Passivation of lithium metal by two-dimensional materials for rechargeable batteries

The present application relates to methods for depositing two-dimensional materials (e.g., MoS2, WS2, MoTe2, MoSe2, WSe2, BN, BN—C composite, and the like) onto lithium electrodes. Battery systems incorporating lithium metal electrodes coated with two-dimensional materials are also described. Methods may include intercalating the two-dimensional materials to facilitate flow of Lithium ions in and out of the lithium electrode. Two-dimensional material coated lithium electrodes provide for high cycling stability and significant performance improvements. Systems and methods further provide electrodes having carbon structures (e.g., carbon nanotubes (CNTs), graphene, porous carbon, free-standing 3D CNTs, etc.) with sulfur coatings.

TECHNICAL FIELD

The present application relates to passivation techniques. More specifically, the present application provides for systems, devices, and methods for creating a battery having a passivation layer to protect its electrodes.

BACKGROUND

There is a growing awareness that current lithium ion battery technologies are reaching their limits in terms of storage and energy capabilities. However, there is still increasing demand for higher energy storage and longer lasting devices. This has challenged the research community to search for next-generation battery systems. Some of the current systems being researched include lithium-air (Li—O2) and lithium-sulfur (Li—S) batteries.

Lithium (Li) metal has been known as the “hostless” material to store Li ions (Li+) without the need for using intercalating and/or conducting scaffold techniques. For this reason, Li metal electrodes exhibit high theoretical specific capacity (3860 mAh g−1) and low redox potential (−3.04 V); thus, they are often regarded as the best choice to use for manufacturing/fabricating anodes for next-generation rechargeable Li batteries. However, Li metal anodes exhibit properties that cause multiple practical issues which inhibit their use. These properties are often associated with uncontrollable dendrite formation during repeated Li deposition/dissolution processes, which can lead to short circuiting the battery and potential overheating and fire.

Several techniques have been implemented to suppress Li dendrite growth and/or to enhance stability of Li metal. For example, methods have sought to do this through: controlling the dendrite growth/deposition of Li through liquid electrolyte modification with additives; adopting Li+ conducting polymer or solid state electrolytes; and applying a layer of alumina (Al2O3) upon the surface of Li metal. A thin layer of Al2O3is a ceramic-based material that lacks the electron conductivity of 2D materials, thus increasing internal resistance of the battery electrode. However, none of the approaches has been shown to be effective in the context of rechargeable batteries.

While the low cost and abundance of sulfur make the concept of Li—S batteries alluring, there are several issues that generally prevent the widespread development of Li—S batteries. For example, sulfur is an insulating material, which provides for poor utilization of the active material and hinders electron transfer during the charge/discharge process. In addition, during the discharge process, Li may react with sulfur to form higher-order soluble polysulfides at the cathode, which creates shuttling of polysulfide between the anode and cathode during the cycling process. The shuttle effect may increase the internal resistance of the battery and contribute to capacity fading. Further, the formation of uncontrolled dendrites resulting from uneven deposition of Li metal may cause safety problems at higher C-rates as well as continuous evolution of a porous Li metal structure, which may lead to corrosion of Li metal. While some approaches have been developed, issues of decreased cell efficiency and increased capacity fading still affect the performance of Li—S batteries when used with a Li anode.

SUMMARY

The present application is directed to systems, methods and devices which passivate Li metal with thin layers of 2D materials (e.g., MoS2, WS2, MoTe2, MoSe2, WSe2, BN, BN—C composite, and the like). Two-dimensional (2D) materials, one atomic thickness film, exhibit low impedance due to their unique interlayer structure that readily intercalates Li ions with minimum energy to substantially increase Li-ion diffusivity and electric conductivity while acting as a passivation layer for Li dendrite growth. Such methods may utilize sputtering or evaporation deposition to create the passivation layer. These methods may form a new phase between Li metal and electrolyte where large amounts of Li atoms may be intercalated in order to facilitate homogenous flow of Li+into and out of bulk Li metal. Unlike other carbon/polymer/ceramic-based protective layers, the unique structural aspects and phase-changing characteristics (e.g., semiconductor and/or metallic traits) of 2D materials such as MoS2, WS2, MoTe2, MoSe2, WSe2, have allowed embodiments of the application to circumvent high impedance and/or poor interfacial-contact related issues. For example, in one embodiment MoS2-coated Li electrodes have demonstrated no Li dendrite growth at a challenging current density over 10 mA cm−2and high capacity retention for over 1000 cycles. The fabricated 2D materials coated Li metal exhibits stable adhesion to the substrate, and yields high cycling stability in 2D materials coated Li metal over bare Li electrode counterparts in rechargeable batteries. Accordingly, embodiments of the present application provide for significant performance improvements in rechargeable batteries.

In an embodiment, a method for passivating lithium metal includes providing a lithium electrode, depositing at least one layer of a two-dimensional material on the lithium electrode, and intercalating the at least one layer of the two-dimensional material with a plurality of lithium ions. In another embodiment, a rechargeable lithium battery includes a first electrode, an electrolyte, and a second electrode, wherein the second electrode comprises a lithium metal having at least one layer of a two-dimensional material deposited thereon.

DETAILED DESCRIPTION

As illustrated byFIGS. 1A-B, methods for fabricating a 2D material coated Li metal electrode are illustrated in accordance with embodiments of the present application. Referring toFIG. 1A, before deposition of a 2D material, Li metal electrode101may be cleaned. Electrode101may include ribbon type Li metal, Li metal coated anodes, or the like. In an embodiment, electrode101may be cleaned with acetic acid, acetone, isopropyl alcohol, deionized water, or the like. In another embodiment, electrode101may be cleaned using a different series of steps and/or cleaning solutions. In certain embodiments, electrode101may have an interface layer102. Interface layer102may be inserted to promote adhesion of 2D materials with electrode101. For example, interface layer102may include a plasma (e.g., Ar, He, H2, N2gas) treated clean surface. In another embodiment, interface layer102may include a deposited metallic layer. A metallic layer may be deposited with a thickness of 1.0 nm to 10 nm. In yet another embodiment, interface layer102may be a functionalized interface layer. For instance, electrode101may be treated in a vacuum with a functional group (e.g., hydrogen, fluorine, C—H bonding).

Next, referring toFIG. 1B, 2D material103is deposited on electrode101(or electrode101with interface layer102). 2D material103may comprise one or more layers of 2D materials such as molybdenum disulfide (MoS2), tungsten disulfide (WS2), molybdenum ditelluride (MoTe2), molybdenum diselenide (MoSe2), tungsten diselenide (WSe2), boron nitride (BN), and/or any other transition metal dichalcogenide monolayer. It is appreciated that different materials may provide for different performance. For example, MoS2provides strong adhesion to Li metal; it also is readily transformed to metallic phase to reduce impedance. In an embodiment, as illustrated byFIG. 1B, metal102(e.g., Mo) is deposited via direct current (DC) sputtering, e-beam evaporation or electro-chemical deposition; subsequently 2D material103may be deposited via sputtering. Using target111(e.g., any of the aforementioned 2D materials) as the target material for magnetron radio frequency (RF) sputtering, successive layers of 2D materials are sputtered onto electrode101to produce a 2D material coated electrode. In an embodiment, sputtering may occur within chamber110with base pressure maintained at or below 10−6Torr, inert gas flow112, and RF power at 10-100 W. Inert gas flow112may be flowed at 1-100 mTorr and comprise argon, helium, or any other gas that has low reactivity with other substances. In other embodiments, evaporation may be utilized to deposit 2D material103on electrode101. Deposition time may be varied from 1 to 30 minutes to adjust the thickness of 2D material103.

FIGS. 2A-Cillustrate methods for intercalating 2D material layers in accordance with embodiments of the present application. It is appreciated that in some embodiments, electrode201may have an interface layer202thereon.FIG. 2Aillustrates an embodiment wherein 2D materials and Li-metal are co-sputtered in a vacuum sputtering chamber by two sputtering guns of Li-metal and 2D materials. Using 2D material target211and Li target212as target materials for sputtering, successive layers of 2D materials and Li are sputtered onto electrode201, resulting in intercalated 2D material203. In an embodiment, co-sputtering may occur within chamber210with base pressure maintained at or below 10−6Torr, inert gas flow213, and RF power at 10-100 W. Inert gas flow213may be flowed at 1-100 mTorr and comprise argon, helium, or any other gas that has low reactivity with other substances. In other embodiments, evaporation may be utilized to deposit intercalated 2D material203on electrode201. Deposition time may be varied from 1 to 30 minutes to vary the thickness of intercalated 2D material203.

FIG. 2Billustrates another embodiment of a method for intercalating 2D material layers in accordance with an embodiment of the present application, wherein a target is made based on a 2D material/Li composite and sputtered accordingly. 2D material target221includes a 2D material and Li metal. In an alternative to the prior embodiment, the combined target is then sputtered, rather than using a co-sputtering method. Using 2D material/Li composite target221as a target material for sputtering, successive layers of the 2D material/Li composite are sputtered onto electrode201, resulting in intercalated 2D material204. In an embodiment, sputtering may occur within chamber220with base pressure maintained at or below 10−6Torr, inert gas flow222, and RF power at 10-100 W. Inert gas flow222may be flowed at 1-100 mTorr and comprise argon, helium, or any other gas that has low reactivity with other substances. Deposition time may be varied from 1 to 30 minutes to vary the thickness of intercalated 2D material204. In other embodiments, evaporation may be utilized to deposit intercalated 2D material204on electrode201.

FIG. 2Cillustrates another embodiment wherein 2D material205is intercalated electro-chemically. For example, electrode201may be deposited with 2D materials according to an embodiment described herein. Electrode201may then be introduced into reaction chamber230, wherein electrode201is faced with Li-metal231in an electrolyte solution (e.g., 1M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in 1:1 DOL/DME solvent). A voltage may then be applied between electrode201and Li-metal231. The applied voltage may be between 1 and 100 V. The distance between electrode201and Li-metal231may be between 1 and 50 mm. Application of the voltage may then cause Li ions of Li-metal231to intercalate the 2D material coated on electrode201, thereby producing intercalated 2D material205.

FIGS. 3A-Cillustrate cross-sectional views of a Li electrode with 2D material deposited thereon in accordance with certain embodiments of the present application. In the embodiment depicted inFIG. 3A, the 2D material coated electrode includes electrode301and 2D material303. 2D material303may include MoS2, WS2, MoTe2, MoSe2, WSe2, BN, BN—C, or the like. In an embodiment, electrode301may first be cleaned and then 2D material may be deposited on electrode301(e.g., sputtered, evaporated, etc.). 2D material303may also be intercalated with Li ions according to any of the forgoing intercalation methods (e.g., co-sputtering 2D material and Li ions, sputtering a 2D material/Li composite, electro-chemically). It is noted that although particular materials are disclosed as being suitable for providing a 2D coated electrode, such particular materials are disclosed for purposes of illustration, rather than by way of limitation, and materials other than those specifically listed herein may be readily utilized to provide a 2D material coated electrode in accordance with embodiments of the present disclosure. In an embodiment, the material(s) selected for 2D material303should tolerate chemicals and temperature cycling which may be required to fabricate electrodes. In certain embodiments, 2D material303may have porous morphology that includes cavities, islands, and pores. Porous morphology may be attributed to various conditions (e.g., non-equilibrium atomic stacking by high energetic bombardment during sputtering.). The porous morphology may offer open paths for electrostatic absorption of electrolyte ions and provide electrochemically active sites for dominant double layer charge storage. This could enable faster charging and/or discharging of the stored charge.

Referring now toFIGS. 3B-C, 2D coated Li electrode may have an interface layer in between electrode301and 2D material303in accordance with certain embodiments of the present application. For example, interface layers can be inserted such that strong adhesion is promoted between 2D material303and electrode301. As illustrated byFIG. 3B, interface layer302amay include a metallic interface layer deposited by any number of methods (e.g., sputtering, evaporation, etc.) to serve as an interface between 2D material303and electrode301. For example, interface layer302amay include transition metals such as molybdenum, tungsten, or any other transition metal. Interface layer302amay be deposited to a certain thickness (e.g., 1-10 nm). As illustrated byFIG. 3C, interface layer302bmay include a functionalized interface layer, e.g., treating electrode301with a functional group (e.g., hydrogen, fluorine, C—H bonding, or the like).

FIG. 4illustrates method400in accordance with an embodiment of the present application. In certain embodiments, method400may correspond to the fabrication processes illustrated and described with reference toFIGS. 1A-Band/orFIGS. 2A-C. At block410, method400includes providing a Li electrode. In an embodiment, the Li electrode may include a lithium composite, lithium oxide, lithium sulfide, or the like. In certain embodiments an interface layer may be inserted, which may provide for better adhesion to a 2D material. For example, interface layers may include plasma treated clean surface, metallic layer, and/or a functionalized layer, as described above. At block420, method400includes depositing at least one layer of a 2D material on the Li electrode. The 2D material may include MoS2, WS2, MoTe2, MoSe2, WSe2, BN, BN—C, or the like and be deposited via a number of methods (e.g., sputtering, evaporation, etc.) as described above.

At block430, method400includes intercalating the at least one layer of the 2D material with a plurality of Li ions. In some embodiments, intercalating the 2D material may occur simultaneously with deposition of the electrode and in other embodiments, deposition of the 2D material may occur after deposition of electrode material. In an embodiment, 2D material and Li-metal are co-sputtered in a vacuum sputtering chamber by two sputtering guns of Li-metal and 2D material. Using a 2D material target and a Li target as target materials for sputtering, successive layers of 2D materials and Li are sputtered onto the Li electrode, resulting in the intercalated 2D material. In another embodiment, a target includes a 2D material and Li metal composite. The composite target is then sputtered, rather than using a co-sputtering method. Using the 2D material/Li composite target for sputtering, successive layers of the 2D material/Li composite are sputtered onto the electrode, resulting in an intercalated 2D material. In yet another embodiment, the 2D material may be intercalated electro-chemically. For example, the electrode may be deposited with 2D materials according to an embodiment described herein, then the electrode may be introduced into a reaction chamber faced with Li-metal in an electrolyte solution. Applying a voltage then causes the intercalation of the 2D materials. The resulting 2D materials coated electrode may then be used in a variety of applications, including rechargeable batteries.

FIG. 5illustrates a Li-ion battery system in accordance with an embodiment of the present application. In an embodiment, Li-ion battery (LIB) system500may include anode501, cathode502, separator503, electrolyte504, negative terminal506, positive terminal507, and casing508. Anode501may include a Li electrode coated with at least one layer of 2D material as described above and illustrated by at leastFIGS. 1A-B,2A-C, and3A-C. Cathode502may include a Li oxide material (e.g., LiCoO2, LiFePO4, LiMn2O4, LiNixMnyCozO2, etc.). In other embodiments, cathode502may include a Li electrode coated with at least one layer of 2D material as described above and illustrated by at leastFIGS. 1A-B,2A-C, and3A-C. Separator503may include polypropylene (PP), polyethylene (PE), or the like. Electrolyte504may include any number of electrolyte solutions (e.g., aqueous, non-aqueous, etc.) which may allow for transporting Li ions between cathode502and anode501. For example, electrolyte504may include various lithium salts (e.g., LiPF6, LiClO4, LiH2PO4, LiAlCl4, LiBF4, etc.) or other electrolyte material. Current collector506may be attached to anode501and current collector507may be attached to cathode502. In an embodiment, current collector506may include copper metal and current collector507may include aluminum metal. Casing508may include a variety of cell form factors. For example, embodiments of LIB system500may be incorporated in a cylindrical cell (e.g., 13650, 18650, 18500, 26650, 21700, etc.), polymer cell, button cell, prismatic cell, pouch cell, etc. Further, one or more cells may be combined into larger battery packs for use in a variety of applications (e.g., cars, laptops, etc.). In certain embodiments, microcontrollers and/or other safety circuitry may be used along with voltage regulators to manage cell operation and may be tailored to specific uses of LIB system500.

In one embodiment, LIB system500was fabricated using cathode502and anode501in an argon-filled glove box under low levels of humidity and oxygen (<0.5 ppm). Electrolyte504included a 1 M solution of lithium hexafluorophosphate (LiPF6) salt in 1:1:1 (volume ratio) mixture solvent of ethylene carbonate (EC), dimethylene carbonate (DMC), and diethylene carbonate (DEC). Separator503included a PP-based membrane. Casing508included a CR 2032 coin-cell, assembled with crimping tool. The charge (delithiation) and discharge (lithiation) cycling tests were performed in a multi-channel battery testing unit at room temperature in the voltage window of 0.01-3.0 V.

FIG. 6illustrates a lithium-sulfur (Li—S) battery system in accordance with an embodiment of the present application. In an embodiment, Li—S battery system600may include anode601, cathode602, separator603, electrolyte604, negative terminal606, positive terminal607, and casing608. Anode601may include a Li electrode coated with at least one layer of 2D material as described above and illustrated by at leastFIGS. 1A-B,2A-C, and3A-C. Cathode602may include sulfur powder as a sulfur electrode and/or a composite with carbon structure (e.g., carbon nanotubes (CNTs), graphene, porous carbon, free-standing 3D CNTs, etc.). Separator603may include polypropylene (PP), polyethylene (PE), or the like. Electrolyte604may include any number of electrolyte solutions (e.g., aqueous, non-aqueous, etc.) which may allow for transporting Li ions between cathode602and anode601. For example, electrolyte604may include 1M LiTFSI in 1:1 DOL/DME with 1% LiNO3additives or other electrolyte solutions. Current collector606may be attached to anode601and current collector607may be attached to cathode602. In an embodiment, current collector606may include copper metal and current collector607may include aluminum metal. Casing608may include a variety of cell form factors. For example, embodiments of Li—S battery system600may be incorporated in a cylindrical cell (e.g., 13650, 18650, 18500, 26650, 21700, etc.), polymer cell, button cell, prismatic cell, pouch cell, etc. Further, one or more cells may be combined into larger battery packs for use in a variety of applications (e.g., cars, laptops, etc.). In certain embodiments, microcontrollers and/or other safety circuitry may be used along with voltage regulators to manage cell operation and may be tailored to specific uses of Li—S battery system600.

In one embodiment, Li—S battery system600was fabricated inside an argon filled glove box constantly maintaining humidity (H2O) and oxygen (O2) concentration less than 0.5 ppm. The electrochemical performance of cathode602(BF 3D-CNTs-S cathode material) was evaluated by a multi-channel battery testing unit in a coin cell with lithium serving as a counter/reference. The size of cathode602was 1 cm×1 cm (1 cm2) with a square geometry. Electrolyte604was prepared by dissolving lithium bis-trifluoromethanesulphonylimide (LITFSI, 99% sigma Aldrich, 1M) and lithium nitrate (LiNO3, 99.99%, sigma Aldrich, 0.25M) salt in the organic solvent of 1,2-dimethoxyethane (DME, 99.5%, sigma Aldrich), and 1,3-dioxolane (DOL, 99%, sigma Aldrich) with 1:1 volumetric ratio. Electrolyte604added to the coin cell was optimized to a volume of 60 μL. Separator603included polypropylene (PP) to isolate anode601and cathode602. A galvanostatic charge-discharge test was carried out at room temperature within a voltage range of 1.5-3.0 V. The C-rate was calculated based on the theoretical specific capacity of sulfur ((Qs=2×9.65×104/(3.6×32.065))˜1672 mAh/g). The cyclic voltammetry and electrochemical impedance spectroscopy (EIS) measurement were performed by a potentiostat.

FIG. 7illustrates Li—S battery system700in accordance with an embodiment of the present application. In an embodiment, Li—S battery system700may include anode701and cathode702. Anode701may include a Li electrode coated with at least one layer of 2D materials as described above and illustrated by at leastFIGS. 1A-B,2A-C, and3A-C. For example, anode701is illustrated byFIG. 7as comprising Li metal with one or more layers of MoS2deposited thereon. As discussed above, anode701may be formed by direct deposition of one or more layer of 2D materials (e.g., MoS2and the like) onto Li metal via sputtering, evaporation, and the like. One or more layer of 2D materials may be uniform and provide negligible impedance such that cells may operate at high current densities with low polarization. In an embodiment, the lithiated MoS2may be edge-oriented flake-like MoS2, which may provide a consistent flow of Li+into and out of the bulk Li metal, a homogenous and stable Li electrodeposition, and suppression of dendrite formation.

In an embodiment, cathode702may include a 3D CNTs/S electrode. As shown inFIG. 7, cathode702may comprise a substrate (e.g., graphene) with a plurality of CNTs thereon, which will be discussed in more detail below. The plurality of CNTs may be coated in sulfur, providing large surface area, an ultra-low resistance path, and strong bonding with a substrate. In one embodiment, initial data of 3D CNTs/S cathode702demonstrated sulfur loading of >8 mg/cm2. In another embodiment including 2D materials coated Li-metal anode701and 3D CNTs/S cathode702, specific capacity was 1100 mAh/g (e.g., >500 Wh/kg) at 0.5° C. with over 1000 charge/discharge cycles.

FIG. 8illustrates a cross-sectional view of an electrode800and corresponding SEM images in accordance with an embodiment of the present application. Electrode800may include a porous 3D CNTs structure (e.g., a plurality of CNTs), which provides a high conduction path and short diffusion length for Li-ions and the ability to absorb polysulfides generated during the cycling process. High loading of CNTs may be achieved by multi-stacking one or more 3D CNTs layers while maintaining structural integrity and conductivity. In an embodiment, treatment of a CNTs surface with a functional group may enhance the bonding strength between CNTs and sulfur (e.g., oxygen terminated CNTs have higher bonding strength with sulfur) such that polysulfide shuttle effect is minimized, as will be discussed in more detail below.

In one embodiment, in a 3-D micro-channeled electrode in a rechargeable battery, the 3D Cu mesh demonstrated surface area improvement of approximately 10 times that of 2D Cu foil, and the loading of CNTs may be increased (e.g., >50 times with a sample of 500 nm thickness). In an embodiment, electrode800may be scalable for various high-energy applications and energy storing technologies. For example, the weight of other battery components is a concern for various applications. In an embodiment, energy/power density and/or specific capacity of a battery may be normalized with the total mass of the battery and/or packaging density. Carbon nanotubes in a 3D structure provide more efficient and versatile energy storage for a variety of platforms.

FIG. 9illustrates aspects of a fabrication process for an electrode in accordance with an embodiment of the present application. In an embodiment, a binder-free 3D CNTs/S cathode structure may be fabricated.FIG. 9at (a) illustrates a plurality of free-standing 3D CNTs and corresponding low magnification SEM image demonstrating same. As shown at (b) ofFIG. 9, an embodiment may include uniformly coating one or more layers of sulfur onto 3D CNTs (e.g., via mechanically pressing at ˜155° C.). The sulfur particles may be uniformly distributed and mechanically pressed to facilitate confinement of sulfur melt into the 3D CNTs structure by capillary action and low surface tension.FIG. 9at (c) illustrates a schematic showing the resulting distribution of sulfur particles into the 3D CNTs. Section (d) illustrates a cross-sectional SEM image of highly dense 3D CNTs. The interconnected CNTs provide large surface area (e.g., >100 m2/g) and narrow pore size distribution (e.g., 2-20 nm).FIG. 9at (e) illustrates a SEM image of as-synthesized binder-free 3D CNTs/S along with corresponding carbon and sulfur EDS mapping. Section (f) illustrates energy-dispersive X-ray (EDX) spectrum of the SEM image shown at (e). The average diameter of these CNTs may range from 100-150 nm. The SEM image (e) and EDX spectrum (f) of an exemplary fabricated 3D CNTs/S cathode demonstrates uniform distribution of sulfur within the conductive network of 3D CNTs.

In one embodiment, a binder-free 3D CNTs/S electrode was fabricated according to the above exemplary process. The binder free cathode design resulted in high sulfur loading of 8.33 mg/cm2(˜55 wt % S in the cathode electrode) with high areal capacity of 8.89 mAh/cm2and specific capacity of 1068 mAh/g at 0.1 C rate (˜1.4 mA/cm2), providing coulombic efficiency of greater than 95% for 150 cycles. The embodiment exhibited specific energy of ˜1233 Wh/kg with a specific power of ˜476 W/kg, with respect to the mass of the cathode.

FIG. 10AandFIG. 10Billustrate graphs depicting number of cycles versus specific capacity of an electrode with various sulfur loading amounts in accordance with an embodiment of the present application. For instance,FIG. 10Aillustrates rate capability of an exemplary cell with different sulfur loading amounts. Further,FIG. 10Billustrates cycling performance of high sulfur loading amount of 55 wt % S (8.33 mg/cm2) sulfur loaded within 3D CNTs.FIG. 10Cillustrates a graph of areal capacity of a 3D CNTs/S electrode in accordance with an embodiment of the present application.FIG. 10Cillustrates a comparison of areal capacity for a binder-free 3D CNTs/S electrode with that of conventional Li—S battery cathode material, demonstrating that an exemplary binder-free 3D CNTs/S cathode structure may achieve higher areal capacity.

The galvanostatic discharge-charge profiles corresponding toFIG. 10Ademonstrate plateaus for all C-rates (e.g., indicating efficient kinetic process with high electrical conductivity within the matrix of 3D CNTs/S structure). Improved reaction kinetics are also demonstrated from the discharge capacity ratio between the lower (Qlower-plateau) and upper plateaus (Qupper-plateau). For example,FIG. 10Ademonstrates the Qlower-plateau/Qupper-plateauratio at 2 C rate for both 37 wt % S and 42 wt % S that are 1.85 and 1.8, respectively, indicating an efficient conversion of soluble polysulfides to non-soluble sulfides at higher C-rates.FIG. 10Aillustrates specific capacity from a high sulfur loading amount of 55 wt % S (8.33 mg/cm2) and the cell delivered initial discharge capacity of ˜1068 mAh/g at 0.1 C (˜1.39 mA/cm2) corresponding to an areal capacity of ˜8.8 mAh/cm2(e.g., higher than conventional Li—S batteries). In an embodiment, after 150 cycles, a cell could still deliver specific capacity of ˜613 mAh/g with an average capacity decay of ˜0.4% per cycle, (e.g., superior to previously reported data shown inFIG. 10C).

FIGS. 11A-Billustrate a flexible 3D metal mesh with a plurality of CNTs thereon in accordance with an embodiment of the present application.FIGS. 11A-Billustrate an embodiment of CNTs on a 3D metal mesh configured such that the embodiment may be scalable and bendable. Further,FIGS. 11C-Dillustrate SEM images of the embodiment demonstrating CNTs on a porous metal mesh structure. An embodiment may be fabricated using CVD of 3D CNTs on a 3D Cu-mesh, and/or any of the fabrication methods discussed herein. It is appreciated that the scalable and bendable structure may be utilized as an electrode in embodiments of Li—S batteries discussed herein, such that a bendable and scalable electrode may be easily adapted to a large variety of shapes, sizes, applications, and the like.

FIG. 12illustrates aspects of a fabrication process of a 3D CNTs anode stack in accordance with an embodiment of the present application. Referring toFIG. 12at (a), in an embodiment, a plurality of 3D CNTs may be grown on a mesh structure (e.g., Cu, graphene, and the like) via CVD and/or other deposition methods discussed herein. For example, a Cu mesh structure (e.g., <200 mesh) may include an average thickness of 50-200 μm and first be cleaned ultrasonically with a sequence of acetone, ethanol, deionized water and the like. The clean Cu mesh structure may then be dried in an oven. In an embodiment, a titanium buffer layer and nickel catalyst may be deposited on the Cu mesh (e.g., using RF magnetron sputtering) at room temperature with varying deposition time (e.g., 1-15 min.) at a given deposition pressure (e.g., 10−3Torr Ar). Next, the 3D CNTs may be synthesized in a thermal CVD system. Growth of highly dense and aligned CNTs may be optimized by using ethylene gas (e.g., 50-150 SCCM) and hydrogen carrier gas (e.g., 10-100 SCCM) at a temperature of 600-800° C. for 10-60 min.

Referring now toFIG. 12at (b), the mesh structure with 3D CNTs may be introduced to an etching process. For example, a CNTs/Cu mesh structure may be etched in a FeCl3etching solution, resulting in a free-standing 3D CNTs structure as illustrated at (c). Further, one or more layers of 3D CNTs may be fabricated by pressing the layers of 3D CNTs by a hot-press to create a multi-stack 3D CNTs (shown at (d)), which may then be utilized as an electrode (e.g., cathode, anode, and the like).

Polysulfide dissolution into the electrolyte may contribute to capacity degradation in Li—S batteries. In an embodiment, to mitigate polysulfide shuttle effect, CNTs surfaces may be treated with functional groups (e.g., oxygen terminated CNTs and the like) to enhance the bonding strength between CNTs and sulfur. For example, a stabilization method of sulfur with CNTs may include introducing functional groups (e.g., carboxylic acids, amines, ketones, alcohol, esters, and the like). Chemical functionalization is based in part on the covalent bond of functional groups with the surface of CNTs as well as the end caps of nanotubes. In an embodiment, oxidation treatment of CNTs with strong acids such as HNO3, H2SO4, and/or a mixture of both with strong oxidants (e.g., KMnO4and the like) may form oxygenated functional groups. In another embodiment, non-covalent interaction with the active molecules may provide for tuning the interfacial properties of CNTs/S. The CNTs may be functionalized non-covalently by aromatic compounds, surfactants, polymers, and/or hydrophobic interactions.