conductive carbon composites were synthesized as follows: Elemental sulfur and multi-walled carbon nanotubes were ground at a weight ratio of 7:3 using a mortar and pestle at room temperature. This sulfur composite was then ball-milled in an argon atmosphere at 200–500 revolutions per minute (rpm) for 4–8 hours (a composite:ball weight ratio of 1:10 is preferred). The milled composite sample was placed in an argon glove box and then placed in a Teflon-lined stainless steel autoclave. The autoclave was then removed and placed in an air oven at 150–200 °C for 12–30 hours to allow the sulfur to melt and diffuse into the carbon nanotube host. After cooling to room temperature, the sulfur composite can be ground with a mortar and pestle and stored for several weeks for further use. Figure 1 shows the thermogravimetric analysis (TGA) plot of the prepared sulfur-CNT composite. The TGA data show that when the sample was heated to 500 °C at a rate of 5 °C / min under argon atmosphere, the total weight of sulfur decreased by 69.5% at 274 °C. The TGA results are in close agreement with the initial sulfur content in the sulfur-CNT composite. The electrolyte was prepared in an argon-filled glovebox by dissolving 1 molar (M) lithium bis(trifluorosulfonyl)imide (LiTFSI) and 0.1 M anhydrous lithium nitrate (LiNO3) in a 1:1 volumetric ratio of dioxolane (DOL) and dimethoxyethane (DME). Pre-cut lithium disks were immersed in the electrolyte before assembling the cell to remove traces of moisture. In an argon-filled glove box, coin cells were assembled using a pre-soaked Li disk, a polypropylene separator (Celgard), and a sulfur cathode. The pre-soaked separator was filled with excess electrolyte and placed on top of the lithium anode, followed by a spacer and bottom cap, followed by the sulfur cathode, spacer, spring, and top cap.The cell assembly was crimped under 90 pounds per square inch (psi) of pressure using an argon gas-powered coin cell crimper. Electrochemical charge-discharge profiles of the coin cells were recorded using MTI and Abrin Instruments Battery Testers at different C-rates over a voltage window of 1.8 V to 3.0 V. Cyclic voltammetry (CV) measurements were recorded using a Gamry Potentiostat 5000E.

[0020] Example 1: Preparation of Li-S cathode (first configuration) A castable slurry was prepared by mixing the prepared sulfur-conductive carbon composite (e.g., S-CNT 7:3), conductive carbon (e.g., SP), and binder (e.g., polyvinylidene fluoride (PVDF)) in a weight ratio of 9:0.5:0.5. This dry solid mixture was ground with a mortar and pestle at room temperature, vortexed, and further ground. Next, the binder dissolved in an organic solvent (e.g., N-methyl-2-pyrrolidone (NMP)) was added to the solid mixture. After the binder solution was added, the entire mixture was ground with a mortar and pestle, followed by the addition of a mixed solvent solution (e.g., DMF:ethanol:NMP = 1:1:1) and grinding. Small amounts of pure NMP were then added sequentially to control the rheological properties of the cathode slurry. A certain amount of the slurry was applied to a battery-grade aluminum (Al) foil as a current collector and coated with a doctor blade. The coated foils were dried directly in air in an oven at 60 °C for 6 to 12 hours. After natural cooling, the coating was vacuum dried in the same oven for 1 to 2 hours to obtain a coating density of 2.0 to 5.0 milligrams per square centimeter (mg / cm). 2 Cathode disks were punched with sulfur loadings ranging from 0.01 to 0.01. The cathodes fabricated in this example are referred to as the first configuration.

[0021] Example 2: Preparation of an active electrocatalytic and polysulfide capture system An electrocatalyst and polysulfide capture system is an integral part of the Li-S cathode according to an embodiment of the present invention. This system includes a high-aspect ratio conductive carbon material (e.g., CNTs, CFs, or carbon nanofibers (CNFs)) and an electrochemically active catalyst, preferably a transition metal (e.g., Ni, Co, Pt, Pd, etc.). For example, a mixture of CNTs and a catalyst metal salt (e.g., a nitrate salt such as palladium nitrate or platinum nitrate) was weighed out in a CNT:pure metal catalyst weight ratio ranging from 99:1 to 95:5 and placed in a glass beaker. The mixture was stirred, and additional deionized (DI) water was added to completely immerse the CNTs into a viscous slurry. The beaker was then bath sonicated for 10–30 minutes and stirred again. The mixture was then dried overnight on a hot plate at temperatures ranging from 100°C to 120°C. The dried CNT:metal salt mixture was lightly ground with a mortar and pestle. Two quartz glass boats were filled with the dried CNT:metal salt mixture and oxidized in air in a tube furnace at 350 °C for 2 h. After natural cooling, the metal oxide on the CNTs was further reduced in a 5% hydrogen / argon gas stream at 400 °C for 2 h. After natural cooling to room temperature, the reduced x%M-CNT (where M = Ni, Co, Pt, or Pd, x = 1 weight percent (wt%) to 5 wt%) of CNT samples were ground with a mortar and pestle and stored as dry powder in glass vials.

[0022] Example 3: Fabrication of Li-S cathode (second configuration) The prepared sulfur-conductive carbon composite (e.g., Sx%M-CNT 7:3) (e.g., x = 1 wt%, M = Pt) was mixed with conductive carbon (e.g., SP) and binder (e.g., PVDF) in a weight ratio of 9:0.5:0.5 to prepare a castable slurry. The dried solid mixture was ground with a mortar and pestle at room temperature, vortexed, and further ground. Next, the binder dissolved in an organic solvent (e.g., NMP) was added to the solid mixture. After the binder solution was added, the entire mixture was ground with a mortar and pestle, followed by the addition of a mixed solvent solution (e.g., DMF:ethanol:NMP = 1:1:1) and grinding. Small amounts of pure NMP were then added sequentially to control the rheological properties of the cathode slurry. A battery-grade Al foil was used as a current collector, and a certain amount of the slurry was applied onto it and coated with a doctor blade. The coated foil was then directly dried in an oven at 60 °C for 6 to 12 h in air. After natural cooling, the coating was vacuum dried in the same oven for 1–2 h to obtain a sulfur loading of 2.0–5.0 mg / cm 2 The cathode fabricated in this example is referred to as the second configuration.

[0023] Example 4: Fabrication of a graded structure Li-S cathode An example of a graded porous structure in a polypropylene separator is detailed below. First, a composite mixture of x% M-CNT (x = 1 wt% to 5 wt%) and cellulose acetate at a weight ratio of 7:3 was prepared. For example, cellulose acetate was dissolved in an organic solvent (e.g., acetone or tetrahydrofuran (THF)) by vortex mixing. The dissolved cellulose acetate solution was then gradually added to a dry powder mixture of M-CNT (prepared as in Example 2) and ground. During grinding, the acetone quickly evaporated, leaving the cellulose acetate with x% M-CNT (x = 1 wt% to 5 wt%), forming a composite mixture that could be collected and stored as a dry powder. Next, an organic solvent (e.g., acetone or THF) was added to the composite mixture, vortex mixed, and then bath sonicated (10–20 min). The mixture was then poured into a vacuum filtration setup containing a pre-cut polypropylene separator (Celgard) with regular filter paper underneath. Filtration was immediately performed using a high-vacuum filtration setup connected to a vacuum filtration pump. The coated polypropylene separator was placed under high vacuum for 1-2 h and then removed. The cellulose acetate film adhered well to the separator, allowing for better dispersion of x%M-CNTs (M = Ni, Co, Pt, or Pdx = 1 wt%-5 wt% of CNTs) and faster solvent filtration through the pores. This resulted in a highly tortuous, graded porous structure, leading to a very fast post-drying process. The prepared graded porous structure film was dried on a hot plate at 40-60 °C for 10-15 h and tested in a Li-S battery. Interconnected electronic conduction pathways were established throughout the cathode. This novel Li-S cathode maximized sulfur utilization by trapping highly ordered polysulfide species (e.g., Li2S8 and Li2S6) within the cathode structure, effectively catalyzing a highly reversible process due to the spatial location of the catalyst. The cross-sectional scanning electron microscope (SEM) image in Figure 2 shows the new structure cathode of a full Li-S cell discharged to 1.50 V. Referring to Figure 2, from bottom to top, the top layers in the image are the Al foil (current collector), the sulfur-rich, electrocatalyst and polysulfide scavenging layer (both layers are fused and interlocked together to form the new structure cathode), and finally the separator.A cross-section of a fully discharged cathode was analyzed by energy-dispersive X-ray spectroscopy (EDX) using line-scan detection, revealing a uniform sulfur distribution (represented by the S in Li2S) throughout the cathode's thickness. The high-magnification SEM surface image in Figure 3(a) shows the spatial distribution of the electrocatalyst (e.g., Pt metal catalyst particles appear as bright dots) on the conductive CNT surface of the new Li-S cathode. The high-resolution transmission electron microscope (TEM) image in Figure 3(b) reveals that the metal catalyst appears as small, dark particles decorating the CNT surface.

[0024] Example 5: Use of the new structure Li-S cathode in a Li-S battery (first configuration) Coin cells (CR2032) were charged with either the baseline or new-structure Li-S cathode (based on the first configuration) at 2.5–5.0 mg / cm in an argon-filled glove box. 2 Cells were assembled using sulfur loadings ranging from 0.01 to 0.10, polypropylene separators (Celgard), and pre-cut Li disks. Cells were crimped under 90 psi pressure using an argon gas-driven coin cell crimper. The assembled cells were then placed in a static state and tested at different C-rates (e.g., charge at C / 10, discharge at C / 5, charge at C / 6, discharge at C / 5, charge at C / 3, discharge at C / 2) within a voltage window of 1.8 V to 3.0 V. Cells were tested with different nanocatalysts (Pt, Pd, Ni, Co) (sulfur loading 3.85 mg / cm). 2 ) are shown in Figure 4. Referring to Figure 4, over 15 cycles, the Pt-containing cell showed higher initial utilization and improved stability compared to the baseline cell. The other nanocatalyst (Pd, Ni, Co) cells showed similar performance with higher initial utilization compared to the baseline cell. The table in Figure 11 summarizes the initial cycle performance of the new structure Li-S cathode (first configuration) compared to the baseline cathode. Figure 5(a) shows the Pt-containing Li-S cell and the Pt-containing Li-S cell with a sulfur loading of 2.50 mg / cm. 2Figure 1 shows the cell performance of a baseline cell charged at C / 10 and discharged at C / 5 for 100 cycles. The new Li-S cathode exhibited a higher initial discharge capacity of 1538 mAh / g compared to 1146 mAh / g for the baseline cathode. The new Li-S cathode exhibited a 7.5% capacity fade at the second cycle, while the baseline Li-S cathode exhibited a 43% capacity fade. The new Li-S cathode stabilized at an average capacity of 1400 mAh / g at 10 cycles, while the baseline Li-S cathode exhibited a continuous capacity fade until stabilizing at 530 mAh / g at 30 cycles.

[0025] After 100 charge-discharge cycles, the capacity of the new Li-S cathode decreased by 13%, while that of the baseline Li-S cathode decreased by 53%. Figure 5(b) shows the results for the high sulfur loading (5.0 mg / cm). 2 ) cells. The Pt-containing cell showed a 37% improved retention over 100 cycles compared to the baseline cell. Figure 5(c) shows the high-rate performance of the new cathode (charged at C / 3 and discharged at C / 2) over 200 cycles using a sulfur loading of 3.40 mg / cm. 2 The results are shown for cells cycled at higher C-rates. The Pt-containing new structure cathode showed a 27% improvement in capacity retention over 200 cycles compared to the baseline cathode.

[0026] The rate capability of the Pt-containing new-structure Li-S cathode (first configuration) was tested against a baseline cathode at different discharge C-rates (e.g., C / 10, C / 5, C / 2, and 1C). After an initial activation cycle at C / 10, the cells were tested for two cycles at each rate. Rate capability data are reported for the high-capacity cycle. Figure 6 shows the discharge capacity (mAh / g) of the new-structure Li-S and baseline cathodes at different discharge rates. The table in Figure 12 shows that the rate capability of the new-structure Li-S cathode was significantly improved when the discharge C-rate was doubled (e.g., from C / 2 to 1C).

[0027] The initial cycle voltage profiles of the new structure cathode and the baseline cathode are shown in Figure 7. The discharge voltage profiles of the baseline and new structure cathodes showed initial discharge capacities of 890 mAh / g and 1538 mAh / g, respectively. The improvement in initial cycle capacity is seen in the high capacity corresponding to the high polysulfide (Li2S8 and Li2S6) depletion plateau (2.30 V). This improvement is also evident in the extended low-voltage plateau (2.10 V) of the new structure cathode, contributing significantly to the overall discharge capacity compared to the baseline cathode. Further electrochemical testing using cyclic voltammetry (0.1 mV / s scan rate) showed higher amplitude reduction / oxidation peaks for the new structure cathode compared to the baseline Li-S cathode (Figure 8), indicating improved reaction kinetics due to the enhanced catalytic activity of the new structure cathode.

[0028] Example 6: Use of the new structure Li-S cathode in a Li-S battery (second configuration) As in Example 5, a coin cell CR2032 was prepared with a sulfur loading of 3.50 mg / cm. 2A baseline or new-structure Li-S cathode (based on the second configuration), a polypropylene separator (Celgardd), and a pre-cut Li disk were assembled in an argon-filled glove box. The electrocatalyst and polysulfide capture layer contained Pt, Pd, Co, and Ni (e.g., 1 wt% Pd in ​​CNT) as the electrocatalyst, and the sulfur-rich layer contained Pt metal (e.g., 1 wt% Pt in CNT). The new-structure Li-S cathode can be referred to as "new-structure cathode-M / Pt" (M = Pt, Pd, Ni, Co). The assembled cells were crimped under 90 psi pressure using an argon-gas-driven coin cell crimper. The assembled cells were then tested at different C rates (e.g., charge at C / 6, discharge at C / 5) within a voltage window of 1.8 V to 3.0 V. Figure 9 shows the cell performance over 15 cycles, measured in terms of discharge capacity (mAh / g), of the novel Li-S cathodes using different catalysts (e.g., Pt, Pd, Ni, and Co), compared to a baseline cathode containing Pt only in the sulfur-rich layer. Referring to Figure 9, similar to Figure 4, all novel cathodes (second configuration) demonstrate higher sulfur utilization (initial discharge capacity) and improved capacity retention compared to the baseline Pt cathode. Furthermore, all novel Li-S cathodes maintain high capacity retention over subsequent cycles. The Pt / Pt cathode exhibited the highest capacity at 15 cycles, followed by Pd / Pt and Ni / Pt. Meanwhile, Co / Pt exhibited slightly lower capacity compared to all novel cathodes in the second configuration. The table in Figure 13 summarizes the initial discharge capacity (mAh / g) and 15-cycle discharge capacity of the novel cathodes in the second configuration compared to the baseline Pt cathode.

[0029] It is to be understood that the examples and embodiments described herein are for illustrative purposes only and that various modifications or changes in light thereof may be suggested to those skilled in the art and are within the spirit and scope of the present application.

[0030] All patents, patent applications, provisional applications and publications mentioned or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent not inconsistent with the explicit teachings of this specification.

Claims

1. 1. A positive electrode for a lithium-sulfur (Li—S) battery, comprising: a sulfur-rich layer comprising at least one first nanocatalyst; an electrocatalyst comprising at least one second nanocatalyst and a polysulfide scavenging layer; The sulfur-rich layer is fused and interlocked with the electrocatalyst and polysulfide scavenging layer in a gradient structure.

2. The cathode of claim 1 , wherein the at least one first nanocatalyst comprises a metal.

3. The cathode of claim 1 , wherein the at least one second nanocatalyst comprises a metal.

4. 4. The cathode of claim 1, wherein the at least one first nanocatalyst comprises at least one of nickel, cobalt, platinum, palladium, ruthenium, iridium, rhodium, silver, gold, nickel oxide, cobalt oxide, platinum oxide, palladium oxide, ruthenium oxide, iridium oxide, rhodium oxide, silver oxide, gold oxide, nickel sulfide, cobalt sulfide, platinum sulfide, palladium sulfide, ruthenium sulfide, iridium sulfide, rhodium sulfide, silver sulfide, gold sulfide, nickel fluoride, cobalt fluoride, platinum fluoride, palladium fluoride, ruthenium fluoride, iridium fluoride, rhodium fluoride, silver fluoride, gold fluoride, nickel carbide, cobalt carbide, platinum carbide, palladium carbide, ruthenium carbide, iridium carbide, rhodium carbide, silver carbide, and gold carbide.

5. 5. The cathode of claim 1, wherein the at least one second nanocatalyst comprises at least one of nickel, cobalt, platinum, palladium, ruthenium, iridium, rhodium, silver, gold, nickel oxide, cobalt oxide, platinum oxide, palladium oxide, ruthenium oxide, iridium oxide, rhodium oxide, silver oxide, gold oxide, nickel sulfide, cobalt sulfide, platinum sulfide, palladium sulfide, ruthenium sulfide, iridium sulfide, rhodium sulfide, silver sulfide, gold sulfide, nickel fluoride, cobalt fluoride, platinum fluoride, palladium fluoride, ruthenium fluoride, iridium fluoride, rhodium fluoride, silver fluoride, gold fluoride, nickel carbide, cobalt carbide, platinum carbide, palladium carbide, ruthenium carbide, iridium carbide, rhodium carbide, silver carbide, and gold carbide.

6. The positive electrode of any one of claims 1 to 5, wherein the positive electrode has sulfur distributed substantially uniformly throughout the graded structure.

7. The positive electrode of any one of claims 1 to 5, wherein the positive electrode has sulfur distributed in a gradient across the graded structure.

8. 8. The positive electrode according to claim 1, wherein the sulfur-rich layer comprises conductive carbon, and the electrocatalyst and polysulfide scavenging layer comprises conductive carbon.

9. 9. The positive electrode of claim 8, wherein the conductive carbon of the sulfur-rich layer is a carbon nanotube, graphene, fullerene, or graphitized carbon.

10. The positive electrode according to any one of claims 8 to 9, wherein the electrocatalyst and the conductive carbon of the polysulfide trapping layer are carbon nanotubes, graphene, fullerene, or graphitized carbon.

11. A current collector; The positive electrode according to any one of claims 1 to 10, which is disposed on the current collector and is in direct physical contact with the current collector; a separator disposed on and in direct physical contact with a first surface of the positive electrode, the first surface of the positive electrode being opposite a second surface of the positive electrode with which the current collector is in direct physical contact; a lithium anode on the separator.

12. a first spacer below the current collector; and a spring below the first spacer; a bottom cap below the spring; a second spacer on the lithium anode; a top cap on the second spacer.

13. preparing a composite mixture of a polymer binder and a carbon-containing material comprising conductive carbon and a metal nanocatalyst present in a predetermined weight percentage; adding a first organic solvent to the composite mixture to provide a first solution; performing vacuum filtration of the first solvent to provide a gradient porous film; drying the gradient porous film to provide a positive electrode; 1. A method of manufacturing a cathode, wherein the cathode comprises a sulfur-rich layer fused and interlocked with an electrocatalyst and a polysulfide scavenging layer in a graded structure.

14. 14. The method of claim 13, wherein the metal nanocatalyst comprises at least one of nickel, cobalt, platinum, palladium, ruthenium, iridium, rhodium, silver, gold, nickel oxide, cobalt oxide, platinum oxide, palladium oxide, ruthenium oxide, iridium oxide, rhodium oxide, silver oxide, gold oxide, nickel sulfide, cobalt sulfide, platinum sulfide, palladium sulfide, ruthenium sulfide, iridium sulfide, rhodium sulfide, silver sulfide, gold sulfide, nickel fluoride, cobalt fluoride, platinum fluoride, palladium fluoride, ruthenium fluoride, iridium fluoride, rhodium fluoride, silver fluoride, gold fluoride, nickel carbide, cobalt carbide, platinum carbide, palladium carbide, ruthenium carbide, iridium carbide, rhodium carbide, silver carbide, and gold carbide.

15. The method according to any one of claims 13 to 14, wherein the predetermined weight percentage is in the range of 0.1 wt% to 10 wt%.

16. The method of any one of claims 13 to 15, wherein the carbon-containing material further comprises sulfur.

17. The preparation of the composite mixture comprises: dissolving the polymer binder in a second organic solvent to provide a second solution; slowly adding the second solution to the dry powder mixture of the carbon-containing material to provide a third solution; 17. The method of any of claims 13 to 16, comprising grinding the third solution and evaporating the second organic solvent to provide a complex mixture.

18. 18. The method of any of claims 13 to 17, wherein drying the gradient porous film comprises drying the gradient porous film at a temperature ranging from 40°C to 100°C for a period ranging from 10 hours to 15 hours.

19. The method further includes preparing the carbon-containing material before preparing the composite mixture, and preparing the carbon-containing material includes: preparing a first mixture of the conductive carbon and a salt of the metal nanocatalyst; adding deionized water to the first mixture to provide a second mixture; bath sonicating the second mixture; After bath sonication of the second mixture, drying the second mixture; After drying the second mixture, grinding the second mixture; oxidizing the second mixture in a furnace after grinding the second mixture to provide an oxidized second mixture; allowing the oxidized second mixture to cool; allowing the oxidized second mixture to cool, and then reducing the oxidized second mixture by providing a gas stream comprising hydrogen to the oxidized second mixture to provide a reduced second mixture; allowing the reduced second mixture to cool; 19. The method of any of claims 13 to 18, comprising allowing the second mixture to cool and then grinding the reduced second mixture to provide a dry powder mixture of the carbon-containing material.

20. The method according to any one of claims 13 to 19, wherein the predetermined weight percentage is in the range of 1 wt% to 5 wt%.

21. oxidizing the second mixture includes oxidizing the second mixture in a furnace at a temperature of about 350° C. for a period of about 2 hours; reducing the second mixture includes providing a gas stream at a temperature of about 400° C. for a period of about 2 hours; 20. The method of claim 19, wherein the gas stream further comprises argon.

22. A positive electrode is produced by carrying out the method according to any one of claims 13 to 21, disposing the positive electrode on a current collector and in direct physical contact with the current collector; the positive electrode being disposed on a separator and in direct physical contact with the separator; A method of making a Li-S battery comprising disposing a lithium anode on said separator.

23. preparing a composite mixture of a polymer binder and a carbon-containing material comprising conductive carbon, sulfur, and a metal nanocatalyst present in a predetermined weight percentage; adding a first organic solvent to the composite mixture to provide a first solution; performing vacuum filtration of the first solvent to provide a gradient porous film; drying the gradient porous film to provide a positive electrode; the positive electrode comprises a sulfur-rich layer fused and interlocked with an electrocatalyst and a polysulfide scavenging layer in a gradient structure; the metal nanocatalyst comprises at least one of nickel, cobalt, platinum, palladium, ruthenium, iridium, rhodium, silver, gold, nickel oxide, cobalt oxide, platinum oxide, palladium oxide, ruthenium oxide, iridium oxide, rhodium oxide, silver oxide, gold oxide, nickel sulfide, cobalt sulfide, platinum sulfide, palladium sulfide, ruthenium sulfide, iridium sulfide, rhodium sulfide, silver sulfide, gold sulfide, nickel fluoride, cobalt fluoride, platinum fluoride, palladium fluoride, ruthenium fluoride, iridium fluoride, rhodium fluoride, silver fluoride, gold fluoride, nickel carbide, cobalt carbide, platinum carbide, palladium carbide, ruthenium carbide, iridium carbide, rhodium carbide, silver carbide, and gold carbide; the predetermined weight percentage is in the range of 1 wt % to 5 wt %; drying the gradient porous film comprises drying the gradient porous film at a temperature ranging from 40°C to 60°C for a period ranging from 10 hours to 15 hours; The preparation of the composite mixture comprises: dissolving the polymer binder in a second organic solvent to provide a second solution; slowly adding the second solution to the dry powder mixture of the carbon-containing material to provide a third solution; 1. A method of making a positive electrode, comprising grinding the third solution and evaporating the second organic solvent to provide a composite mixture, The method further comprises preparing the carbon-containing material prior to preparing the composite mixture; The preparation of the carbon-containing material comprises: preparing a first mixture of the conductive carbon and a salt of the metal nanocatalyst; adding deionized water to the first mixture to provide a second mixture; bath sonicating the second mixture; After bath sonication of the second mixture, drying the second mixture; After drying the second mixture, grinding the second mixture; After grinding the second mixture, oxidizing the second mixture in a furnace to provide an oxidized second mixture; allowing the oxidized second mixture to cool; allowing the oxidized second mixture to cool, and then reducing the oxidized second mixture by providing a gas stream comprising hydrogen to the oxidized second mixture to provide a reduced second mixture; allowing the reduced second mixture to cool; allowing the second mixture to cool and then grinding the reduced second mixture to provide the dry powder mixture of carbon-containing material.

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