Method for producing thick sulfur cathode for Li-S batteries

Abstract

A method for producing sulfur cathodes for Li-S batteries utilizes dry or semi-dry blending of the components (sulfur, carbon, and a binder). The resulting structure binds adjacent particles together without covering them, i.e., by connecting portions of a particle to other adjacent particles, providing a solution for the successful cycling of thick and ultra-thick sulfur cathodes. This method provides a robust, thick cathode in which the particles are firmly bonded while minimizing polymer surface coverage, providing ample space for expansion during lithiation. Bridging bonds are formed within the cathode.

Description

Technical Field

[0001] The present invention relates to Li-S batteries, and more particularly to a method for producing a thick sulfur cathode with high electron mobility and ion mobility. Background Art

[0002] Li-S batteries are considered a potential replacement for rechargeable Li-ion battery technology. This is because the theoretical energy density of these batteries is 2600Wh / kg, which is much greater than the energy density of Li-ion batteries (~200Wh / kg). Li-S batteries are also advantageous because all the components: Li (metal-anode), sulfur and carbon (cathode) are relatively cheap and abundant. However, to achieve battery-grade energy content closer to that of lithium-ion batteries, the amount of sulfur in the electrodes must be increased, otherwise the battery will not be able to take advantage of its large theoretical energy density. Currently produced sulfur cathodes, in particular, can provide >3-4mgs / cm 2 The approach of thicker sulfur cathodes is hampered by a number of issues, including a lack of ion mobility (getting the electrolyte to the reaction sites) and / or electron mobility. These issues lead to poor sulfur utilization, underdeveloped / underdeveloped electrochemistry (leading to poor performance at high C rates), and low coulombic efficiency—all of which contribute to overall poor performance whenever a thick (high sulfur content per unit area) sulfur cathode is used.

[0003] Several methods for producing these cathodes have been reported in patent literature and peer-reviewed publications, such as US9577243 B2 (Use of Expanded Graphite in Li-Sulfur Batteries). Typically, the carbon and sulfur sources are homogenized by melting sulfur (~155-200°C). Subsequently, a castable formulation is prepared from the mixture using a binder material, typically in a large amount of solvent such as water, ethanol, NMP, etc. Several other variations of this method are described in the literature (US20120119161A1, CN105470518A, WO2013049663A1), including the use of S and C particles alone (without forming a composite material by melting sulfur). However, in all these cases, two phenomena are unavoidable: surface tension / capillary forces from the dissolved binder system completely or partially cover the carbon and sulfur particles. This microstructure inevitably leads to low electrolyte accessibility and sulfur utilization, negative effects that have never been so detrimental in thin cathodes. Furthermore, during lithiation, the cathode undergoes volume expansion that is difficult to accommodate within the dense microstructure reported in the prior art, leading to structural fragmentation of the electrode. It is important to note that the binder is required to significantly enhance the electrode's processability and mechanical integrity during volume expansion during cycling. However, capillary forces in the solvent-binder system force the polymer to cover the carbon's reactive surfaces and pore interiors, limiting its overall electronic conductivity and reactivity. Electrodes in lithium-based batteries consist of active materials responsible for transferring energy by absorbing and releasing lithium ions, conductive agents providing electrical conductivity throughout the electrode network, and binders that bind the two together and to the current collector. For the electrode active materials to electrochemically react and transfer energy, ions must be transferred through the electrolyte, and electrons must be transferred through the conductive agent. Electrolyte diffusion issues or loss of contact between the active material and the conductive agent can lead to localized deactivation and capacity loss. Among all high-capacity electrodes, Si anodes in Li-ion batteries experience the most severe electrode pulverization. While boasting a specific capacity an order of magnitude higher than that of graphite, Si anodes undergo a substantial volume change of 400% during Li ion absorption and release, effectively degrading the electrode's integrity within a few cycles. As the main limitation to the realization of Si batteries, the literature focuses on exploring binders to maintain the integrity of the electrodes during cycling. -1 Compared with the silicon anode of Li-S battery system, 1670mAhg -1 The sulfur cathode exhibited a volume change of approximately 78%, much smaller than that of the Si anode, but still sufficient to cause the insulating sulfur particles to disconnect from the cathode conductive network and lose capacity. The negative effects of electrode disintegration became more pronounced with increasing areal sulfur loading of the cathode, a key parameter for achieving commercial levels of areal capacity.

[0004] In contrast to the traditional literature on Li-S batteries, which has focused on unusual cathode host materials with polysulfide confinement / adsorption capabilities, recent literature has demonstrated an appreciation for the importance of cathode integrity. Numerous papers have focused their attention on improved binder systems that demonstrate improved performance compared to PVDF-based cathodes. Given the more mature Si literature and the higher volume changes experienced by Si anodes, one approach has been to utilize suitable binders that have proven quite successful in the composition of Si anodes. This inspiration has led to the investigation of several binder systems, including but not limited to gum arabic, CMC / SBR guar and xanthan gums, and cross-linked CMC-citric acid. Despite their advantages over PVDF, these transformations have not resulted in reasonably stable Li-S batteries. This is primarily because, in addition to volume changes, Li-S systems face the highly studied problem of polysulfide shuttling, as well as the equally important issue of the insulating properties of the active materials. Reactions between lithium and sulfur in Li-S systems are problematic because the byproducts of the multistep discharge reactions, or so-called polysulfides, are highly soluble in the battery's liquid electrolyte, leading to unique shuttling phenomena in this system. Due to this shuttling effect, high-order polysulfides diffuse through the battery's thin-film separator to the anode side, where they react with lithium to form low-order polysulfides that migrate back to the cathode side. This effect significantly contributes to the loss of active material, reduced coulombic efficiency, and rapid capacity fade during cycling.

[0005] Novel binder systems have been rigorously designed to incorporate polysulfide-absorbing functionality into the binder, such as conductive, elastic, and electroactive nanocomposite binders composed of polypyrrole and polyurethane (PPyPU) (4.6 mg) and modified cyclodextrin (C-β-CD) (3 mg). A general conclusion drawn from studies aimed at retarding polysulfide shuttling is that binders with polar / electronegative functional groups are better suited for sulfur cathodes. However, few attempts have been made to address the latter issue, namely the insulating nature of the active material, in thick cathodes. Dissolved binders tend to create a continuous network spanning most of the electrode, which remains permeable to Li ions in LIB electrodes if the amount of binder used is very small (1-2%). In silicon anodes or sulfur cathodes, a relatively high proportion of binder is required to hold the electrodes together (5-30%), which effectively reduces a large portion of the active surface through the common approach of using dissolved binder systems—a major reason for the low utilization of highly loaded electrodes.

[0006] It is clear then that in order to achieve optimal electronic and electrochemical performance in thick sulfur cathodes, the design rules for their fabrication should be revisited to maximize the number of electrochemically available reaction sites. However, a new design should be able to find its way from the laboratory to industry. In order for Li-S chemistry using extremely cheap sulfur as the active material to flourish in the vast space beyond LIBs, the other two main electrode components, binders and conductive agents, cannot be separated from the typical LIB electrode formulation unless used for more specialized applications.

[0007] It is an object of the present invention to provide a method for producing thick sulfur cathodes that alleviates the above problems, or at least provides the public with a useful alternative. Summary of the Invention

[0008] In a first aspect, the present invention provides a method for producing a sulfur cathode for a rechargeable energy storage battery, the method comprising the steps of mixing a sulfur-containing source, a conductive agent, and a binder in a dry state to form a dry blend.

[0009] Preferably, the sulphur-containing source comprises from 5% to 95% sulphur by volume, preferably more than 50% sulphur, more preferably more than 65% sulphur, most preferably more than 75% sulphur.

[0010] Preferably, the sulfur-containing source comprises about 80% sulfur by volume.

[0011] Preferably, the sulfur source is selected from the group consisting of crystalline sulfur, colloidal sulfur, Li2S and MoS2.

[0012] Preferably, the dry blend comprises from 1% to 40% binder by volume, preferably less than 20% binder, more preferably less than 15% binder, most preferably less than 10% binder.

[0013] Preferably, the dry blend comprises about 5% binder by volume.

[0014] Preferably, the binder is selected from: polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), colloidal binders such as gum arabic, xanthan gum and guar gum, binders based on natural cellulose, polysaccharides such as Na-CMC, Li-CMC, sodium alginate, polyacrylates, aliphatic polymers such as polyvinyl butyral (PVB), aromatic polymers such as styrene-butadiene rubber.

[0015] Preferably, the polysaccharide-based binder is selected from the group consisting of: CMC, sodium alginate and CNC.

[0016] Preferably, the dry blend comprises from 0% to 50% by volume of a conductive agent, preferably less than 35% by volume of a conductive agent, more preferably less than 20% by volume of a conductive agent, most preferably less than 15% by volume of a conductive agent.

[0017] Preferably, the dry blend comprises about 10% by volume of the conductive agent.

[0018] Preferably, the conductive agent is a carbon-based material, such as high surface area activated carbon, highly conductive expanded graphite, CNTs, CNFs, graphene, or a conductive polymer.

[0019] Preferably, the conductive agent is selected from the group consisting of carbon black, activated carbon and graphite.

[0020] Preferably, the method further comprises the step of mixing the dry blend with a solvent to form a processable mixture.

[0021] Preferably, the amount of solvent added to the dry blend is below the solubility of the binder, preferably much below the solubility of the binder.

[0022] Preferably, the solvent is selected from the group consisting of: water, NMP, alcohol-based solvents and DMF.

[0023] Preferably, the method further comprises the step of processing the mixture onto a current collector to form the sulfur cathode.

[0024] In a second aspect, the present invention provides a rechargeable energy cell comprising a lithium anode, a separator and a sulfur cathode produced according to the method, wherein the cell further comprises a polysulfide retaining layer.

[0025] Preferably, the retention layer is coated on the sulfur cathode.

[0026] Preferably, the polysulfide retaining layer is independent from between the sulfur cathode and the separator.

[0027] Preferably, the polysulfide retaining layer is coated on the separator support.

[0028] Preferably, the polysulfide retaining layer is high surface area carbon.

[0029] Preferably, the high surface area carbon is selected from the group consisting of: graphene, carbon and CNT.

[0030] Preferably, the polysulfide retaining layer is a functional polymer.

[0031] Preferably, the functional polymer is selected from the group consisting of gum arabic, CMC and sodium alginate.

[0032] In yet another aspect, the present invention provides a rechargeable energy cell comprising a lithium anode, an electrolyte and a sulfur cathode produced according to any one of the preceding claims, wherein the electrolyte comprises an organic solvent, preferably (DME) and 1,3-dioxolane (DOL).

[0033] Preferably, the solvent comprises a mixture of DME and DOL, such as a 50:50 (v / v) mixture.

[0034] Preferably, the electrolyte comprises a soluble lithium salt, providing ionic conductivity between the anode and the cathode.

[0035] Preferably, the lithium salt includes at least one selected from lithium bis(trifluoromethane)sulfonyl imide (LiTFSI) and lithium trifluoromethanesulfonate, and preferably includes LiTFSI.

[0036] Preferably, the lithium salt is present in the electrolyte at a concentration between 0.1M and 5.0M, preferably between 0.25M and 1M, such as about 1.0M.

[0037] Preferably, the electrolyte comprises lithium nitrate (LiNO3) at a concentration between 0.05M and 1M, for example 0.5M.

[0038] It should be noted that any one of the aspects described above may include any features of any of the other aspects described above, and may include any features of any of the embodiments described below, as appropriate. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The preferred features, embodiments and variations of the present invention will be understood from the following detailed description, which provides sufficient information for those skilled in the art to implement the present invention. The detailed description should not be construed as limiting the scope of the foregoing invention in any way. The detailed description will be with reference to the following multiple drawings.

[0040] Figure 1A FIG1 is a low-resolution scanning electron microscope image showing the microstructure of a cathode produced using the method of the present invention.

[0041] Figure 1B The portion showing the link formed using the method of the present invention Figure 1A High-resolution images.

[0042] Figure 1C Schematic diagram of the resulting microstructure showing the formed links.

[0043] Figure 2 Viscosity curves comparing cathode slurries made according to the present invention with prior art cathode slurries are shown.

[0044] Figure 3 Raman spectroscopy analysis comparing cathode slurries made according to the present invention with prior art cathode slurries is shown.

[0045] Figure 4 Conductivity analysis comparing cathode slurries made according to the present invention with prior art cathode slurries is shown.

[0046] Figures 5A to 5C A comparative graph of the discharge of the cathode is shown.

[0047] Figure 6 The long-term cycling performance of the cathode under high load is demonstrated.

[0048] Figure 7 Shown is the cycling performance of a cathode formed from colloidal sulfur, CMC binder, and expanded graphite as a conductive agent, with the inset showing an SEM image of the implemented bridging mechanism.

[0049] Figures 8A to 8C SEM images showing the bridging mechanism achieved at enhanced resolution for a cathode formed from colloidal sulfur, CMC binder, and activated carbon as a conductive agent.

[0050] Figure 9 The cycling performance of a cathode formed of colloidal sulfur, CMC binder, and activated carbon as a conductive agent is shown.

[0051] Figures 10A to 10D SEM images showing the bridging mechanism achieved at enhanced resolution for a cathode formed from colloidal sulfur, PVDF binder, and expanded graphite as a conductive agent.

[0052] Figures 11A to 11C The cycling performance of cathodes formed from colloidal sulfur, PVDF binder, and expanded graphite as a conductive agent is shown for different concentrations of undissolved PVDF binder.

[0053] Figure 12 The cycling performance of the ultrahigh-loading cathode prepared by the binder-free method at a 0.1C rate in terms of gravimetric capacity, areal capacity, and Coulombic efficiency is shown. DETAILED DESCRIPTION

[0054] The following detailed description of the present invention refers to the accompanying drawings. Wherever possible, the same reference numerals will be used throughout the drawings and the following description to refer to the same or like parts.

[0055] The present invention provides a method for producing thick sulfur cathodes that overcomes the limitations of prior art methods. The method forms 'bridging' bonds between particles, overcoming the aforementioned drawbacks. A key feature is that the electrode produced by this method consists of particles that are not covered by a binder, thereby creating an open structure with accessible sulfur and carbon. This enables high discharge capacity in Li-S coin cells with sulfur loadings of 4-18 mgs / cm 2The sulfur and carbon particles of the produced cathode are connected by "bridging" bonds, resulting in a microstructure that is significantly different from that produced by state-of-the-art techniques, in which the particles are confined within a polymer binder network. The structure achieved by this method provides ample space for particle growth and volume expansion during cycling and prevents the cathode's microstructure from collapsing, enabling very impressive cycling performance. In addition, the slurry exhibits rheological characteristics that make it suitable for electrode manufacturing using conventional processes used in battery manufacturing.

[0056] The novel manufacturing process of the present invention for producing electrodes moves away from the wet mixing step reported in most sulfur-containing cathodes reported to date, and instead employs a combination of dry mixing (sulfur, carbon, and binder) and semi-dry (with a small amount of solvent) homogenization. The method ensures the formation of electrodes with rheological properties in which capillary forces play a minor role and viscoelastic forces enable the formation of bridging bonds between active particles. This method is the first known disclosure of a slurry formulation that allows for bridging bonds in sulfur cathodes. Another step of the present disclosure is the use of colloidal sulfur particles, which eliminates the need for the melt mixing step described in the prior art. The sulfur in the form of colloidal particles ensures a uniform distribution of particles in the microstructure and is compatible with the process described in the present disclosure.

[0057] The following describes a novel method for preparing sulfur cathodes using sodium carboxymethylcellulose (Na-CMC), a high modulus binder rich in carboxyl groups, resulting in very stable, high-loading sulfur cathodes. Unlike many commonly used methods for preparing cathode slurries, which follow a wet mix scheme using a pre-dissolved binder solution, the undissolved / partially dissolved Na-CMC and its high content of carboxyl groups allow for a sufficient number of bridging bonds between particles to hold them together without over-coating them. This method enables the successful fabrication of very thick cathodes (up to 20 mgs cm -2 and above), and enables very high active material utilization due to the significant increase in the free reaction surface of the active material, while providing space to accommodate volume changes during cycling due to the high degree of freedom of expansion of the particles.

[0058] The general design rule for electrodes in LIBs is straightforward: the more active material that fills a confined volume, the more energy can be obtained. Therefore, any components other than the active material, such as binders, electrolytes, separators, and conductive additives, should be minimized. However, the very different energy transfer mechanisms and the higher specific capacities in sulfur cathodes or Si anodes necessitate different design principles for battery components. A higher proportion of conductive additive (typically high-surface-area carbon) requires the use of more binder and electrolyte. While this negatively impacts the battery's energy density, this can be offset by using a higher areal loading of active material, which, in contrast to commercial LIB electrodes, results in a higher specific capacity. The study demonstrates that it is feasible to fabricate relatively robust, thick sulfur cathodes with the aid of a cellulose-based binder. A uniform distribution of carbon and sulfur in the cathode is crucial, as sulfur, Li2S, and polysulfide intermediates are insulating and cannot participate in redox reactions if they lose physical contact with the conductive carbon.

[0059] This approach shifts from the commonly used network mechanism to a bridging mechanism, in which a thick, hard binder binds adjacent particles together without covering them—that is, by connecting portions of a particle to other adjacent particles—providing a solution for successful cycling of thick and ultrathick sulfur cathodes. This approach provides robust, thick cathodes in which particles are firmly bonded while minimizing polymer surface coverage and, equally importantly, have ample space to expand during lithiation. To achieve these efficiencies in thick sulfur cathodes, the present method provides at least 10% cellulose-based binder.

[0060] The method utilizes the abundant carboxyl functional groups of Na-CMC and the ideal submicron-sized colloidal sulfur particles (rather than micron-sized elemental sulfur). Dry blending of S / C / CMC yields a highly homogeneous electrode mixture, eliminating the need for wet mixing. Compared to conventional electrode manufacturing practices, this mixture forms a highly robust, ultra-thick cathode with minimal addition of water (sufficient to wet the CMC particles, which are uniformly distributed throughout the electrode mixture).

[0061] The present invention relates to the method for preparing the electrodes, not to the materials themselves or their proportions. The method can be applied with compositions matching those used in contemporary research. To demonstrate the invention and its advantages over the prior art, four thick sulfur cathodes (≥7 mg cms) were produced with the same composition (70% S, 20% C, 10% CMC) but with different slurry preparation methods (slurry formulations). -2 ). For comparison purposes, these cathodes will be referred to as cathodes AD.

[0062] Cathodes A and B were prepared according to the first and second methods of the present invention, in which bridging bonds were established. For both cathodes, all ingredients were mixed at once for 48 hours, and then deionized water was added to form a slurry. In cathode A, water was gradually added to the mixture just enough to wet the CMC particles so that the CMC particles could establish bonds with their adjacent particles and obtain a castable paste. It was found that the amount of water required for this was approximately 1.5 mL / g electrode material, equivalent to 65 mg CMC / mL water, which is well above the solubility limit of CMC in water at room temperature. For cathode B, the amount of water added to the S / C / CMC mixture was approximately 5 mL / g electrode material.

[0063] Figure 1A Shown is a scanning electron microscope image of cathode A at a resolution that reveals its microstructure. Figure 1B Displayed at a higher resolution Figure 1A , which discloses a bridging linkage 10 produced as a result of the method of the present invention. Figure 1C Schematic diagram of the resulting structure.

[0064] Cathodes C and D were prepared based on the most commonly proposed prior art hybrid approach for making LIB electrodes, which is typical of the Li-S literature: the active materials and conductive agents, both in powder form, are mixed to establish a good conductive network, and then blended in a pre-dissolved binder solution to provide good particle-to-particle and particle-to-current collector adhesion. In cathode C, the pre-dissolved binder solution was a solution of 20 mg CMC / mL in water; in cathode D, it was a solution of 20 mg CMC / mL in a cross-linking solution. The mixture was continuously mixed for several hours to ensure good dispersion. For cathodes C and D, the amount of solvent added to the S / C / CMC mixture was approximately 5 mL / g of electrode material, similar to cathode B. For cathode C, deionized water was used as the solvent, while for cathode D, a cross-linking solution with a pH of 3 was used.

[0065] The cathode has been subjected to various tests which reveal the improved properties achieved as a result of the method of the invention.

[0066] The first test passes in 0.01s -1 The viscosity of the cathodes was measured under shear rate to compare the rheological properties of the cathodes. The viscosity of cathode A was 45,100 Pa.s, the viscosity of cathode B was 379 Pa.s, the viscosity of cathode C was 0.782 Pa.s, and the viscosity of cathode D was 17 Pa.s. The viscosity curves of the slurries used to make the four different electrodes are shown in Figure 2. For the slurry prepared to make cathode A, a significant shear thinning behavior was observed. Over time, the viscosity dropped sharply with increasing shear rate, revealing that the solid content of the slurry was very high. The shear thinning behavior in the slurries of cathodes B and cathodes D was not as dramatic due to the lower solid content. On the other hand, the viscosity curve obtained for the slurry prepared to make cathode C seemed to approach a relatively constant Newtonian viscosity at higher shear rates, indicating that the particles were fully dispersed. At very low shear rates (0.01s -1 ), the viscosity of each slurry also shows clear differences: greater than 50,000 and nearly 500 for the slurries in our work (cathode A and B, respectively), compared to typical practice in the literature (cathode D). The viscosity differences in the high shear rate region are not as drastic due to shear thinning effects. Since slurry A has a very high solid content and a water content below the minimum required to dissolve CMC, its viscosity is expected to be very high. However, given that the two slurries are identical in terms of solid content, the clear difference between the viscosity of slurries B and C is interesting and directly demonstrates the influence of slurry preparation on the rheological behavior of electrode slurries.

[0067] like Figure 3 As shown, Raman spectroscopy analysis of the four cathodes was performed to identify the presence of isolated materials, indicating ineffective mixing. The active material fraction in a typical sulfur cathode (50-80%) is much lower than that in LIB electrodes (80-97%), resulting in a much higher proportion of inactive materials. Furthermore, inactive materials bear a greater burden in sulfur cathodes. Sulfur and polysulfides are insulating, while the active materials in LIB electrodes are conductive or semiconductors, highlighting the necessity of good contact between carbon and sulfur. In fact, isolated sulfur / polysulfide particles in the cathode network are just as detrimental as polysulfides permanently dissolved in the electrolyte. This isolation can occur both during cathode fabrication and during cycling, with the latter seemingly unavoidable to date. Similar to conductive agents, binders play a more important role beyond simply bonding particles to each other and to the current collector, accommodating volume changes and maintaining cathode integrity during cycling. Unsurprisingly, uniform distribution of components is even more critical in sulfur cathodes than in LIB electrodes. It is important to note that wet mixing of the active material / carbon mixture in a pre-dissolved binder solution is not necessarily the best approach to achieve such uniform distribution, as suggested by the extensive literature on LIB electrodes. Figure 3Raman spectroscopic analysis of electrode materials collected from four cathodes, the electrode mixtures of cathodes A and B, and the individual components is shown. Clearly, no vibrational modes assignable to sulfur or CMC are found in any of the four cathodes or in the electrode mixture (S / C / CMC), demonstrating the effectiveness of dry blending as expected. Elemental mapping analysis of cathode B is consistent with this observation. The collected elemental mapping images also show matching spatial distributions of sulfur, carbon, and Na-CMC, demonstrating the uniform distribution of all three components across the cathodes prepared via the dry blending method.

[0068] The intensity of the D band corresponds to the degree of disorder of the carbon material used as a conductive agent in the electrode mixture and is generally attributed to the destruction of the lattice symmetry and the hybridization of the sp3 orbitals of carbon. -1 The G-band intensity within the region IG corresponds to the degree of order in the system, which is a result of the hybridization of the planar sp2 orbitals of carbon in crystalline graphite. In this context, the ratio ID / IG can be used to quantitatively compare the extent of the presence of surface functional groups in cathode carbon. It is worth noting that the ID / IG of cathodes prepared by the dry mixing method is lower than that of cathodes prepared by the common method of wet mixing in a pre-dissolved binder solution, which is probably because in the latter case, the carbon is more sp3 carbon induced by the surface functional groups of the large amount of dissolved CMC. Figure 4 It can be seen that a clear correlation is also observed between the bulk conductivity and the peak ratio (ID / IG) of the cathode, with higher conductivity being measured when lower ID / IG values ​​are observed.

[0069] In addition to the quantitative studies described above, detailed SEM studies at a wide range of magnifications were performed to further elucidate the effect of slurry preparation on the microstructure of the thick sulfur cathodes. Low-magnification SEM images showed crack-free, robust microstructures for all cathodes. However, even at this low magnification, the cathodes from these groups showed significant microstructural differences. Cathodes C and D showed very compact microstructures, as expected from the way they were prepared, demonstrating the effectiveness of the dissolved CMC binder in producing crack-free electrodes. Quite differently, cathode B showed a discontinuous network of large clusters, while cathode A showed a continuous network of resolvable small particles. However, at low magnification, no bonding between the clusters in cathode B and between the particles in cathode A could be observed.

[0070] High-resolution SEM analysis provides a powerful tool for clearly understanding the binding mechanisms in cathodes from these groups. In cathodes C and D, the polymer coating is evident on all particles. Entrapping the particles within a continuous polymer network not only reduces the active surface area available for redox reactions, but also, in such compact microstructures, microstructurally induced ion transport limitations can negatively impact battery performance at high C rates for thick, dense electrodes. Equally important, there appears to be very little space to buffer volume changes in these dense cathodes, and microstructural fragmentation and particle isolation are expected to occur upon cycling. As evident in the high-resolution SEM images, this approach effectively reverses the network mechanism observed in electrodes fabricated with dissolved binder systems or cross-linking via uniformly distributed bridging bonds across the cathode. In these cathodes, not only is a large portion of the surface available for redox reactions, but the particles or clusters appear to have ample buffer space, rather than being confined between a few adjacent particles.

[0071] Importantly, the two dry-blending methods increased the cathode areal density to as high as 20 mg.cm -2 On the other hand, when ultra-thick cathodes were made from slurries using pre-dissolved binder solutions, delamination of the coating from the thin Al current collector occurred.

[0072] like Figure 5A As shown in Figure 2, the cathode was tested for cycling performance at different cycling rates, from a rate as slow as 0.1C to achieve high capacity, to a rate as fast as 0.2C to evaluate the cathode response at high current. Obviously, although cross-linking helps to make a dense high-loading cathode, it leads to poor performance indicators. The absorption of the electrolyte is the main cause of Li + The main mechanism of diffusion is Figure 5B and 5C The cathode discharge regime observed in the , suggests that electrolyte access is severely restricted by the continuous polymer network spanning the cathode. Furthermore, the underdeveloped lower plateau of the cross-linked cathode demonstrates a lack of available reaction surface—a consequence of the polymer coating on the particles.

[0073] Depend on Figure 5A As can be seen, the cathode prepared by the typical practice of using a pre-dissolved binder solution showed good indicators at lower rates of 0.1C and 0.2C, but performed poorly at a rate of 0.5C. The SEM images showed that its polymer network was not continuous, allowing good permeability of Li ions through the cathode and forming a well-developed upper plateau. However, the coating on the particles showed its adverse effect on the electronic properties, resulting in a short lower discharge plateau. Figure 5BAs shown, in direct comparison with this cathode, the cathode prepared by our dry mixing method shows a significantly more developed lower plateau at the same solvent amount. Figure 5C As can be seen, this difference is more pronounced at the higher rate of 0.5C – a direct result of the much less coverage of the particles by the binder.

[0074] However, when the binder dissolution in the cathode slurry was minimized, the performance improved significantly. Cathodes made from ultra-high viscosity slurries did not appear to present manufacturing challenges or cycling difficulties. Figure 6 The researchers demonstrated long-term cycling performance of these cathodes at high and ultra-high loadings. In addition to cycling stability, impressive coulombic efficiencies (CEs) were achieved in both cases. This combination of excellent cycling performance and CEs above 99% is unique in the literature for high-loading sulfur cathodes.

[0075] More examples of structures and properties achieved with various materials are shown in Figures 7 to 12 middle.

[0076] In another embodiment, the sulfur cathode is prepared from colloidal sulfur with minimally dissolved CMC binder and expanded graphite (Ex-Gr) as a conductive agent. Compared to porous activated carbon, the expanded graphite used has near-zero porosity and eliminates the stress absorption effect of the conductive agent. Figure 7 The SEM analysis shown in the inset of Figure 1B The cycling performance is also comparable, suggesting that there is no need to use highly porous carbon to absorb the cycling stress of the ET electrode. Figure 7 The graph shows excellent cycling performance at 0.2C; after 200 cycles, the CE is still close to 100%.

[0077] In another example, where the cathode is formed from colloidal sulfur, CMC binder, and activated carbon as a conductive agent, the use of colloidal sulfur is shown to be important for our approach and the successful establishment of bridging bonds. Replacing submicron-sized colloidal sulfur particles with several micron-sized crystalline sulfur particles in the cathode formulation results in an inhomogeneous microstructure and poor performance indicators due to the often-overlooked coarsening effect of crystalline sulfur and the strong chain-linking tendency of S atoms (34). Figures 8A to 8C SEM images showing the bridging mechanism at enhanced resolution for a cathode made using a dry-mix / undissolved binder formulation but using crystalline sulfur instead of colloidal sulfur, a common cathode component, illustrate the challenges of achieving a uniform distribution of components and a crack-free microstructure due to the coarsening effect of crystalline sulfur. Particle coarsening, the macroscopic observation of particle size increase, is a combination of processes that increase overall particle size and affect particle size distribution. Crystalline sulfur is the ground state or atomic form of sulfur, S.0 , and are almost always used as the active material in sulfur cathode compositions. Sulfur atoms have a strong tendency to chain, leading to the formation of polymeric forms that can exist as rings or as chains of different sizes and configurations, but are most stable as 8-membered rings (S8) with a crown configuration. These rings quickly aggregate to form very small but visible forms of sulfur, typically in the particle size range of tens to hundreds of nanometers to a few microns. It is obvious then that in a system where one of the elements tends to aggregate, dry mixing will not allow for a uniform distribution of all the components. Additionally, bridging bonds may not be sufficient to bind such large clusters of adjacent particles. In Figure 8A The presence of sizable binder-only areas demonstrates a lack of homogeneity, while the presence of macrocracks indicates a lack of structural integrity—a direct result of particle coarsening. Figure 8B 、 8C and 8D show different bonding mechanisms at the cathode: successful bridging bonding ( Figure 8B and 8C ), and to a greater extent, network mechanisms ( Figure 8D ) and unsuccessful bridging bonding ( Figure 8D ), which illustrates the importance of using colloidal sulfur. Figure 9 The cathode prepared by crystalline sulfur showed poor cycling performance.

[0078] In yet another example, the effect of undissolved PVDF binder on cathodes formed from colloidal sulfur and expanded graphite as a conductive agent was explored. Another important factor related to the aggregation behavior of ET cathodes is that while cellulose maintains its adhesive properties (although not completely dissolved) in dry mixing methods, other conventional binders such as PVDF lose their adhesiveness under such conditions. Furthermore, to our knowledge, there are no reports on the formation of bridging bonds in the presence of solvents other than water. Figures 10A to 10D SEM images of the bridging mechanism for a PVDF-based cathode fabricated using a dry-blended / undissolved binder formulation are shown at enhanced resolution. We note that electrode fabrication is not straightforward when using PVDF as a binder, particularly in the presence of relatively large surface areas of conductive agents, where very poor adhesion to Al foil is observed ( Figure 10A and 10B ). The use of expanded graphite as a conductive agent resulted in relatively good coatings, which allowed the stamping of some thick and relatively thick electrodes from non-uniformly coated cathodes and the investigation of their performance in button cell assemblies. However, visual observation of the coatings and SEM images indicated that PVDF, in contrast to cellulose, loses its adhesive properties when not fully dissolved in the solvent (NMP). Interestingly, very little agglomeration was observed, and the colloidal sulfur particles and expanded graphite powder largely retained their physical properties ( Figure 10C and 10D ).

[0079] Figures 11A to 11C The cycling performance of cathodes formed of colloidal sulfur, PVDF binder, and expanded graphite as a conductive agent for different concentrations of undissolved PVDF binder is shown. The cycling performance of high-loaded and ultra-high-loaded sulfur cathodes using undissolved PVDF binder is shown. -2 ( Figure 11A ) and 6.1 mg cm -2 ( Figure 11B ) and a rate of 0.2C, it exhibits relatively high capacity and very good capacity retention, and after 100 cycles, the CE is still close to 100%. -2 ( Figure 11C ) and 0.1C rate, average capacities and high capacity retention were observed, and after 100 cycles, the CE was still above 99%. It is believed that the extremely open microstructure of this cathode is able to promote electrolyte diffusion and accommodate cycling stress. However, the usual electronic wiring on the electrode hinders the desired sulfur utilization. From a production point of view, for practical applications, such as for soft pack battery configurations, a uniform, pinhole-free and strong coating on large-sized 2D metal current collectors is the only solution suitable for the industry, which is obviously not achievable using PVDF. Even so, it can be concluded that dry mixing still exhibits its most important advantage in the case of PVDF: an open structure in which the particles are not strictly confined between adjacent particles.

[0080] Figure 12 The cycling performance of the ultrahigh-loading cathode prepared by the binder-free method at a 0.1C rate in terms of gravimetric capacity, areal capacity, and Coulombic efficiency is shown.

[0081] The present invention also relates to a rechargeable energy cell manufactured according to the method, comprising a lithium anode, a separator, and a sulfur cathode produced according to the method, wherein the cell further comprises a polysulfide retention layer (also known as a carbon-coated separator). The retention layer can be applied to the sulfur cathode or separately between the sulfur cathode and the separator. The polysulfide retention layer can be applied to a separator support and is preferably a high-surface-area carbon, such as graphene, carbon, or carbon nanotubes. The retention layer can also be a functional polymer, such as gum arabic, carbon methyl sulfide (CMC), and sodium alginate.

[0082] In another aspect, the present invention provides a rechargeable energy cell comprising a lithium anode, an electrolyte, and a sulfur cathode produced according to the method, wherein the electrolyte comprises an organic solvent, preferably (DME) and 1,3-dioxolane (DOL). In a preferred embodiment, the solvent comprises a mixture of DME and DOL, such as a 50:50 (v / v) mixture. The electrolyte comprises a soluble lithium salt that provides ionic conductivity between the anode and the cathode. The lithium salt comprises at least one selected from lithium bis(trifluoromethane)sulfonyl imide (LiTFSI) and lithium trifluoromethanesulfonate, and preferably comprises LiTFSI. The lithium salt may be present in the electrolyte at a concentration between 0.1M and 5.0M, preferably between 0.25M and 1M, such as about 1.0M. The electrolyte may comprise lithium nitrate (LiNO3), which is reported to inhibit the redox shuttle reaction of polysulfides at the anode, thereby improving the coulombic efficiency of the battery. In some embodiments, the concentration of LiNO3 present in the electrolyte may be between 0.05M and 1M, such as 0.5M.

[0083] The reader will now appreciate the present invention, which provides a new method for producing sulfur electrodes that results in improved performance and durability compared to known prior art methods. In general, the present invention comprises the steps of dry mixing all ingredients, including active material, binder, conductive agent (and any other additives); semi-dry treating the mixture by adding a minimum amount of solvent to obtain a castable paste, but with the binder still largely undissolved, preferably to the greatest extent possible; and casting the ultra-high viscosity paste onto a current collector. By placing a minimum amount of binder between adjacent particles, the method allows increased space for material expansion (expansion-resistant structure), provides additional porosity for rapid diffusion of ions, and maximizes the number of electrochemically available reaction sites (material not covered with binder). Although specific examples of materials and solvents have been described, they should not be considered limiting as the method is applicable to a wide range of materials and solvents.

[0084] Further optimizations and improvements may be made to the present invention without departing from the scope of the present invention. Although the present invention has been shown and described in what are considered to be the most practical and preferred embodiments, it will be appreciated that departures may be made within the scope of the present invention, which is not limited to the details disclosed herein but is given the full scope of the claims to cover any and all equivalent devices and apparatuses. Throughout this specification, any discussion of the prior art should not be taken as an admission that such prior art is widely known or forms part of the common general knowledge in the field.

[0085] In this specification and claims (if any), the word "comprise" and its derivatives including "include" and "comprising" include each of the stated integers but do not exclude the inclusion of one or more additional integers.

Claims

1. A method for producing a sulfur cathode for a rechargeable energy storage battery, the method comprising the following steps: mixing the sulfur source, the conductive agent, and the binder in a dry state to form a dry mix; combining the dry blend with a solvent to form a processable mixture; wherein the amount of solvent added to the dry blend is lower than the solubility of the binder; The binder is selected from polysaccharides, aliphatic polymers or aromatic polymers.

2. The method of claim 1, wherein the binder is selected from the group consisting of polytetrafluoroethylene, polyvinylidene fluoride, and polyacrylate.

3. The method of claim 1, wherein the binder is selected from the group consisting of: a gum binder or a natural cellulose based binder.

4. The process of any one of claims 1 to 3, wherein the solvent is selected from the group consisting of: water, NMP, alcohol-based solvents or DMF.

5. The method of any one of claims 1 to 3, further comprising the step of processing the mixture onto a current collector to form the sulfur cathode.

6. The method of claim 1, wherein the sulfur-containing source comprises 5% to 95% sulfur by volume.

7. The method of claim 6, wherein the sulfur-containing source comprises from 50% to 95% sulfur by volume.

8. The method of claim 7, wherein the sulfur-containing source comprises from 65% to 95% sulfur by volume.

9. The method of claim 8, wherein the sulfur-containing source comprises from 75% to 95% sulfur by volume.

10. The process of any one of claims 1 to 3, wherein the sulfur-containing source comprises 80% sulfur by volume.

11. The method of any one of claims 1 to 3, wherein the sulfur-containing source is selected from the group consisting of crystalline sulfur, colloidal sulfur, Li2S, or MoS2.

12. The method of claim 1, wherein the dry blend comprises 1% to 40% binder by volume.

13. The method of claim 12, wherein the dry blend comprises 1% to 20% binder by volume.

14. The method of claim 13, wherein the dry blend comprises 1% to 15% binder by volume.

15. The method of claim 14, wherein the dry blend comprises 1% to 10% binder by volume.

16. A method according to any one of claims 12 to 15, wherein the dry blend comprises 5% binder by volume.

17. The method of claim 3, wherein the gum binder is selected from the group consisting of gum arabic, xanthan gum, and guar gum.

18. The method of any one of claims 1, 3, and 12 to 15, wherein the polysaccharide is selected from: Na-CMC, Li-CMC, or sodium alginate.

19. The method of any one of claims 1 and 12 to 15, wherein the aliphatic polymer is polyvinyl butyral.

20. The method of any one of claims 1 and 12 to 15, wherein the aromatic polymer is styrene-butadiene rubber.

21. The method of claim 1 or 3, wherein the polysaccharide is selected from the group consisting of CMC, sodium alginate, and CNC.

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