Alkaloid materials as a lithium-sulfur battery additives and applications thereof

Abstract

A method for making a sulfur cathode, comprising steps oF mixing sulfur and an amount of a purine compound in a weight ratio of sulfur to purine of from less than 10:1 to greater than 2:1 to form a powder mixture; mixing the powder mixture with a solvent to form a slurry; and drying the slurry onto a current collector to form the sulfur cathode. A cathode for use in a lithium sulfur battery including sulfur; and an purine selected from the group consisting of caffeine, nicotine, morphine, codeine, quinine, atropine, ephedrine, strychnine, colchicine, theobromine, theophylline, papaverine, reserpine, coniine, harmine, ibogaine, vinblastine, vincristine, yohimbine, capsaicin, camptothecin, mitragynine, apomorphine, psilocybin, scopolamine, tetrahydrocannabinol (THC), salvinorin A, mescaline, muscarine, preferably, the purine is caffeine. Also disclosed are electrolytes and batteries.

Description

[0001] PURINE MATERIALS AS A LITHIUM-SULFUR BATTERY ADDITIVES AND APPLICATIONS THEREOF STATEMENT OF GOVERNMENT INTEREST

[0002] This invention was made with government support under grant nos. NSF 1919177 and 1804374 from the National Science Foundation. The government has certain rights in the invention.

[0003] CROSS REFERENCE TO RELATED APPLICATIONS

[0004] This application claims the benefit of U.S. Provisional Application No. 63 / 636,274, filed on April 19, 2024, and U.S. Provisional Application No. 63 / 636,291, filed on April 19, 2024, the entire disclosures of which are hereby incorporated by reference in their entirety as if set forth fully herein.

[0005] BACKGROUND

[0006] Batteries with higher energy density are being developed to meet the demand from electric vehicles (EVs). The batteries that are currently leading the EV revolution are lithium-ion batteries (LIBs). Although LIBs are a mature technology, they have reached their theoretical energy density threshold of around 150 - 250 Wh kg'1. This means that opportunities to reduce weight and cost are limited for these expensive batteries. Therefore, a next generation battery chemistry that is cheaper and that provides a higher energy density is desired. For this purpose, lithium-sulfur batteries (LSBs) are a promising technology due to the expectation that such batteries can provide an 8-fold increase in theoretical capacity (up to 1675 mAh g'1and a theoretical energy density around 2,600 Wh kg'1), compared to LIBs (-200 mAh g'1). Also, due to the natural abundance of sulfur, its price is around $100 / ton making it much more affordable than the current materials used in LIBs [1], Furthermore, sulfur has a low environmental impact.

[0007] However, LSBs have their own challenges. One significant challenge is the polysulfide shuttling effect where intermediate polysulfide species that are formed during battery discharge can dissolve in the liquid electrolyte and shuttle from the cathode to the anode. As a result, the active material becomes inactive and ultimately results in capacity fade. At the lithium anode, the poly sulfides reduce rapidly to insoluble products that precipitate out of the electrolyte, thus decreasing the amount of accessible active material in the battery. Ultimately, this results in adecline in battery capacity as cycling continues. To address this shuttling effect, various strategies have been proposed.

[0008] One approach is to employ electrolyte additives, inert cosolvents, and phase separating electrolytes.8'10Another less commonly used technique is to add molecules, such as an organic oligoaniline, to create complexes with polysulfides that are too large to diffuse through the conventional separator pores.11Other methods focus on the cathode structure to physically encapsulate sulfur and prevent it from leaving the cathode.12'14Thiourea has been used as a liquid electrolyte additive to inhibit the shuttling effect. In this work Rafie et al. demonstrated that thiourea forms sulfur complexes that prevent sulfur from leaving the cathode [2], Zheng et al. added an “inert” fluoroalkyl ether cosolvent to the traditional DME-DOL ether electrolytes. The inert cosolvent has a low donor number so polysulfide solubility decreases, thereby preventing shuttling [3],4Ren and Manthiram developed a dual-phase electrolyte wherein two distinct electrolytes phase separate. One electrolyte phase promotes the poly sulfide redox reaction, while the other shields the lithium anode from poly sulfides.5Other methods focus on the cathode structure itself. Ji et al. first demonstrated the concept of melt diffused sulfur entrapment in a nanostructured cathode that contained channels just 3 nm wide [4, 5], Li et al. used “lotus root” inspired nanofibers with channels within the fibers that were 60 nm in diameter. These small channels encapsulated the sulfur and prevent the shuttling effect [6], In a recent discovery by Pai et al., the sulfur active material itself was altered. Most commonly, the a-orthorhombic phase of sulfur is used because it is the most stable, y-monoclinic sulfur was synthesized since it was stable in natural ambient conditions [7], This sulfur could undergo direct conversion from elemental sulfur to Li2S and enabled the use of a carbonate electrolyte. Notably, this reaction was possible with unconfined sulfur, distinguishing it from other entrapment and encapsulation studies. In a cathode additive approach, Cardoza et al. studied 1 -dimensional titania lepidocrocite-based nanofilaments that could anchor polysulfides both through polar-polar and Lewis acid-base interactions.16Natural materials like gelatin, zein proteins,17amino acids, and soy protein18have also seen success due to their polar functional groups. Other methods include improving necessary battery components such as using functionalized cathode binders19, 20and separators.21

[0009] Polymer electrolytes are another method to tackle the shuttling effect. One of the polymers that addresses this challenge is poly(methyl methacrylate) (PMMA). PMMA is used because it contains carbonyl groups that help anchor polysulfides via a polar-polar interaction between the oxygen in the carbonyl group and the lithium in the poly sulfide [8-10], Another polymer that is known for inhibiting the polysulfide shuttling effect is pentaerythritoltetraacrylate (PETEA). Liu et al. first presented this polymer and illustrated its polysulfide adsorption capability utilizing carbonyl groups

[0011] ,

[0010] Additionally, filler molecules are a method to further enhance the properties of polymer electrolytes. These filler molecules are compounds that are homogenously dispersed in the polymer matrix. For example, Mukkabla et al. used SiCh nanoparticles as a filler molecule. The polar surface groups of SiCh help to adsorb polysulfides

[0012] ,

[0011] In an approach to mitigate polysulfide shuttling with natural materials, Chen et al. used gelatin and zein protein compounds in carbon nanofiber (CNF) interlayers. Additionally, gelatin was used as a cathode binder. The polar groups on the proteins with negatively charged oxygen atoms provided the best binding sites for the polysulfides. Gelatin was the better compound because its proteins had shorter side chains which allowed polysulfides to more easily access the electronegative oxygen atoms, unlike in the case of zein proteins

[0013] , Fu et al. leveraged the concept of amino acids and used soy protein as a poly sulfide anchoring compound. Soy protein was very effective because it contains a very large number of amino acid sequences

[0014] ,

[0012] To effectively study the interactions of these materials with polysulfides, in-operando techniques are highly desirable. In-operando Fourier transform infrared (FTIR) spectroscopy is a powerful technique for studying bond signatures and changes in materials and does not require an X-ray source. However, few studies exist in the literature due to the stringent requirements of the system including simulating mechanical coin cell crimping pressure, hermetic sealing, demonstration of proper electrochemical performance in the model system, and coupling of optical and electrochemical signals. Of the few studies leveraging such techniques for lithium-sulfur batteries, most have investigated polysulfide speciation.22'25Some works also investigated electrolyte decomposition.26’27Beyond these, in-operando FTIR studies studying LSBs in real time are limited. The present invention demonstrates the utility of this technique beyond speciation to tackle the mechanistic understanding of polysulfide interaction with the molecular additive caffeine.SUMMARY OF THE INVENTION

[0013] The present invention relates to cathodes for use in lithium sulfur batteries (LSB) comprising of a purine compound, for example, caffeine. Purine compounds, such as caffeine may be used as additives to provide benefits for LSBs. The present invention demonstrates that caffeine can improve how long the battery lasts per charge. Caffeine can also be obtained from food waste, making it a green solution for LSB additives. Further, purine compounds such as caffeine can be employed as a powder additive to the cathode slurry which allows for easy incorporation of the additive at a large, commercial scale using existing industry infrastructure.

[0014] Experimental data indicates that the carbonyl functional group of caffeine may interact with intermediate lithium polysulfide molecules during the redox reaction that occurs in an LSB during discharge. This interaction decreases the amount of active material lost due to the “shuttling effect” by mitigation reduction of polysulfides at the anode after shuttling from the cathode. Inhibition of the shuttling effect, in turn, increases the amount of available active material that remains in the battery after multiple cycles and therefore increases the long-term capacity of LSBs. Preliminary galvanostatic cycling data has confirmed the inhibition of the shuttling effect. The occurrence of a reversible interaction between polysulfides and the carbonyl group of caffeine is also supported by in situ FTIR data.

[0015] The present invention may be described by the following sentences:

[0016] 1. In a first aspect, the present invention relates to a method for making a sulfur cathode, comprising steps of:

[0017] a) mixing sulfur and an amount of a purine compound in a weight ratio of sulfur to purine of from less than 10:1 to greater than 2: 1 to form a powder mixture; b) mixing the powder mixture with a solvent to form a slurry; and

[0018] c) drying the slurry onto a current collector to form the sulfur cathode.

[0019] 2. In a second aspect, the present invention relates to a method for making a sulfur cathode, comprising steps of:

[0020] a) mixing sulfur and an amount of a purine compound in a weight ratio of sulfur to purine of from less than 10:1 to greater than 2: 1 to form a powder mixture; b) melting the powder mixture into a porous material to form the sulfur cathode.

[0021] 3. The method of any one of sentences 1 - 2, wherein the weight ratio of sulfur to purine is from 8: 1 to 3:1 or 6:1 to 3.5:1 or about 4:1.The method of sentence 2, wherein the porous material is selected from the group consisting of carbon nanofibers; metal-organic frameworks; mesoporous silica; porous polymers, such as titanium oxide, iron oxide, aluminum oxide; porous ceramic materials, such as alumina and zirconia; graphene; and carbon nanotubes.

[0022] The method of any one of sentences 1 and 3, further comprising mixing carbon black in step a) to form the powder mixture.

[0023] The method of any one of sentences 1, 3, and 5, further comprising dissolving a polymer binder in the solvent.

[0024] The method of any one of sentences 1 - 6, wherein the purine compound is caffeine.

[0025] The method of any one of sentences 1, 3, 6, and 7, wherein the solvent is present in an amount such that the slurry has a viscosity range of from about 1 to 100 Pa'S, as determined by a rotational viscometer.

[0026] The method of any one of sentences 1, 3, and 5 - 8, wherein the drying step is carried out at a temperature of from about 20°C to about 60°C, or at about 40°C.

[0027] The method of any one of sentences 1 - 9, wherein the purine is selected from the group consisting of caffeine, theobromine, theophylline, guanine, hypoxanthine, xanthine, theophylline, theobromine, uric acid, isoguanine, preferably, the purine compound is caffeine.

[0028] The method of any one of sentences 1, 3, 5 - 8, and 10, wherein the polymer binder is selected from the group consisting of poly(vinylidene fluoride-co-hexafluoropropylene), poly(vinylidene fluoride), poly(acrylonitrile-co-methyl acrylate) (PANMA), polyethylene oxide (PEO), polyacrylonitrile, polytetrafluoroethylene, styrene-butadiene rubber, polyacrylic acid, chitosan, sodium alginate, sodium carboxyl methyl cellulose (CMC), polyvinyl alcohol (PVA), polyethyleneimine (PEI), and combinations thereof.The method of any one of sentences 1, 3, 5 - 8, and 10 - 11,, wherein the solvent is selected from the group consisting of N-methyl-2-pyrrolidone, dimethoxyethane (DME), 1,3-dioxolane, propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and combinations thereof.

[0029] In a third aspect, the present invention relates to a cathode made by the method of any one of sentences 1 - 12.

[0030] In a fourth aspect, the present invention relates to a cathode for use in a lithium sulfur battery, comprising:

[0031] sulfur; and

[0032] a purine selected from the group consisting of caffeine, theobromine, theophylline, guanine, hypoxanthine, xanthine, theophylline, theobromine, uric acid, isoguanine, preferably, the purine is caffeine;

[0033] wherein the cathode has a weight ratio of sulfur to purine of from less than 10:1 to more than 2:1.

[0034] In a fifth aspect, the present invention relates to a lithium sulfur battery comprising the cathode of any one of claims -13 - 14.

[0035] In a sixth aspect, the present invention relates to an electrolyte for use in a battery, comprising:

[0036] a purine compound selected from the group consisting of caffeine, theobromine, theophylline, guanine, hypoxanthine, xanthine, theophylline, theobromine, uric acid, isoguanine, preferably, the purine compound is caffeine; and

[0037] a carbonate solvent selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and combinations thereof.

[0038] The electrolyte of sentence 16, further comprising a conductive salt, wherein the conductive salt is optionally selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and combinations thereof.18. The electrolyte of any one of sentences 16 - 17, wherein the purine compound is caffeine.

[0039] 19. The electrolyte of any one of sentences 16 - 18, further comprising a polymer binder, wherein the binder is optionally selected from the group consisting of poly(vinylidene fluoride-co-hexafluoropropylene), poly(vinylidene fluoride), poly(acrylonitrile-co- methyl acrylate) (PANMA), polyethylene oxide (PEO), sodium carboxyl methyl cellulose (CMC), polyvinyl alcohol (PVA), polyethyleneimine (PEI), and combinations thereof.

[0040] 20. In a seventh aspect, the present invention relates to a lithium sulfur battery comprising the liquid electrolyte of any one of sentences 16 - 19.

[0041] 21. The battery of sentence 20, wherein the battery has a ratio of microliters of electrolyte to milligrams of sulfur of from about 10 to about 40, or from about 20 to 30.

[0042] 22. The battery of any one of sentences 20 - 21, wherein the battery has a ratio of microliters of electrolyte to milligrams of sulfur of from about 10 to about 40, or from about 20 to 30.

[0043] BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Fig. 1 shows the chemical structure of a caffeine molecule.

[0045] Fig. 2 shows a scanning electron microscopy (SEM) image with energy-dispersive X-ray spectroscopy (EDS) elemental mapping overlaid to highlight the presence of caffeine in a carbon-sulfur slurry. The caffeine crystal near the center of the image indicates the presence of oxygen atoms (highlighted using a dotted line ••••) and nitrogen atoms (highlighted using thick dashed line - ). The surrounding material shows sulfur in clusters (labeled) and carbon. Carbon is not identified in the image as the majority of the image includes carbon throughout except where identified.

[0046] Fig. 3 shows an SEM image of caffeine incorporated into a carbon-sulfur slurry. It shows the presence of the caffeine crystal and that no adverse effects to the slurry morphology resulted from the addition of the caffeine.Fig. 4 shows an SEM image of a carbon nanofiber (CNF) mat with caffeine and sulfur melt diffused into it. The large crystalline structures in the middle of the image are likely to be caffeine because they appear similar to the structures seen in the SEM image of Fig. 3.

[0047] Fig. 5A shows Fourier transform infrared spectroscopy (FTIR) spectrum of an in situ lithium-sulfur coin cell with a CNF cathode with sulfur and caffeine melt diffused into it discharging.

[0048] Fig. 5B shows Fourier transform infrared spectroscopy (FTIR) spectrum of an in situ lithium-sulfur coin cell with a CNF cathode with sulfur and caffeine melt diffused into it charging.

[0049] Fig. 6A shows FTIR spectrum of an in-situ lithium-CNF coin cell discharging, with a carbon nanofiber (CNF) cathode that has only caffeine melt diffused into it.

[0050] Fig. 6B shows FTIR spectrum of an in-situ lithium-CNF coin cell charging, with a carbon nanofiber (CNF) cathode that has only caffeine melt diffused into it.

[0051] Fig. 7A shows a plot showing the shift in wavenumber, corresponding to the voltage during discharge.

[0052] Fig. 7B shows a plot showing the shift in wavenumber, corresponding to the voltage during charging.

[0053] Fig. 8A shows galvanostatic cycling of a lithium-sulfur coin cell with caffeine added to the sulfur cathode.

[0054] Fig. 8B shows a corresponding voltage curve from the 150th cycle of the galvanostatic cycling shown in Fig. 8A.

[0055] Fig. 9 shows cyclic voltammetry (CV) results for coin cells with lithium anodes and cathodes with caffeine and sulfur and cathodes with caffeine only.

[0056] Fig. 10A shows FTIR spectra of caffeine powder and caffeine powder dissolved in DME:D0L 1 : 1 ether solvent in the region of 800-11800 wavenumber.

[0057] Fig. 10B shows FTIR spectra of caffeine powder and caffeine powder dissolved in DME:DOL 1 : 1 ether solvents in the region of 1400-1800 wavenumber.

[0058] Fig. 11 A shows an FTIR spectrum of an in-operando lithium-sulfur coin cell with a slurry cathode with sulfur and caffeine (left) with spectra corresponding to discharging and charging portions of the CV.

[0059] Fig. 1 IB shows an FTIR spectrum of an in-operando coin cell, with a slurry cathode that has caffeine and no sulfur.

[0060] Fig. 11C is a plot showing the wavenumber shifting associated with the C2 (bottom line, corresponding to the far left x axis) and Ce carbonyl peaks (middle line, correspondingto the second to the left x axis) corresponding to the voltage and current during in-operando CV sweeps (right x axis.)

[0061] Fig. 12 shows an in-operando IR spectra showing polysulfide evolution during the first discharge of the in-operando CVs.

[0062] Fig. 13 A shows a CV comparison of a cathode with caffeine only to a cathode with both caffeine with sulfur.

[0063] Fig. 13B shows a shuttle current test for sulfur cathodes with varying ratios of sulfur incorporated therein.

[0064] Figs. 14A-I show SEM images of caffeine incorporated into a carbon-sulfur slurry. Image (a) shows 2,000x magnification, and image (b) shows 200x magnification. Image (c) shows an SEM image of the region shown in image (a) at l,500x magnification with EDS elemental mapping overlaid to highlight the presence of caffeine in the cathode with individual elemental maps of (d) oxygen, (e) sulfur, (f) nitrogen, and (g) carbon. Postmortem SEM images of a cathode with the caffeine additive after galvanostatic cycling at (h) 4,000x magnification and (i) 200x magnification.

[0065] Fig. 15A shows galvanostatic cycling comparison of lithium-sulfur coin cells with and without caffeine incorporated into the cathode.

[0066] Fig. 15B shows the corresponding voltage curve from the 5thcycle of the galvanostatic cycling comparison of lithium-sulfur coin cells with and without caffeine incorporated into the cathode of Fig. 15 A.

[0067] Fig. 15C shows the corresponding voltage curve from the 250thcycle of the galvanostatic cycling comparison of lithium-sulfur coin cells with and without caffeine incorporated into the cathode of Fig. 15 A.

[0068] Fig. 16 shows in-operando FTIR spectra from the second cycle of a lithium-sulfur battery with caffeine as a cathode additive showing the caffeine carbonyl peak shifting during discharge and charge.

[0069] Fig. 17 shows in-operando FTIR spectra from the third cycle of a lithium-sulfur battery with caffeine as a cathode additive showing the caffeine carbonyl peak shifting during discharge and charge.

[0070] Fig. 18 shows a schematic of the in-operando FTIR coin cell setup featuring an ATR diamond crystal, aluminum contacts, and connections to the potentiostat.

[0071] Fig. 19 shows a schematic of a coin cell setup featuring a sulfur cathode with the incorporated caffeine anchoring the polysulfides.Fig. 20 shows a spectra comparing caffeine powder to a 4: 1 cathode with caffeine incorporated therein showing identical peaks in the range of 1,400 cm’1to 1,800 cm’1.

[0072] Fig. 21A shows an X-ray photoelectron spectroscopy N1 s spectra of a cathode with caffeine.

[0073] Fig. 2 IB shows an X-ray photoelectron spectroscopy Ols spectra of a cathode with caffeine.

[0074] Fig. 21C shows an X-ray photoelectron spectroscopy S2p spectra of a cathode with caffeine.

[0075] Fig. 2 ID shows an X-ray photoelectron spectroscopy N1 s spectra of a cathode without caffeine.

[0076] Fig. 2 IE shows an X-ray photoelectron spectroscopy Ols spectra of a cathode without caffeine.

[0077] Fig. 2 IF shows an X-ray photoelectron spectroscopy S2p spectra of a cathode without caffeine.

[0078] Fig. 22 shows UV-vis spectra and corresponding photographs for polysulfide solution with and without exposure to caffeine powder.

[0079] Fig. 23 shows Li2S nucleation measurements comparing a cathode without caffeine and a cathode with caffeine.

[0080] Fig. 24 shows EIS spectra showing the resistance in coin cells with and without caffeine.

[0081] Fig. 25 shows the rate capability test for coin cells with and without caffeine.

[0082] Fig. 26A shows fitted Gaussian curves to spectra from various points during in-operando cycling at 2.8V during the first discharge.

[0083] Fig. 26B shows fitted Gaussian curves to spectra from various points during in-operando cycling at 2.0V during the first discharge.

[0084] Fig. 26C shows fitted Gaussian curves to spectra from various points during in-operando cycling at 2.6V during the third discharge.

[0085] DETAILED DESCRIPTION

[0086] Alkaloids are employed as an additive in lithium-sulfur batteries as cathode additives, liquid electrolyte additives, and / or as filler molecules in polymer electrolytes. Alkaloid compounds are an attractive material because of the abundance of polar functional groups, e.g. carbonyl groups, in these compounds. These polar groups can be used to anchor polysulfides and thereby reduce the loss of active material from the cathode. Furthermore, these alkaloidcompounds also contain amide groups that have lone electron pairs. These amide groups may also behave as polysulfide anchoring functional groups

[0015] ,

[0087] One advantage of the present concept is that the purine compounds are naturally derived and represent a bio-based, green approach to LSB improvements. Purine compounds can be obtained from food waste, like coffee grounds. Another advantage to using, for example, caffeine as an additive to the cathode is that it requires very little adaptation of the current industrial infrastructure. Purines in powder form, such as caffeine powder, can be mixed in with the other solid powders used for making battery components during slurry processing.

[0088] Further, compared to conventional LSB cathodes that only contain carbon, sulfur, and a binder, addition of the purine compound(s) improves the capacity and longevity of the battery. Finally, other methods to inhibit polysulfide shuttling are energy intensive and / or require complex processing. For example, creating nanometer scale channels within a carbon nanofiber host is difficult and melting sulfur into those pores is energy intensive. By contrast, the purine compounds can be added as a powder and mixed together with carbon and sulfur in the existing mixing step and no additional energy is required to activate the cathode, beyond the energy employed in the existing drying steps.

[0089] Cyclic voltammetry was employed to study the impact of purines such as caffeine on the reaction kinetics. Electrochemical impedance spectroscopy was employed to study the impact of caffeine on cell resistance, and shuttle current tests were used to quantitatively measure the poly sulfide anchoring effectiveness, and effects of additional galvanostatic cycling to better understand rate capability and capacity retention. Scanning electron microscopy coupled with energy dispersive spectroscopy was used to detect polysulfide interactions with caffeine, Fourier-transform infrared spectroscopy was also used to study interactions between polysulfides and purines such as caffeine, and postmortem X-ray photoelectron spectroscopy was employed to observe changes in anode solid electrolyte interphase (SEI) composition.

[0090] Motivation for Incorporating Caffeine in Lithium-Sulfur Batteries

[0091] Caffeine is a compound of great interest regarding the polysulfide redox reaction because its functional groups (carbonyl, amine, amide, and imine) have lone electron pairs that interact with polysulfides (Figure 1). Furthermore, the polar carbonyl group adsorbs polysulfides and could reduce the polysulfide shuttling effect. This adsorption effect corresponded directly with the appearance of polysulfides in the battery, both during charging and discharging. Leveraging this information, this anchoring phenomenon was confirmed using a polysulfide shuttling test in a coin cell format and with caffeine incorporated into aconventional sulfur cathode to make complete Li-S cells that showed improved capacity retention.

[0092] Additionally, caffeine does not have long side chains which could prevent polysulfides from reaching its functional groups. This allows caffeine to chemically adsorb polysulfides. When added to the cathode side of the battery, this keeps soluble intermediate polysulfides from shuttling to the anode. This prevents the polysulfides from reducing to insoluble compounds at the anode. For this reason, methylxanthines, and more generally purines are useful in lithium-sulfur batteries. The initial experimental results using in situ FTIR, cyclic voltammetry, and galvanostatic cycling support the conclusion that polysulfide shuttling and precipitation are reduced by use of such additives.

[0093] In-Situ FTIR

[0094] In situ FTIR is a technique that is used to view how materials behave inside a coin cell as it cycles. The peaks at around 1,655 cm’1and 1,555 cm’1of Figure 5A were assigned to the asymmetric and symmetric stretching of the carbonyl groups, respectively

[0016] , Density Function Theory (DFT) studies and experimental calculations done by Srivastava et al. support this assignment. Additionally, the peak at around 1,705 cm’1was assigned to the in-phase stretching of the carbonyl groups

[0017] , When the battery was discharged from 2.6V to 1.8V, the carbonyl peaks shifted down in frequency by about 6 cm’1, suggesting that the carbonyl group was interacting with another compound while cycling (Figure 5A). The resolution of the FTIR was about 0.5 cm’1, which makes a change of 6 cm’1significant. This shift is an indication of a polysulfide interaction. The return of these carbonyl peaks to their original locations after charging indicates the full reversibility of this phenomenon that can be repeated cycle after cycle (Figure 5 / i)

[0095] In contrast to the in situ FTIR coin cells with sulfur and caffeine in the cathode, a coin cell with only caffeine in the coin cell showed no changes in its carbonyl peaks while charging and discharging (Figures 6 A - 6B).

[0096] Figure 7A shows the carbonyl peak wavenumber shifting from about 1,655 cm'1to 1,649 cm’1as the cell discharges from 2.6V to 1.8V. At about 2.2V, after the long chain intermediate polysulfides have formed, the peak position begins to shift, which suggests that the peak shift occurs in response to polysulfide interactions. Figure 7B shows the charging behavior of the cell and the carbonyl peak shift from 1,649 cm’1to 1,655 cm’1, the original peak position. This shows complete reversibility of the phenomenon, which is critical for preserving the caffeine molecule’s ability to continually anchor polysulfides for many cycles.Galvanostatic Cycling

[0097] Figure 8A shows a galvanostatic cycling plot for a reference coin cell with just sulfur in the cathode (S ref) and two replicates of coin cells with sulfur and caffeine in the cathode in a weight ratio of 5: 1 sulfurcaffeine. The cells were cycled at a C / 2 rate after formation cycles. A clear improvement was seen in the cells containing caffeine. At 150 cycles, the cells with caffeine provided a capacity of over 700 mAh g’1, whereas the conventional cell provided a capacity slightly under 600 mAh g'1. This represents an improvement in capacity of at least 15%.

[0098] Figure 8B shows the voltage curve that corresponds with the 150thcycle of the embodiment of Figure 8A. In this figure 8B, the 2.3 V plateau is noticeably shorter in the cell without caffeine. This plateau represents the formation of long chain intermediate polysulfides. The shortening of this peak indicates that less sulfur is being converted, or that there is less available sulfur in the cell. This data suggests that caffeine mitigates both of these issues. This is supported by the increased capacity and the larger 2.3 V plateau in cells with caffeine added to the cathode. Additionally, the polarization in the cell without caffeine is much higher than in the cell with caffeine, which indicates a higher resistance to the conversion of short chain intermediate polysulfides to the final product of Li2S.

[0099] Cyclic Voltammetry

[0100] Figure 9 shows CV of coin cells with CNF cathodes with caffeine only and caffeine with sulfur. The darker, straight line, indicating only caffeine, shows no features whatsoever. This indicates that the lithium-ions and commercial ether electrolyte did not produce a redox reaction. The coin cell with caffeine and sulfur exhibited the conventional Li-S plateaus at ~2.3 V and ~2.1 V. The absence of additional peaks indicates that the caffeine had a minimal impact, if any, on the reaction kinetics in the coin cell.

[0101] In situ FTIR was used to investigate the fundamental mechanism by which caffeine was able to interact with polysulfides. A significant shift in carbonyl peak position indicated that the functional groups of caffeine reversibly adsorb and desorb polysulfides as the battery discharges and charges. When comparing CVs of cells with only caffeine and caffeine with sulfur, the caffeine only cell showed no features that would indicate a redox reaction with other battery components. An improvement in galvanostatic cycling capacity was also observed. Compared to the reference coin cell with no caffeine, the cell with the addition of caffeine provided a 15% improvement in capacity at 150 cycles.Examples

[0102] Materials and Methods

[0103] Materials

[0104] Caffeine and poly(vinylidene fluoride) (PVDF) were purchased from Sigma Aldrich. Sulfur (99.5%, sublimed, 100 mesh), lithium (99.9%, 450 pm thick and 15.6 mm diameter), and carbon black Super P Conductive (99+% metals basis) were purchased from Alfa Aesar. N-methyl-2-pyrrolidone (NMP extra dry, 99.5%) was purchased from Thermo Fischer Scientific. Commercial ether electrolyte (Dimethoxy ethane: 1,3 Dioxolane 1:1 by volume with IM lithium bi s(trifluorom ethane) sulfonamide and 1 wt% lithium nitrate (DME:DOL 1 : 1 vol% with IM LiTFSI and 1 wt% LiNCh)) was purchased from Gotion. Trilayer commercial battery separators (Celgard 2325) were purchased from Celgard.

[0105] Cathode fabrication

[0106] Cathodes for Li-S cells were produced using a slurry casting method. Conventional cathodes with no caffeine had a composition of 50% sulfur, 40% carbon, and 10% polyvinylidene fluoride (PVDF). When incorporating caffeine, the sulfur content was kept constant with respect to the conventional cathodes. The mass of caffeine was added with respect to the mass of sulfur in ratios of 10:1, 4:1, and 2:1 (sulfur: caffeine). Hereinafter, cathodes with caffeine in them are referred to by their ratio (for example, a cathode with a 4: 1 ratio of sulfur to caffeine is referred to as “4: 1 cathode” or “4: 1 cell”).

[0107] To create the cathode, carbon black and sulfur were first mixed as dry powders in a planetary mixer (Thinky ARM-310). In examples where caffeine was included, caffeine was also mixed as a dry powder in the planetary mixer prior to further mixing with dry carbon and sulfur powders. Separately, PVDF was dissolved in N-methyl-2-pyrrolidone (NMP). Once dissolved, the PVDF solution was added to the dry powder mixture and mixed in the planetary mixer. NMP was added until the slurry viscosity was appropriate for blade casting on aluminum foil. Once cast, the slurry was dried at 40 °C for 6 hours under vacuum followed by a further 12 hours under vacuum without heat. 11 mm diameter circular cathodes were punched from the dried slurry. The average areal sulfur loading for these cathodes was 1.5 mg-S-cm'2. Cathodes for in-operando FTIR Li-S cells were produced by casting the cathode slurry mixture directly onto a Celgard film and were then dried in the same manner. The sulfur loading of these cathodes was the same.Electrochemical Characterization

[0108] Cathodes were transferred to an argon-filled glovebox (MBraun LABstar, O2 and H2O levels <1 ppm). The Li-S coin cells were fabricated using CR2032 casings, circular lithium anodes, a Celgard 2325 separator, and Gotion commercial ether electrolyte. The electrolyte was added at an E / S ratio of 20 (E = microliters of electrolyte, S = milligrams of sulfur).

[0109] Coin cells for in-operando FTIR were also assembled in the argon-filled glovebox (O2 and H2O levels <1 ppm). They were assembled with the slurry cast on a Celgard cathode, a Celgard 2325 separator, lithium anode, and Gotion commercial ether electrolyte. These cells had an E / S ratio of 30 to help prevent components from drying out in the coin cell while cycling. The assembled coin cells were rested at room temperature with open circuit voltage for 12 hours before initiating electrochemical experiments. FTIR spectra (FTIR-ATR, Nicolet iS50, ThermoFisher) and CV(Biologic VMP3) were collected simultaneously. CV was performed at a scan rate of 0.02 mVs'1between 1.8 V and 2.6 V.

[0110] Galvanostatic cycling tests were performed on a battery cycler (Neware BTS 4000) with a cycling profile of 2 cycles at C / 10, 2 cycles at C / 5, followed by extended cycling at C / 2 (1C = 1,672 mAh g-1suifur) between 1.8 and 2.6 V. Cyclic voltammetry (CV) was performed on a potentiostat (Biologic VMP3) at a scan rate of 0.5 mV s'1between 1.8 and 2.6 V with respect to Li / Li+. Electrochemical impedance spectroscopy (EIS) was conducted on a Biologic potentiostat from 1 MHz to 100 mHz with a 10 mV potential amplitude.

[0111] Polysulfide shuttle current tests were performed by resting cells for 2 hours at open cell voltage (OCV), performing 2 C / 10 cycles, charging them to 2.6 V, allowing them to rest for 10 minutes to stabilize at an OCV, then holding that OCV with chronoamperometry for 4 hours to measure the current response. Electrochemical impedance spectroscopy was conducted on the Biologic potentiostat from 1 MHz to 100 mHz with a 10 mV potential amplitude. Li2S nucleation tests were performed by discharging a Li-S coin cell galvanostatically at a C / 10 current rate from OCV to 2. IV, then using chronoamperometry to hold the cell at 1.95 V for 3 hours and measuring the current response.

[0112] Material Characterization

[0113] The morphology of the cathodes was observed using a scanning electron microscope (SEM, Thermo-Fisher Apreo 2S) with a 10 mm working distance. Elemental mapping was achieved using energy dispersive X-ray spectroscopy (EDS, ChemiSEM Technology, ThermoFisher). The caffeine cathode bond signatures were collected with a Fourier transform infraredspectrometer (FTIR, Nicolet iS50, Thermo-Fisher) using an extended range, attenuated total reflection (ATR) diamond. UV-vis spectroscopy was performed to measure polysulfide solution intensity (Cary 5000). X-ray photoelectron spectroscopy was performed to study the cathode surface chemical environment (XPS, Thermo Scientific Nexsa G2).Results and Discussion

[0114] In-Operando FTIR Analysis

[0115] To fundamentally understand the phenomenon seen in the electrochemical analysis, an in-operando FTIR cell was used to observe spectral changes in the battery as it cycled. Figure 18 shows the physical setup of the in-operando FTIR cell.17The first feature is using an ATR accessory on a stainless steel puck with aluminum contacts. This acts as the positive current collector, replacing the conventional bottom CR2032 coin cell cap. The second feature is a freestanding cathode slurry coated on a Celgard separator. This eliminates the need for a conventional aluminum current collector that would otherwise block the IR signal. To study caffeine as a cathode additive with in-operando FTIR, a model system was created by casting a slurry of sulfur and caffeine on Celgard. Using this platform, the carbonyl peak position of the caffeine-sulfur cell was observed as the battery cycled. While previous works observed polysulfide adsorption to amine groups,18'20the present inventors were unable to deconvolute the characteristic tertiary amine peaks caffeine possesses at 1,025 cm'1and 975 cm'1, from peaks produced by the ether electrolyte (Figure 10A). Therefore, it was decided to focus on the carbonyl groups instead.

[0116] In Figure 10B peaks in the spectra for pure caffeine powder at around 1,655 cm'1and 1,555 cm'1were assigned to the asymmetric and symmetric stretching of the carbonyl groups, respectively.21Additionally, the peak at 1,705 cm'1was assigned to the in-phase stretching of the carbonyl bond.22In the following examples, the focus is on just one of these peaks, the peak at 1655 cm'1, to represent the carbonyl functional group. This peak gets shifted slightly to 1,642 cm'1when caffeine is solvated in ether solvent, but it is still clearly distinguishable. Furthermore, there are no ether solvent peaks in this region to mask the carbonyl peak.

[0117] To fundamentally understand the phenomenon seen in the electrochemical analysis, in-operando FTIR was employed to observe spectral changes in the battery as it cycles. Figure 18 illustrates the physical setup of the in-operando FTIR cell. The cathode for this system was created by casting a 4:1 slurry on a Celgard separator. To identify peaks of interest, baseline caffeine peak positions were established with raw materials. In the spectra for pure caffeine powder (Figure 10A & 10B), the peak at 1,647 cm'1can be assigned to the carbonyl stretching mode primarily localized at position C6 (Figure 1) and the peak at 1,694 cm'1is likewise assigned to position C2.33, 34These peaks are slightly shifted when caffeine is solvated in ether, but remain clearly distinguishable. Amines are another polar group of interest for their ability to interact with polysulfides, as suggested by DFT calculations.35'39In our observed spectra(Figure 17), the characteristic tertiary amine peaks of caffeine at 1,025 cm’1and 975 cm’1are masked by the signal from the ether,40

[0118] Figure 11A illustrates the changes in FTIR spectra of a 4: 1 cathode during the first in-operando CV cycle (the second and third are shown in Figures 16 and 17). In the top half of the figure, spectra lines from top to bottom (corresponding to 3.1 V to 1.9 V) represent the cathode state as the battery progresses through its discharge. In the bottom half, the spectra lines from top to bottom (corresponding to 1.9 V to 2.6 V) represent the cathode state as the battery charges. To the right of the figure, is the vertical representation of the corresponding CV performed on the in-operando cell. The CV curve is color coded, so that each spectrum matches with its corresponding voltage location. As shown, when the battery is discharged from 2.6 V to 1.8 V, the C6 carbonyl peak shifts from 1,655 cm’1to 1,642 cm’1. The C2 carbonyl peak also shifts from 1,707 cm’1to 1,696 cm’1. Figure 11B represents an identical cell prepared without any sulfur added, exhibiting a carbonyl stretching region that is unperturbed irrespective of the applied voltage. Therefore, the assignment of the origin of the spectral shifts to a polar interaction between caffeine and the lithium polysulfides.

[0119] Furthermore, the peak shifting phenomenon correlates strongly with certain voltages during the discharge and charge. Again, in Figure 11 A, both carbonyl peaks initially start to shift at around 2.35 V. This voltage corresponds to the onset of long chain lithium polysulfide formation,41illustrating that the shift occurs only once poly sulfides are present. Upon analyzing the S-S region of the in-operando FTIR data, polysulfide formation was observed that corresponds precisely with this voltage and carbonyl peak shift (Figure 12). Interestingly, the way the two carbonyl peaks shift is slightly different. The C6 carbonyl peak first shifts by 8 cm’1between 2.35 V and 2.1 V. In contrast, the C2 carbonyl peak shifts by 11 cm’1in this voltage window, which is the full magnitude of its shift during this discharge. Continuing the discharge from 2.1 V, long chain polysulfides are further reduced.41From 2.1 V to 1.9 V, the C6 carbonyl peak shifts a further 5 cm’1. Meanwhile, the C2 carbonyl peak remains fixed at 1,696 cm’1. This behavior is demonstrated again in subsequent cycles. For both the C2 and C6 carbonyl peaks, the larger initial peak shift beginning at 2.35 V could be because there are more unoccupied sites on the caffeine molecules. As polysulfide reduction continues, the available sites diminish. The rapid initial wavenumber shift may also be assisted by the absence of long side chains on caffeine. If long side chains existed, they might sterically hinder the polysulfide adsorption.17

[0120] The difference in peak shifting behavior may be due to the difference in the polarity of the two carbonyl groups on caffeine. In a study performed by De Taeye et al.,42they showedthat the carbonyl groups were the preferred polar binding sites on caffeine by calculating formation constants and enthalpies of formation. A computational study also showed the carbonyl group as the preferred interaction site.43Additionally, De Taeye et al. determined that the C2 carbonyl group had a greater ionization potential than the C6 carbonyl, making it the more likely location for polar interactions between the two carbonyls. On the basis of the strong polar interaction potential, it is speculated that the carbonyl groups on caffeine are interacting with the lithium of the polysulfides to bind them via a polar-polar interaction, thereby hindering their mobility. A more rapid shift at the C2 carbonyl site and a more gradual shift at the C6 carbonyl site were observed. This indicates that the C2 carbonyl site interacts more strongly with the lithium polysulfides, confirming the trends from the previous studies.42, 43As the discharge prolongs, the C6 carbonyl site has more time to bind polysulfides which could explain why it exhibits an ultimate wavenumber shift similar to that of the C2 carbonyl.

[0121] As the discharge concludes at voltages of 1.9 V to 1.8 V, the wavenumber of both the C2 and C6 carbonyl peaks returns to near its original value. This behavior suggests that lithium polysulfides are released from caffeine as they are reduced to Li2S. The bottom portion of Figure 11A shows that the carbonyl peak shifting phenomenon also occurs during the charging phase of the CV. As the cell is charged, redox activity beginning around 2.3 V indicates that Li2S is oxidizing back into lithium polysulfides. Corresponding with this voltage, the carbonyl peaks again show a large shift which further supports the idea that caffeine binds lithium polysulfides. As the charging completes and polysulfides become elemental sulfur again, the carbonyl peaks demonstrate a reversal to near their original peak positions. The carbonyl peaks shifting back to near its original value suggests that caffeine molecules are not permanently binding many polysulfides during charging and discharging.

[0122] In this example, 3 consecutive cycles were conducted on the in-operando FTIR cell with corresponding CV data (Figure 11C). The difference in the C6 and C2 carbonyl peak shifting behavior during discharge can be seen. The precise magnitude of the carbonyl peak shifts for each cycle can be found in Table 1. After the first cycle, a hysteresis was observed in the carbonyl peak wavenumbers, and in the third cycle, there appeared to be an unrecovered drop in the C2 carbonyl peak. This is because the middle position of the carbonyl peaks that becomes broadened was being tracked. The broadening may be due to the emergence of multiple caffeine species, causing the peak to span a larger wavenumber range (Figure 11C).

[0123] Prior to the onset of cycling, the liquid electrolyte solvated a majority of the caffeine additive. Therefore, the single carbonyl peak positions were initially observed. However, as cycling commenced, the liquid electrolyte began to deplete due to SEI formation.44, 45SEI reactionsthat persist throughout cycling may lower the amount of caffeine that can remain solvated and thus the emergence of a solid was observed, and likely was a microcrystalline phase of caffeine. Furthermore, it has been found that after the initial discharge, lithium polysulfides remain in the electrolyte, even when the battery is charged.41This likely lead to a third species of caffeine that may be due to an interaction with these remaining polysulfides. Since these three populations of caffeine appear in similar locations in the IR spectrum, it would manifest as a peak broadening effect. Nevertheless, upon close inspection of the spectra, a reversal in peak shifting in each cycle was observed.

[0124] The presence of polysulfides was observed by the appearance of peaks in the S-S IR region (Figure 12). The peak at 510 cm'1is the C-F vibration from the electrolyte salt and is prominent during the OCV resting phase. As elemental sulfur began to reduce, long chain polysulfides appeared and were visible at a peak around 502 cm'1. This peak diminished as the long chain polysulfides were reduced to shorter chain polysulfides and other peaks began to appear at lower wavenumbers (500 cm'1to 475 cm'1). These peaks also correspond to the carbonyl peak shifting phenomenon where the carbonyl peak shift at voltages were observed when the polysulfides began to appear. At the end of the CV, the electrolyte peak began to reappear.

[0125] Table 1. Difference in carbonyl peaks wavenumber as a result of the shifting behavior exhibited during each cycle.

[0126]

[0127] • H to L = High to Low

[0128] • L to H = Low to High

[0129] Table 1 shows the total change in peak wavenumber for each carbonyl peak. The C6 carbonyl peak corresponds to the 1,655 cm'1location and the C2 carbonyl peak corresponds to the 1,700 cm'1location. Each half of a given cycle has two parts, discharge and charge.

[0130] The three in-operando cycles are each delineated into these two distinct parts. Furthermore, each charge and discharge exhibited a shift from a higher wavenumber value to a lower wavenumber value (denoted as “High to Low”) and then a reversal from the lower wavenumber to a higher wavenumber (denoted as “Low to High”). The magnitude of these shifts is indicated by the value listed in the box.Caffeine as an Additive in a Conventional Slurry

[0131] Figure 13B shows CVs of a conventional cathode with caffeine only (carbon and binder are present, but there is no sulfur) and a cathode with both caffeine and sulfur. The CV profile for the caffeine only cathode showed no features in the curve indicating an absence of redox activity. This demonstrates that caffeine itself is not electrochemically active in the voltage window of 2.6 V to 1.8 V and is stable with other lithium-sulfur battery components. Furthermore, caffeine does not contribute to the battery capacity. The CV with caffeine and sulfur exhibits the expected two-peak behavior of the Li-S chemistry and any improvement in the battery performance is therefore due to the caffeine polysulfide anchoring phenomenon, as observed in the in-operando FTIR experiments.

[0132] To study the effect of caffeine in a LSB, caffeine was incorporated as a solid powder additive into the cathode, with the carbonyl groups of caffeine remaining unchanged during the fabrication process, as seen by FTIR and X-ray photoelectron spectroscopy (XPS) measurements. Caffeine was added with respect to the mass of sulfur, keeping sulfur loading constant, in ratios of 10:1, 4:1, and 2:1, sulfur: caffeine. For the cathode process according to the present invention, the 2:1 slurry resulted in a nonuniform cathode. Therefore, this formulation was not considered in the experiments and the 4: 1 ratio was considered the upper limit for the amount of caffeine added to the cathode.

[0133] Figure 13B shows a shuttle current test for a reference cathode without caffeine, a 10: 1 cathode, and a 4:1 cathode. Achieving higher caffeine mass loadings in the cathode with the present fabrication method was not possible due to cathode cracking. In the cell, polysulfides shuttle due to concentration gradients. This test measures the applied current necessary to maintain the open circuit voltage (OCV) in the cell to combat polysulfide shuttling. In the cell, polysulfides shuttle from the cathode to the anode because of a concentration difference. At the anode, they reduce quickly, but some reduced polysulfides can still shuttle back to the cathode. This is again because of concentration differences. The flux of reduced species to the cathode would decrease the voltage if nothing was done. In this test, the OCV was held constant by applying a current at the cathode to re-oxidize the reduced species that shuttled back from the anode.24Therefore, a lower current value indicates a lower concentration of polysulfides shuttling between the cathode and anode.

[0134] The reference cathode showed a current of 7.5 pA at 4 hours. Comparatively, both cells with caffeine in the cathode showed a lower current value over four hours. The 4:1 cathode showed a current of 2.3 pA, a 67% improvement or 3-fold improvement over the referencecathode without caffeine. Ostensibly, this represents a 3-fold reduction in polysulfide shuttling in the cell at OCV. The 10:1 cathode showed a current of 6.0 pA which was a minor improvement relative to the reference, but much less than the 4:1 cathode. The 4:1 ratio of sulfur to caffeine was clearly the most effective in this test. UV-vis measurements showing a decrease in polysulfide peak intensity after exposure to caffeine support these findings.

[0135] Figure 14A shows an SEM image of caffeine crystals incorporated into a conventional carbon and sulfur cathode slurry. Distinct crystal structures can be seen protruding from the cathode surface. It illustrates how the caffeine powder initially used in the cathode slurry formulation exhibited a self-assembling behavior during the cathode fabrication process.

[0136] Figure 14B shows the same region at a lower magnification and reveals that the crystal structures tend to form in clusters on the cathode surface. These clusters are uniformly dispersed across the cathode surface. Figure 14C shows the SEM image from Figure 14A with an EDS elemental map overlaid. The caffeine crystal near the center of the image has oxygen and nitrogen atoms from its carbonyl and tertiary amine groups, respectively. The surrounding material shows sulfur and carbon. Figures 14D - 14G respectively show the individual elemental maps of oxygen, sulfur, nitrogen, and carbon. Postmortem SEM images were also collected. The postmortem image for the cathode with caffeine incorporated shows that the caffeine crystals become buried below the cathode surface (Figure 14H). This may be due to the precipitation buildup of both electrolyte degradation products and Li2S on the cathode surface as cycling continues.25'28Figure 141 shows that no protruding caffeine structures remained on the cathode, as were seen before cycling. This is due to the combination of some caffeine dissolution into the ether electrolyte and the molecules becoming buried after material rearrangement.

[0137] Figure 15A shows a galvanostatic cycling plot comparing lithium sulfur coin cells with various ratios of caffeine in the cathode cycled at a C / 2 rate. The 10:1 cell showed a minimal difference in capacity retention performance compared to the cell with no caffeine and did not have a pronounced effect, supporting the findings from the shuttle current test. This galvanostatic test reflects more factors than shuttle current alone and evidently the small improvements in shuttle current at lower caffeine mass-fraction are insufficient to differentiate it from the control by this measure. By contrast, the 4:1 cell showed a clear improvement. At 300 cycles, the 4:1 cell provided a capacity of 500 mAh g'1, whereas the conventional cell had a capacity of 354 mAh g'1. This represents a 40% improvement in capacity retention at 300 cycles. The increased capacity may be due to higher sulfur utilization, enabled by lithium polysulfide binding via caffeine molecules (Figure 19). Still,due to the severe polysulfide shuttling issue, capacity fade was observed in all cells. It was especially prominent in the earlier cycles and lessened once the lithium anode was more passivated. The caffeine additive mitigated this effect. However, due to volume expansion in the cell and parasitic side reactions, passivation layers may be fractured and capacity fade persisted throughout cycling.

[0138] Figure 15B shows the voltage curve corresponding to the 5thcycle of galvanostatic cycling and Figure 15C from the 250thcycle. These figures both show that the 2.3 V plateau remained larger throughout cycling in the 4: 1 cell. Since the 2.3 V plateau is where polysulfides are first formed in the cell, this indicates that the caffeine is binding long-chain lithium polysulfides and retaining active material at the cathode. This anchoring effect is supported by the shuttle current test. The 4:1 cell showed a slightly greater overpotential at the 2.1 V plateau that grew from the 5thto the 250thcycle. Still, the 4: 1 cell had a longer 2.1 V plateau that reflected the higher sulfur utilization and greater capacity achieved by the 4: 1 caffeine cell. The larger overpotential at the 2.1 V plateau required slightly more energy for polysulfide conversion and deposition at the cathode since they were adsorbed to caffeine molecules. This may be caused by the reduction process occurring in more localized areas near caffeine, due to lithium polysulfides interacting with it. This may in turn create localized areas of insulating species on the cathode that could cause an increase in overpotential. This hypothesis is supposed by IJ2S nucleation measurements and electrochemical impedance spectroscopy (EIS) (Figs. 23, 24). Additionally, as caffeine anchors polysulfides overtime, sulfur redeposition at the cathode may become more concentrated in areas near the caffeine and could make it more difficult for the polysulfide conversion to proceed. Nevertheless, the polysulfide binding phenomenon displayed by caffeine provided an overall benefit to the Li-S cell chemistry, especially at lower C-rates (Fig. 25). Furthermore, the conversion of polysulfides back to sulfur did not appear to be hindered, as shown by the similarly shaped charging voltage profiles of Fig. 15B and 15C.

[0139] Figure 20 shows that a sulfur cathode with caffeine incorporated into it still exhibits the distinct caffeine carbonyl peaks after cathode processing. This demonstrated that the carbonyl groups of caffeine are unchanged by the cathode manufacturing process and that there were no other components with interfering signals.

[0140] Figures 21A - 21F show that the XPS spectra for the cathode with caffeine (Figure 21 A) showed the peaks related to the imidazole group (401.4 eV and 403 eV) and amine groups (399.5 eV) of caffeine in the Nls whereas the cathode without caffeine showed no signal (Figure 22D). In the Ols spectra for the cathode with caffeine, there was a strongsignal assigned to the carbonyl group at 533.6 eV (Figure 21B). In the spectra for the cathode without caffeine (Figure 2 IE) the peak intensity for the carbonyl group was much lower since this signal came from the carbon black and no additional caffeine. Furthermore, the S2p spectra for both the cathode with and without caffeine remained alike (Figure 21C, 21G), showing that there is no effect on the active material.

[0141] Figure 22 shows UV-vis spectra of a 2 mM solution of Li2S4 lithium polysulfide before and after exposure to caffeine powder. Polysulfide solution was made by vigorously mixing stoichiometric amounts of elemental sulfur (Ss) and lithium sulfide (Li2S) in a solution of DME:DOL (1:1 v / v). When exposed to caffeine, the solution became clearer and there was a corresponding reduction in intensity for the polysulfide peak in the UV-vis spectra at around 420 nm.3This supports the fact that caffeine can adsorb polysulfides.

[0142] Figure 23 shows that in the cell without caffeine, the peak current achieved was higher than in the cell with caffeine. The duration of nucleation was longer when caffeine was present. This illustrates that more capacity was achieved in the cell with caffeine, but nucleation was achieved more easily in the cell without caffeine. This supports the idea that Li2S may deposit in localized areas in the cathode when caffeine is present, creating slightly more resistance to deposition.

[0143] The EIS curves for both cells with and without caffeine had a similar resistance before cycling. (Figure 23) After cycling, the resistance in both cells decreased due to the rearrangement of sulfur. In the cell with caffeine, there was a slightly larger charge transfer resistance. This is to be expected, as the caffeine molecules are binding polysulfides. Still, the overall benefit of increased capacity in the cell is advantageous.

[0144] 5 cycles each at C-rates of C / 10, C / 5, C / 2, 1C, and 2C were performed. The cell with caffeine performed better than the cell without caffeine at the lower C-rates (C / 10, C / 5, C / 2). However, the cell with caffeine did not perform as well at 1C and 2C. The cell without caffeine was able to cycle at 1C, although both cells were unable to perform well at 2C. Both cells recovered after returning to the C / 2 rate. The faster C-rates require faster deposition and reduction processes which were more limited in the cell with caffeine. These results support the results corresponding to the voltage curves and the increased overpotential at 2.1 V (Figure 15).

[0145] Fitted Gaussian curves show that 3 distinct species of caffeine emerged by the third cycle. (Figure 26A - 26C) At 1707 cm’1the solvated caffeine species existed. This was the dominant species before cycling began. At 1,697 cm’1the solid microcrystalline phase of caffeine existed. This phase became apparent after a few cycles. At 1,684 cm’1the caffeinespecies that interacted with polysulfides existed. This appeared distinctly during the discharge of the first cycle and then persisted even in the charged state, since poly sulfide species remained after their formation.

[0146] The present invention utilized in-operando FTIR to investigate the fundamental mechanism by which caffeine interacts with polysulfides. A significant shift in the caffeine carbonyl peak position indicates that these functional groups can reversibly bind lithium polysulfides as the battery discharges and charges. This shift correlated strongly with the 2.3 V discharge plateau of the in-operando CV, suggesting that caffeine readily binds to these species as they are formed in the battery. After analyzing the different carbonyl peak shifting behaviors, it was determined that the C2 carbonyl was the preferred binding site of the two carbonyl groups of caffeine. Additional CV tests of cells with caffeine also showed no features that would indicate a redox reaction between caffeine and other Li-S battery components. Furthermore, a shuttle current test was used with caffeine to qualitatively investigate the extent of its polysulfide shuttling suppression capabilities. Finally, Li-S coin cells were built with caffeine as a cathode additive and demonstrated a 40% improvement in galvanostatic cycling capacity at 300 cycles.

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Claims

WHAT IS CLAIMED IS:

1. A method for making a sulfur cathode, comprising steps of:d) mixing sulfur and an amount of a purine compound in a weight ratio of sulfur to purine of from less than 10:1 to greater than 2: 1 to form a powder mixture; e) mixing the powder mixture with a solvent to form a slurry; andf) drying the slurry onto a current collector to form the sulfur cathode.

2. A method for making a sulfur cathode, comprising steps of:c) mixing sulfur and an amount of a purine compound in a weight ratio of sulfur to purine of from less than 10:1 to greater than 2: 1 to form a powder mixture; d) melting the powder mixture into a porous material to form the sulfur cathode.

3. The method of any one of claims 1 - 2, wherein the weight ratio of sulfur to purine is from 8:lto 3:1 or 6:1 to 3.5:1 or about 4:1.

4. The method of claim 2 wherein the porous material is selected from the group consisting of carbon nanofibers; metal-organic frameworks; mesoporous silica; porous polymers, such as titanium oxide, iron oxide, aluminum oxide; porous ceramic materials, such as alumina and zirconia; graphene; and carbon nanotubes.

5. The method of any one of claims 1 and 3, further comprising mixing carbon black in step a) to form the powder mixture.

6. The method of any one of claims 1, 3, and 5, further comprising dissolving a polymer binder in the solvent.

7. The method of any one of claims 1 - 6, wherein the purine compound is caffeine.

8. The method of any one of claims 1, 3, 6, and 7, wherein the solvent is present in an amount such that the slurry has a viscosity range of from about 1 to 100 Pa'S, as determined by a rotational viscometer.

9. The method of any one of claims 1, 3, and 5 - 8, wherein the drying step is carried out at a temperature of from about 20°C to about 60°C, or at about 40°C.

10. The method of any one of claims 1 - 9, wherein the purine is selected from the group consisting of caffeine, theobromine, theophylline, guanine, hypoxanthine, xanthine, theophylline, theobromine, uric acid, isoguanine, preferably, the purine compound is caffeine.

11. The method of any one of claims 1, 3, 5 - 8, and 10, wherein the polymer binder is selected from the group consisting of poly(vinylidene fluoride-co- hexafluoropropylene), poly(vinylidene fluoride), poly(acrylonitrile-co-methyl acrylate) (PANMA), polyethylene oxide (PEO), polyacrylonitrile, polytetrafluoroethylene, styrene-butadiene rubber, polyacrylic acid, chitosan, sodium alginate, sodium carboxyl methyl cellulose (CMC), polyvinyl alcohol (PVA), polyethyleneimine (PEI), and combinations thereof.

12. The method of any one of claims 1, 3, 5 - 8, and 10 - 11,, wherein the solvent is selected from the group consisting of N-methyl-2-pyrrolidone, dimethoxyethane (DME), 1,3-dioxolane, propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and combinations thereof.

13. A cathode made by the method of any one of claims 1 - 12.

14. A cathode for use in a lithium sulfur battery, comprising:sulfur; andan purine selected from the group consisting of caffeine, nicotine, morphine, codeine, quinine, atropine, ephedrine, strychnine, colchicine, theobromine, theophylline, papaverine, reserpine, coniine, harmine, ibogaine, vinblastine, vincristine, yohimbine, capsaicin, camptothecin, mitragynine, apomorphine, psilocybin, scopolamine, tetrahydrocannabinol (THC), salvinorin A, mescaline, muscarine, preferably, the purine is caffeine;wherein the cathode has a weight ratio of sulfur to purine of from less than 10:1 to more than 2:1.

15. A lithium sulfur battery comprising the cathode of any one of claims -13 - 14.

16. An electrolyte for use in a battery, comprising:a purine compound selected from the group consisting of caffeine, theobromine, theophylline, guanine, hypoxanthine, xanthine, theophylline, theobromine, uric acid, isoguanine, preferably, the purine compound is caffeine; anda carbonate solvent selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and combinations thereof.

17. The electrolyte of claim 16, further comprising a conductive salt, wherein the conductive salt is optionally selected from the group consisting of lithium hexafluorophosphate (LiPF6), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and combinations thereof.

18. The electrolyte of any one of claims 16 - 17, wherein the purine is caffeine.

19. The electrolyte of any one of claims 16 - 18, further comprising a polymer binder, wherein the binder is optionally selected from the group consisting of poly(vinylidene fluoride-co-hexafluoropropylene), poly(vinylidene fluoride), poly(acrylonitrile-co- methyl acrylate) (PANMA), polyethylene oxide (PEO), sodium carboxyl methyl cellulose (CMC), polyvinyl alcohol (PVA), polyethyleneimine (PEI), and combinations thereof.

20. A lithium sulfur battery comprising the liquid electrolyte of any one of claims 16 - 19.

21. The battery of claim 20, wherein the battery has a ratio of microliters of electrolyte to milligrams of sulfur of from about 10 to about 40, or from about 20 to 30.

22. The battery of any one of claims 20 - 21, wherein the battery has a ratio of microliters of electrolyte to milligrams of sulfur of from about 10 to about 40, or from about 20 to

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