WO2012100301A1

WO2012100301A1 - Sulfur-carbon material and use as cathodes for high energy batteries

Info

Publication number: WO2012100301A1
Application number: PCT/AU2012/000067
Authority: WO
Inventors: Da-Wei Wang
Original assignee: The University Of Queensland
Priority date: 2011-01-27
Filing date: 2012-01-27
Publication date: 2012-08-02

Abstract

A sulfur-carbon material, in one example the sulfur-carbon material forms at least part of a cathode in a battery, such as a lithium-sulfur battery. Also, a method of producing the sulfur-carbon material, including a two stage process of adsorbing sulfur into a porous carbon material, and then selectively or preferentially extracting sulfur from the porous carbon material, based on pore size, thereby leaving sulfur remaining in micropores of the porous carbon material.

Description

SULFUR-CARBON MATERIAL AND USE AS CATHODES FOR HIGH ENERGY BATTERIES

Technical Field

[001] The present invention generally relates to a new sulfur-carbon material, for example for use as a cathode material in high energy batteries, such as lithium-sulfur batteries, and more particularly to selectively confined sulfur carbon-based cathodes for use in high energy batteries.

Background

[002] In a carbon-constrained world, current concerns of addressing the pressing environmental pollution issues and the limited fossil energy resources have increased the importance of clean energy technologies. Lithium-sulfur batteries (LSBs) can deliver a significant energy density of theoretically 2500 W h kg^-1 or 2800 W h Γ¹. This class of high energy batteries comprehensively outperforms lithium ion batteries with varying configurations. The outstanding performance of LSBs arises from the distinct non- topotactic cathode reaction of S₈ + 16 Li «→ 8 Li₂S, which offers an extreme capacity of 1675 mA h g^_1, more than five times of the theoretical upper limit of 300 mA h g^_1 for conventional cathode materials. Importantly, LSBs are intrinsically protected from overcharging and lithium dendrite short-circuiting, and promise high levels of safety.

[003] The delayed introduction of LSBs to market is caused by limited stability, especially upon high power operation, due to several critical disadvantages of known sulfur cathodes. First, an especially troublesome issue is the formation of soluble long chain polysulfide ions (S„ ) on reduction of elementary sulfur (Sg) or upon oxidation of the low-order sulfides. In fact, even sulfur is weakly soluble in aprotic electrolytes. The dissolution of polysulfide in electrolyte associated with its redox shuttle between anode and cathode causes severe loss of coulombic efficiency and a reduction in the amount of usable active cathode material. [004] Second, the soluble polysulfide irreversibly deposits as insulating lithium sulfides (Li₂S₂/Li₂S) coverage on the cathode surface, preventing the penetration of lithium ions and also reducing the electrode conductivity. [005] Third, it is noted that sulfur (d = 2.03 g cm^-3) expands during reduction to lithium sulfide (Li₂S, d = 1.67 g cm^"3), while the latter contracts upon oxidation. The volume change of around 20% during the reversible redox conversion causes deterioration of the electrode integrity. [006] Fourth, sulfur is inherently dual -insulating to both electrons and lithium ions. The electrical conductivity of sulfur at room temperature is 5 * 10^~30 S cm^-1, more than 20 orders of magnitude lower than that of normal lithium transition-metal oxides (>10^~4 S cm^-1). Solid state sulfur normally exists as cyclic crown-shaped Sg molecules with a S-S chain length of ca. 2.06 A. A lack of intrinsic voids in the crystallographic structure of sulfur limits the conductivity of solvated lithium ions. These problems result in severely degraded capacity during fast discharge/recharge and/or extended cycling operations.

[007] Despite a great deal of research, the stable fast discharge/recharge of LSBs has not been successful in the past decade. Conducting polymer-sulfur composites were expected to deliver a molecular level mixed state with sulfur, and porous oxides were used as chemical adsorbents to keep polysulfide from dissolution. But their poor conductivity limits high power applications.

[008] Carbon materials combine good electrical conductivity and high porosity; porous carbons have been demonstrated to immobilize sulfur (see US Patent Publication No. 2009/0311604). Immobilization of sulfur in pores with varying sizes has shown quite different cathode behavior, while the challenge of stable high rate capacity remains unmet due to either the weak adsorption potential of mesopores or the blocked porous networks post sulfur-filling.

[009] There is a need for a new sulfur-carbon material and/or method of production thereof. There is also a need for a new sulfur-carbon based cathode for use in batteries, such as lithium-sulfur batteries, and/or a method of production thereof, which address or at least ameliorate one or more problems inherent in the prior art. [010] The reference in this specification to any prior publication (or information derived from the prior publication), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from the prior publication) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Brief Summary

[011] In a broad form there is provided a sulfur-carbon material and a method of producing a sulfur-carbon material. In one example aspect, the sulfur-carbon material forms at least part of an electrode, such as a cathode. In another example aspect, the cathode is part of a battery, such as a lithium-sulfur battery. [012] According to a particular example form there is provided a method of producing a sulfur-carbon material, including the steps of adsorbing sulfur into a porous carbon material, and selectively or preferentially extracting or removing sulfur from pores larger than micropores of the porous carbon material, while leaving sulfur remaining in micropores of the porous carbon material.

[013] According to another particular example form there is provided a sulfur-carbon material, comprising a porous carbon material having a distribution of different pore sizes including micropores and mesopores, wherein sulfur is present in the micropores and the rriesopores are substantially free of sulfur.

[014] In particular non-limiting examples the porous carbon material includes micropores less than a few nanometers in size, micropores less than a couple of nanometers in size, micropores less than about 1.6 nm in size, and/or sub-nanometer micropores less than about 1.0 nm in size. In other particular non-limiting examples the porous carbon material includes pores larger than micropores such as mesopores greater than several nanometers in size, and/or mesopores greater than about 6 nm in size.

[015] In further non-limiting examples, the sulfur is adsorbed as molten sulfur, and/or the sulfur is selectively extracted by exposure to a sulfur solvent. In a further non-limiting example, selectively extracting sulfur is by preferential dissolution of sulfur adsorbed into pores larger than micropores, such as the mesopores, over dissolution of sulfur adsorbed into micropores by the sulfur solvent. In further non-limiting examples, the sulfur is substantially removed from the mesopores and the mesopores are mostly free or devoid of sulfur. In a further non-limiting example, the sulfur and the porous carbon material are solids and ground together and then heated so as to adsorb the sulfur into the porous carbon material. In still a further non-limiting example, the porous carbon material with adsorbed sulfur is immersed into the sulfur solvent, which is then infiltrated through a membrane, and then remaining porous carbon material is dried. In one example form, the sulfur solvent is carbon disulfide.

[016] Preferably, though not necessarily, the sulfur solvent is carbon disulfide, and/or the porous carbon material is graphitic open framework (GOF). Brief Description Of Figures

[017] Example embodiments should become apparent from the following description, which is given by way of example only, of at least one preferred but non-limiting embodiment, described in connection with the accompanying figures. [018] Figure 1 illustrates an example method of preparing a sulfur-carbon material;

[019] Figure 2 illustrates a schematic demonstration of adsorption and extraction toward substantially sub-nanometer sulfur immobilization; [020] Figure 3A illustrates a low-magnification TEM image of example GOF showing an interconnected open porous network;

[021] Figure 3B illustrates a high-resolution TEM image of example GOF showing the homogeneous distribution of graphite nanoribbons with thickness ca. 5 nm and abundant micropores within the open porous network;

[022] Figure 4A illustrates an example distribution of pore volume and corresponding sulfur occupation ratio against varying pore size regimes; [023] Figure 4B illustrates wide angle XRD patterns of pure sulfur and sulfur confined in pores of different sizes, showing the phase evolution of sulfur (rhombic-monoclinic- amorphous) corresponding with an example implementation of the present adsorption- extraction method;

[024] Figure 5 illustrates a zero-loss low-magnification image of example S_mj_Cro/GOF showing an open meso-macroporous texture;

[025] Figure 6 illustrates an example EELS spectrum of sulfur (165 eV) and carbon (284 eV) edges with an energy window of 20 eV;

[026] Figures 7A and 7B illustrate element mapping distributions of (A) carbon and (B) sulfur elements in a porous texture of an example composite material, the contrast among porous regions and elemental regions suggests the element localization dependent on pore size;

[027] Figure 8A illustrates a zero-loss low-magnification image showing the porous texture of example S_micro/GOF maintained after a first discharge to 1.5 V vs. Li⁺/Li°; [028] Figures 8B and 8C illustrate mapping distributions of (B) carbon and (C) sulfur elements in a porous texture of an example composite material after discharge, the contrast between porous regions and sulfur element regions suggests the distribution of lithium sulfides depending on pore size; [029] Figure 9A illustrates the discharge capacity of an example S_mj_Cro/GOF cathode against cycles under varying current densities;

[030] Figure 9B illustrates the cycle stability of the example S_mj_cro/GOF cathode evaluated for 50 cycles at room temperature at 750 raA g^~', 1.5 A g^-1, and 3 A g^-1 within 1.5 V - 2.8 V vs. Li⁺/Li°;

[031] Figure 10 schematically illustrates the electrode process of a micropore confined sulfur-GOF cathode, regions labeled with S_mj_Cro represent amorphous sulfur confined in micropores, balls indicate lithium ions in mesopores, balls with arrows illustrate facilitated electron transfer through graphitic nanoribbons;

[032] Figure 11 illustrates pore size distribution of example GOF, S_meso GOF and Smicro/GOF composites;

[033] Figure 12 illustrates XPS spectra;

[034] Figure 13 illustrates (A, C) SEM images and (B, D) EDS mapping pictures of (A, B) S_meso/GOF and (C, D) S_miero/GOF;

[035] Figure 14 illustrates thermogravimetric analyses of example GOF and sulfur/GOF composites; [036] Figure 15 illustrates electrochemical characteristics of example sulfur/GOF composites;

[037] Figure 16 illustrates XPS S2p3/2 spectrum of S_micro/GOF before and after a first discharge;

[038] Figure 17 illustrates SEM images of (top) S_micro/GOF and (bottom) S_meSo GOF example cathodes after a first discharge;

[039] Figure 18 illustrates an electrochemical impedance spectroscopy;

[040] Figure 19 illustrates (A) reversible discharge-recharge profiles of a second cycle, (B) cycle stability of S_mj_cro/GOF cathode at 150 raA g^'1, and (C) coulombic efficiency and capacity retention ratio versus cycle number tested at 3 A g^-1 for 550 cycles (started from the second cycle);

[041] Figure 20 illustrates discharge-recharge profiles of an example S_meso/GOF material for a second cycle.

Preferred Embodiments [042] The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments. [043] A new sulfur-carbon material has been produced, with an example use as a cathode material in batteries, such as high energy lithium-sulfur batteries.

[044] Referring to Fig. 1, there is illustrated a method 100 of producing a sulfur-carbon material. Initially and optionally, sulfur is melted at step 110 for improved adsorption into a porous carbon material. At step 120, sulfur is adsorbed into a porous carbon material. At step 130, sulfur is selectively or preferentially extracted from some but not all pores of the porous carbon material depending on pore size. This consequently provides step 140 of preferentially leaving sulfur remaining in micropores of the porous carbon material, whilst preferentially removing sulfur from pores larger than micropores, for example from mesopores of the porous carbon material. Selective or preferential removal of sulfur from larger pore sizes or volumes can be achieved using a sulfur-extraction method using a sulfur solvent. After the sulfur has been adsorbed into the porous carbon material, then the sulfur solvent can be used to selectively dissolve sulfur from the pores larger than micropores, such as the mesopores; The sulfur solvent containing dissolved sulfur can be infiltrated through a membrane. The remaining sulfur-carbon material can be then slowly dried.

[045] In preferred, but non-limiting examples, the porous carbon material includes a range of pore sizes, including sub-nanometer pores less than 1 nm in size and pores greater than 1 nm in size. As used herein, a "micropore" is a pore less than a few nanometers in characteristic size. Also as used herein, a "mesopore" is a pore greater than several nanometers in characteristic size. Preferably, a micropore is less than a couple of nanometers in characteristic size. Most preferably, a micropore is less than about 1.6 nm in size. Furthermore, a micropore, in one form, can also be considered as a sub-nanometer pore less than about 1.0 nm in size. Most preferably, a mesopore is greater than about 6 nm in size. In a particular example, selective extraction of sulfur means that sulfur is preferentially, substantially or dominantly removed from mesopores and any larger sized pores, that is for example from pores greater than about 6 nm in size. [046] The method thus provides a sulfur-carbon material, which includes a porous carbon material that has a distribution of different pore sizes including micropores and mesopores, and where sulfur is dominantly, or at least substantially, present in the micropores, whereas the mesopores are mostly, or at least substantially, free or devoid of sulfur. This provides a two stage process to produce the sulfur-carbon material, firstly loading or adsorbing the sulfur into all, substantially all or most, of the pores of the porous carbon material, and secondly, then selectively or preferentially removing sulfur from those pores larger than a certain pore size (or volume). [047] In a preferred application, sulfur is selectively or preferentially confined in micropores, which may be sub-nanometer pores/spaces, in a porous carbon material to provide at least part of a cathode. This cathode material can be produced using the adsorption-extraction method developed by the Applicant. In contrast to the known method of non-selective loading of sulfur into a porous carbon matrix, the present method uses site-targeted immobilization of sulfur substantially, preferentially or dominantly in micropores, preferably sub-nanometer pores, which restricts the soluble polysulfides inside sub-nanometer spaces or micropores (in one example the spaces/micropores are smaller than a picoliter), retains the open network of the porous carbon material (in one example a Graphitic Open Framework (GOF)) for fast ion transport and displays enhanced cathode reaction efficiency, durability and kinetics.

[048] The resulting novel sulfur-carbon material can be used in an example application as a cathode, or part thereof, in batteries such as lithium-sulfur batteries (LSBs) with higher energy density and safety level, for example to replace Li-ion batteries. In one particular example, sulfur is selectively confined in micropores conjugated with a carbon framework, for example a GOF, which efficiently improves cathode durability and reaction kinetics. This enhanced sulfur-carbon cathode improves efficiency by restricting soluble polysulfides inside micropores, particularly inside sub-nanometer spaces, and produces reversible high activity of the sulfur-carbon cathode. In a particular example form, the sulfur confined in micropores of the GOF provides a cathode having an ultrafast recharge of LSBs in about 4 minutes with durable capacity around about 200 mA h g^-1 upon cycling for over 500 times. [049] Referring to Fig. 2, there is illustrated a schematic demonstration of adsorption and extraction toward substantially sub-nanometer sulfur immobilization. Step I represents the melt of sulfur with a GOF mixture allowing the adsorption of melted sulfur into the GOF. Step II represents the removal of solid sulfur from a meso-macroporous system, and the adhesion of sulfur in small micropores. As used herein, a "macropore" is a pore much greater than a mesopore in characteristic size.

Sulfur-Carbon Material Synthesis

[050] An example method of producing the sulfur-carbon material involves sulfur being selectively immobilized into micropores or sub-nanometer pores of GOF via a two-step adsorption-extraction method. Sulfur powder was firstly melted, for example at about 155 °C, to allow for better incorporation of sulfur into interwoven channels in GOF particles (the resulting material referred to as "S_meso GOF"). Subsequently, a sulfur solvent, such as carbon disulfide in which sulfur has a high solubility of 24 wt% at 22 °C, was used to dissolve and extract sulfur loosely adsorbed in the large pores, e.g. mesopores and any macropores, of the GOF (the resulting material referred to as "S_mj_Cro GOF").

[051] In various embodiments, other porous carbon materials can be used, for example microporous carbon, mesoporous carbon, hierarchical porous carbon, activated carbon, activated carbon nanofibers, carbon aerogels, carbon nanotubes, expanded graphite, graphene nanosheets, graphene oxide nanosheets, carbide-derived carbon and zeolite- templated carbon. Also, a variety of other sulfur solvents can be used, for example dialkyl disulfides, which can be used with a range of catalysts or co-solvents added, water, ethanol, acetone, carbon disulfide, carbon tetrachloride, toluene, dimethylformamide, methylpyrrolidone, natural rubber, synthetic elastomer and liquid sulfur dioxide.

[052] The following more detailed example is intended to be merely illustrative and not limiting to the scope of the present invention. The synthesis of GOF with graphitic nanoribbons and open porous network was as follows (1): 1 g of 20 wt % Ni(N0₃)₂-6H₂0 solution was dropped into 2 g of 10 wt % NaOH solution, and stirred continuously at room temperature for 4 h for the formation of Ni(OH)₂ precipitate. Subsequently, 1 g of 20 wt % ethanol solution of phenolic resin was added drop wise under vigorous stirring. The mixed system of resin, alkaline, salt and hydroxide precipitates was evaporated slowly for 24 h in a glass utensil at 60 °C under ambient pressure to obtain an inorganic filler containing resin composite. The composite was carbonized at 600 °C in a tubular furnace under inert argon atmosphere. After carbonization, the inorganic species were removed with 3 M HC1 solution at 100 °C. [053] The S_meSo GOF composite material was prepared following a melt-adsorption strategy. GOF powder (1 g) and sulfur (1 g) were ground together, and heated at 155 °C for 12 h in an inert gas, preferably Argon gas. To prepare the S_mj_cro/GOF composite material, a sulfur-extraction method using carbon disulfide was used. Firstly, the S_m.so GOF was immersed into 30 ml carbon disulfide and stirred for 10 minutes at room temperature, other stirring times and temperatures could be used. The carbon disulfide containing dissolved sulfur was quickly infiltrated through a membrane driven by atmospheric force. A trace amount of clean carbon disulfide was dropped to soak the exterior surface of S_mj_cro/GOF and remove the physically adsorbed sulfur. The remaining S_mj_Cro/GOF was dried slowly in a vacuum oven at 50 °C for 8 h, although other temperatures and times could be used.

[054] Referring to Fig. 3 A, an open porous texture of a GOF material is visible from transmission electron microscopy (TEM) images. The homogeneous graphitic ribbons have average thickness around 5-20 nm, and micropores can be observed surrounding the large pores and the graphitic ribbons (refer to Fig. 3B). The large pores, i.e. mesopores and macropores, with weak adsorption potential can hardly hold sulfur against its strong affinity with sulfur solvents, such as carbon disulfide (CS₂). As a consequence, sulfur densified in micropores, including sub-nanometer pores, of the produced sulfur-carbon material with high adsorption potential remaining mostly after CS₂ extraction. [055] Referring to Fig. 4A and Fig. 1 1, a statistical comparison of pore volume change during the sulfur adsorption-extraction process, together with the pore volume occupation ratio by sulfur, is demonstrated with respect to different pore size regimes. It is clear that GS₂ extraction does not remove sulfur in micropores (less than about 1.6 nm), such as sub- nanometer spaces/pores (less than about 1.0 nm). More precisely sulfur remains in micropores about 0.6-0.7 nm, with a peak value about 0.65 nm, but sulfur is effectively extracted from the mesopores and macropores, with partial sulfur removal from micropores about 1.0 - 1.6 nm in size. [056] Referring to Fig. .12 and Fig. 13, x-ray photoelectron spectroscopy (XPS) spectra and low-magnification energy dispersive spectroscopy (EDS) element mapping images together confirm the inner-particle incorporation of sulfur, with no visible sulfur aggregates on the external surfaces of the GOF particles. The sulfur confined in small micropores (< 1.6 nm) showed sublimation temperature higher by 100 °C than that in large mesopores (> 6 nm), demonstrating the strong adsorption potential in small micropores.

[057] Referring to Fig. 4B, x-ray diffraction (XRD) peaks revealed the rhombic- monoclinic phase transition of sulfur solidified in large pores from the melt. The well resolved peaks corresponding to monoclinic sulfur disappeared completely after CS₂ extraction, indicating the amorphous nature of sulfur confined by small micropores.

[058] In Figs. 5, 6, 7A and 7B, there is illustrated element mapping images of sulfur and carbon in an example S_mj_Cro/GOF cathode using electron energy loss spectroscopy (EELS). The existence of sulfur in the GOF matrix is confirmed from the EELS spectrum, and the homogeneous distribution of sulfur is observed mostly in micropores, which are located around the large pores highlighted by dotted lines. The recognizable contrast difference between the microporous region and some large pores clearly suggests the significant micropore confinement of sulfur, but only slightly by large pores.

[059] Referring to Figs. 15A-D, the reversible cathode reaction was observed for sulfur confined in sub-nanometer pores, while the existence of sulfur in large mesopores deteriorated the cathode durability completely only after the first cycle of cyclic voltammetry test. Electrochemical impedance spectroscopy further revealed the smaller resistance of sub-nanometer confined sulfur cathode due to the release of open framework consisting large pores for fast ion transport.

[060] The dramatically different electrochemical behavior of a S_micro/GOF cathode from a Smeso GOF cathode indicates the feasibility of confined sulfur-lithium sulfide transitions in micropores or sub_^nanometer pores. To detect the nanoconfined lithium sulfide formation, EELS element mapping was adopted to track the sulfur distribution after discharge (refer to Fig. 8). Since only lithium sulfides are responsible for the sulfur signal after discharge according to XPS (refer to Fig. 16), the detection of sulfur signals in EELS clearly indicates the presence of lithium sulfides. The sulfur signals remain around the large pores; in other words, lithium sulfides are detected in small micropores after full discharge to 1.5 V, which is the basis of a nanoconfined cathode reaction.

[061] Referring to Fig. 17, SEM images of S_micro/GOF (top) and S_meso/GOF (bottom) cathodes after the first discharge to 1.5 V (vs. Li⁺/Li°) demonstrate the total surface coverage of the Smeso/GOF cathode by solid insulating lithium sulfides, while the Smicro GOF cathode maintains a porous morphology due to the nanoconfinment of soluble polysulfides. [062] Referring to Fig. 18, electrochemical impedance) spectroscopy recorded after discharge confirms the superior cathode kinetics of S_micro/GOF to that of S_meso/GOV, which is reasonably attributed to the unfilled porous framework due to the nanoconfined lithium sulfide formation. The nano-confinement effect of micropores or sub-nanometer pores is thus believed to be twofold. First, the strong adsorption potential can constrain the highly soluble polysulfide inside small micropores, which curtails the redox shuttle mechanism to a large degree and produces a high coulombic efficiency. Second, the deposition of insoluble Li₂S2/Li2S from polysulfides is thus restricted in nanoconfined spaces, which releases the stress from volume change, prevents the coverage of cathode with dual- insulating sulfide layer, and retains the good electric/ionic connections with the current collector and electrolyte.

[063] The entrapment of active sulfur/sulfides in the micropores or sub-nanometer pores is vitally important for the stable utilization of sulfur cathodes. Nevertheless the high rate capacity of the nanoconfined sulfur cathode is more crucial for high energy/power applications. LSBs were assembled in a button cell with a metallic lithium foil anode and the nanoconfined sulfur-GOF cathodes. The capacity values are reported only regarding the sulfur mass; that is a general mathematical treatment in LSBs research. The S_mj_cro/GOF cathode can deliver a reversible capacity around 200 mA h g^"1 under an ultrahigh current density of 3 A g^-1 for 550 cycles or probably even longer with a coulombic efficiency close to 100% (refer to Figs. 9A and 9B, and Fig. 19).

[064] This high current density operation accounts for an equivalent battery recharge time of ca. 4 minutes, which is basically applicable for on-board use. In stark contrast, the unconstrained dissolution of polysulfide ions and dense coverage of cathode surface by insoluble Li2S2/Li₂S caused the absolute loss of capacity even in the second cycle of S_meso/GOF (refer to Fig. 20). This high rate performance of the produced sulfur-carbon cathode is significant with its high discharge voltage (1.5 V) accompanying with the long- lasting and high energy density behavior, compared to the poor stability of mesoporous sulfur/carbon cathodes.

[065] The cycling stability of the produced sulfur-carbon cathode was noticed to weaken upon increasing the discharge rate. However, this did not prove troublesome because the redox transitions of sulfur-polysulfides-lithium sulfides are all confined in tiny spaces. On the other hand, in the GOF scaffold comprising an open framework with a hierarchical porous system (6-100 nm), the migration of lithium ions would be much faster with smaller active sites/electrolyte distance, which are obviously critical for high rate performance. Moreover, the homogeneous distribution of graphitic naiioribbons embedded in the GOF matrix should be a critical advantage in facilitating electron conduction. As a consequence, the combination of sub-nanometer pores with a graphitic open framework provides high performance sulfur-carbon cathodes for ultrafast and durable high energy batteries (refer to Fig. 10).

[066] The selective immobilization of sulfur in micropores results in an ultrafast and durable sulfur-carbon cathode. Utilization efficiency of sulfur can be increased by increasing total volume of sub-nanometer micropores or by modifying the cathode surface with hydrophilic polymer, with either approach advanced LSBs using carbon based and sulfur nanoconfined cathodes can exceed previously known limits. Further Examples

[067] The following further examples provide a discussion of particular embodiments. The examples are intended to be merely illustrative and not limiting to the scope of the present invention. Materials Characterizations

[068] XPS analysis was performed on ESCALAB 250 instrument with Al Ka radiation (15 kV, 150 W) under a pressure of 4x 10^-8 Pa. X-ray diffraction (XRD) was conducted on D-MAX/2400 instrument (Cu α, λ=0.154056 nm). TEM and EELS were performed on a Tecnai F30. SEM and EDS were carried out on a FEI Nova NanoSEM 430, 15 kV. Porous parameters were determined using a Micromeritics ASAP 2010 M instrument at 77 K. Before measurement of GOF, the powder was degassed at 200 °C until a manifold pressure of 2 mm Hg was reached. It was not feasible to degas the sulfur/GOF composite at this temperature due to the possibility of sulfur sublimation, and only room temperature degassing was applied for 12 h. The total pore volume and pore size distribution were determined based on the NLDFT method. Thermogravimetric analysis was performed on a NETZSCH STA 449C thermo balance in argon with a heating ramp of 10 °C min^"1 to 500 °C. Electrochemical Evaluations

[069] Sulfur/GOF composite cathodes were comprised 80 wt% active composite, 10 wt carbon black and 10 wt% poly(vinylidene fluorid) binder. The cathode materials were slurry-cast from N-methyl-2-pyrrolidone onto an aluminum foil current collector. The electrolyte was composed of a 1 M LiPF₆ solution in ethylene carbonate, diethyl carbonate and ethylmethyl carbonate (EC/DMC/EMC, 1 :1 :1 vol) electrolyte. The electrodes after discharge were washed extensively with DMC to remove soluble sulfur-containing species for the characterizations of morphology and elemental composition.

[070] Referring to Fig. 11, there is illustrated pore size distribution of the GOF, Smeso/GOF and S_mj_Cr0/GOF composites. Selective immobilization of sulfur in pores with varying sizes can be realized via this adsorption-extraction method and was evident according to the pore size distributions. The sulfur melt was imbedded into the voids of the GOF by capillary forces, whereupon it solidified causing the significant loss of pore volume. This infiltration process clearly resulted in the disappearance of 6-16 nm mesopores in the GOF. The melt-filled GOF is thus denoted as S_meso/GOF. Additionally, the pore volume in the range beyond 16 nm also decreased sharply due to the presence of sulfur. After CS₂ extraction, sulfur was removed completely from 6-16 nm mesopores and mostly from larger pores as was clear from the similar pore volume distribution to that of the original GOF, and only micropores were still occupied by sulfur in the extracted sample, which is labeled as S_mi_Cro GOF. For both S_meSo/GOF and S_micro/GOF, the 0.65 nm micropores were filled with sulfur. It is noted that no mesopores in the range between 1.6-6 nm were measured for the GOF based on liquid nitrogen cryosorption analysis. [071] Referring to Fig. 12, there is illustrated XPS spectra showing clear evidence of sulfur incorporation within GOF. All the XPS profiles were standardized based on a C ls binding energy of 284.6 eV. [072] Referring to Fig. 13, there is illustrated (A, C) SEM images and (B, D) EDS mapping pictures of (A, B)S_meso/GOF and (C, D) S_micro/GOF.

[073] Referring to Fig. 14, there is illustrated thermogravimetric analyses of GOF and sulfur/GOF composites. The sulfur sublimed gradually when heating the composites in Argon from room temperature to 500 °C. The proportional weight loss determined at 500 °C is used here to represent the sulfur mass ratio. Before CS₂ extraction, 43 wt% of sulfur was confined, while only 16 wt% was left thereafter. The weight loss of pure GOF within the same temperature range has been taken into account in the calculations. The weight loss rate of sulfur from the composites can be divided into three regions. Region I lies below 150 °C and exhibits weight decreasing at a rate comparable with that of GOF, which is due to the removal of adsorbed water. In Region II, S_meSo GOF shows more significant weight loss than, that of S_mj_Cro/GOF. Thus, region II corresponds to the evaporation of sulfur loosely confined in the large pores (> 6 nm). The slow weight loss trend in region III is observed for both samples, and can be attributed to the removal of sulfur firmly confined in the small pores (< 1.6 nm).

[074] Referring to Fig. 15, there is illustrated electrochemical characteristics of sulfur/GOF composites. Fig. 15A shows first cycle CV and Fig. 15B shows second cycle CV curves for S_mj_cro/GOF and S_meso/GOF cathodes recorded at 0.5 mV s^"1 at room temperature. The complete loss of current responses of S_meSo GOF cathode indicates the thorough deactivation of active sulfur materials due to the unsuccessful sulfur confinement in large pores. Fig. 15C shows CV profiles of S_mj_cro/GOF from the first to fifth cycles. Fig. 15D shows EIS Nyquist profiles of S_mj_Cro GOF and S_meSo/GOF cathodes recorded from 10 kHz to 10 mHz at room temperature at cathode polarization potentials of open circuit potential (2.8 V vs. Li⁺/Li°). The much smaller real impedance of S_mj_Cro/GOF cathode demonstrates its feasibility for high rate operations.

[075] There are two peaks in the first cathodic reduction process of S_mj_Cro/GOF (see Fig. 15A). The small peak at 2.43 V (vs. Li⁺/Li°) corresponds to the conversion from elemental sulfur (Sg) to lithium polysulfide anions (Li₂S_x; where x is typically 4-6). However, this peak is not observed in the second cathodic reduction cycle (see Fig. 15B). Considering the distribution of sulfur present in pores larger than 6 nm in S_mj_Cro/GOF (see Fig. 4A), the small peak is most likely caused by the reduction of a trace, of external sulfur into polysulfide ions, which are weakly adsorbed and easily dissolved into the electrolyte. The tiny magnitude of this reaction with respect to the strong reduction peak at 1.62 V (vs. Li⁺/Li°) suggests the considerable confinement of sulfur in small pores. The cathodic peak at 1.62 V is in accordance with the deep reduction of polysulfide ions to insoluble Li₂S2 Li2S. In mesopore-confined sulfur composites, the conversion of polysulfides to lithium sulfide occurs at around 2.0 V. This potential hysteresis is also observed in micropore-rich carbon-sulfur cathodes. The low potential reduction can be attributed to the extra electrode polarization required to overcome the nanoconfinement barrier of strong adsorption energy. In stark contrast, the cathodic reaction of S_meso GOF initiates at 2.4 V and reaches the peak current at 1.74 V. Two slow plateaux are noticed with a transition point about 2.2 V. The upper branch (2.4-2.2 V) indicates the formation of polysulfide ions from sulfur located in large pores. The lower oblique branch (2.2-1.74 V) originates from the slow kinetics of lithium sulfide formation on the outer surface. The probable surface coverage by exterior lithium sulfide limits the full reduction of polysulfide ions and hinders the approach of electrons and ions to sulfur confined in subnanometer pores. The incomplete conversion of nanoconfined sulfur could be responsible for its higher peak potential (1.74 V, compared to 1.62 V of S_mj_cro/GOF), where the reaction is actually terminated.

[076] As indicated in Fig. 15B, the second cycle of S_meso/GOF shows no current response, and clearly confirms the terminated cathode activity by exterior lithium sulfides. During the second cathodic reaction process, the major 1.62 V peak of S_mj_Cro/GOF was shifted to a higher potential within 1.66-1.78 V. This potential shifting is mainly attributed to the formation of complexes with lower adsorption energy after the first anodic oxidation of lithium sulfides. Most importantly, the anodic oxidation from the first to the fifth cycles keeps almost constant (see Fig. 15C), and indicates the excellent redox stability of lithium sulfides confined in small pores. On the basis of the CV results, the unconstrained dissolution of polysulfide ions and dense coverage of cathode surface by insoluble Li₂S₂/Li₂S caused the absolute loss of capacity in the second cycle of S_meSo/GOF; while the restored redox activity of S_mj_Cro/GOF indicates the positive effect of small pore confinement on improving the stability of sulfur cathode.

[077] As seen in Fig. 15D, both the open circuit (2.8 V vs. Li⁺/Li°) EIS spectra comprise a semicircle at high frequency and an inclined tail in the low frequency region. The interfacial charge-transfer resistance recognizable from the semicircle is due to the redox formation of high-order polysulfides and low-order lithium sulfides. The monolayer adsorption of sulfur in sub-nanometer pores realizes a molecular level intimate electronic contact with GOF scaffold, while the bulky loading of sulfur in large pores fails to build up efficient circuits with conductive GOF. As a consequence, the S_meso GOF cathode gives nearly four times greater resistance than the S_mj_cro/GOF cathode. The mass transfer kinetics, mainly of lithium ions, inside the porous cathode can be compared qualitatively with reference to the slope of the low-frequency tail. The CS₂ extraction frees the occupied large pores, and surely results in the facilitated ion transfer in S_mj_cro/GOF cathode with a bigger tail slope. The combination of intimate electric connection and open ionic transport as a result of selective nanoconfinement of sulfur in pores less than 1 nm, could be utilised in a cathode providing high rate capability.

[078] Referring to Fig. 16, there is illustrated XPS S2p3/2 spectrum of S_micro GOF before and after the first discharge.

[079] Referring to Fig. 17, there is illustrated SEM images of sulfur/GOF composites recorded after the first discharge to 1.5 V vs. Li⁺/Li°. SEM images of (top) S_mj_Cro/GOF and (bottom) S_meso/GOF cathodes morphology after the first discharge. The porous texture of Smicro GOF cathode, in stark contrast to the dense surface coverage of Sm_eso/GOF cathode, highlights the effectiveness of sub-nanometer pore confinement on entrapping soluble polysulfide ions, and hence restricting the formation of solid lithium sulfides coverage.

[080] Referring to Fig. 18, there is illustrated an electrochemical impedance spectroscopy. Nyquist profiles of Smj_cro/GOF and Smeso/GOF cathodes were recorded from 10 kHz to 10 mHz at room temperature at cathode polarization of full discharge potential (1.5 V vs. Li⁺/Li°) after discharge but before recharge in the first cycle. The much smaller real impedance of S_mjcro/GOF cathode demonstrates the less occupied porosity by solid lithium sulfides coverage as shown in Fig. 17. [081] Referring to Fig. 19, there is illustrated (A) reversible discharge-recharge profiles of the second cycle, (B) cycle stability of S_mi_Cro/GOF cathode at 150 mA g^-1, and (C) coulombic efficiency and capacity retention ratio versus cycle number tested at 3 A g^"1 for 550 cycles (started from the second cycle).

[082] Referring to Fig. 20, there is illustrated discharge-recharge profiles of S_meSo/GOF of the second cycle. [083] Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

[084] Although a preferred embodiment has been described in detail, it should be understood that various changes, substitutions, and alterations can be made by one of ordinary skill in the art without departing from the scope of the present invention.

Claims

The claims.

1. A method of producing a sulfur-carbon material, including the steps of:

adsorbing sulfur into a porous carbon material; and,

selectively extracting sulfur from pores larger than micropores of the porous carbon material while leaving sulfur remaining in micropores of the porous carbon material.

2. The method of claim 1, wherein the micropores are less than a couple of nanometers in size.

3. The method of claim 1 or 2, wherein the micropores are less than about 1.6 nm in size.

4. The method of any one of claims 1 to 3, wherein the micropores are sub-nanometer pores less than about 1.0 nm in size.

5. The method of any one of claims 1 to 4, wherein the pores larger than micropores are mesopores greater than several nanometers in size.

6. The method of any one of claims 1 to 4, wherein the pores larger than micropores are mesopores greater than about 6 nm in size.

7. The method of any one of claims 1 to 6, wherein the sulfur is adsorbed as molten sulfur.

8. The method of any one of claims 1 to 7, wherein selectively extracting sulfur is by exposure to a sulfur solvent.

9. The method of claim 8, wherein selectively extracting sulfur is by preferential dissolution of sulfur adsorbed into the pores larger than micropores over dissolution of sulfur adsorbed into micropores by the sulfur solvent.

10. The method of claim 5 or 6, wherein sulfur is substantially removed from the mesopores.

11. The method of claim 5 or 6, wherein the mesopores are mostly free or devoid of sulfur.

12. The method of any one of claims 1 to 11, wherein the sulfur and the porous carbon material are solids and ground together and then heated so as to adsorb the sulfur into the porous carbon material.

13. The method of claim 8, wherein the porous carbon material with adsorbed sulfur is immersed into the sulfur solvent, which is then infiltrated through a membrane, and then remaining porous carbon material is dried.

14. The method of claim 8, wherein the sulfur solvent is carbon disulfide.

15. The method of claim 8, wherein the sulfur solvent is selected from the group consisting of dialkyl disulfides, dialkyl disulfides with a catalyst or co-solvent added, water, ethanol, acetone, carbon tetrachloride, toluene, dimethylformamide, methylpyrrolidone, natural rubber, synthetic elastomer and liquid sulfur dioxide.

16. The method of any one of claims 1 to 15, wherein the porous carbon material is a , Graphitic Open Framework (GOF) material.

17. The method of any one of claims 1 to 15, wherein the porous carbon material is selected from the group consisting of hierarchical porous carbon, activated carbon, activated carbon nanofibers, carbon aerogels, carbon nanotubes, expanded graphite, > graphene nanosheets, graphene oxide nanosheets, carbide-derived carbon and zeolite- templated carbon.

18. A sulfur-carbon material, comprising a porous carbon material having a distribution of different pore sizes including micropores and mesopores, wherein sulfur is present in the micropores and the mesopores are substantially free of sulfur.

19. The sulfur-carbon material of claim 18, wherein the micropores are less than about 1.6 nm in size and the mesopores are greater than about 6 nm in size.

20. A cathode, at least partially formed of the sulfur-carbon material of claim 18 or 19.

21. A lithium-sulfur battery, including a cathode at least partially formed of the sulfur- carbon material of claim 18 or 19.