WO2018183556A1

WO2018183556A1 - Lithium-sulfur electrode and method

Info

Publication number: WO2018183556A1
Application number: PCT/US2018/024921
Authority: WO
Inventors: Juchen GUO; Haiping Su
Original assignee: The Regents Of The University Of California
Priority date: 2017-03-28
Filing date: 2018-03-28
Publication date: 2018-10-04

Abstract

A carbon-sulfur electrode material and methods are shown. In one example, the carbon-sulfur electrode material is used as an electrode in a battery, such as a lithium ion battery.

Description

LITHIUM-SULFUR ELECTRODE AND METHOD

Related Applications

[001] This application claims priority to United States Provisional Patent

Application Number 62/477,731, entitled "CRITICAL DESIGN FACTORS OF HIGH PERFORMANCE LITHIUM-SULFUR CATHODES," filed on March 28, 2017, which is incorporated herein by reference in its entirety.

Technical Field

[002] This invention relates to electrode materials, microstructures and

methods. In one example, this invention relates to sulfur and carbon based electrodes for lithium ion batteries.

[003]

Background

[004] Improved batteries, such as lithium ion batteries are desired. One

example of a battery structure that can be improved is a cathode structure.

Brief Description of the Drawings

[005] FIGS. 1A-1D show (a) SEM images of GO@C; (b) N2 adsorption-desorption isotherms and (c) pore size distribution (PSD) of sample GO@C, GO@C2H, GO@C3H, GO@C4H, GO@C5H, and GO@C6H; (d) TGA curves of S- GO@C2H, S-GO@C3H, S-GO@C4H, S-GO@C5H, and S- GO@C6H.according to an example of the invention.

[006] FIGS. 2A-2B show (a) Cycle performance and coulombic efficiency (b) of S- GO@C2H, S-GO@C3H, S-GO@C4H, S-GO@C5H, and S-GO@C6H according to an example of the invention.

[007] FIGS . 3 A-3 C show (a) Structural formula of three different binders PVP, D 11 , and PDADMAC; (b) Cycle performance and coulombic efficiency (c) of sample S-GO@C5H with three different binders according to an example of the invention.

[008] FIGS. 4A-4B show (a) UV-vis spectra and (b) photograph of the polysulfide solution before and after exposure to S-GO@C5H with different binders according to an example of the invention. [009] FIGS. 5A-5B show (a) In situ CV curves (at 0.5 mV/s) and the mass changes during the oxide process measured by EQCM. (b) In situ galvanostatic discharge curves (at 0.5 C) and the mass changes measured by EQCM according to an example of the invention.

[0010] FIGS. 6A-6D show XPS spectra of S 2p in lithiated (a) and non-lithiated (b) S- GO@C5H with PVP binder; and XPS spectra of S 2p in lithiated (c) and non- lithiated (d) S-GO@C5H with PDADMAC binder according to an example of the invention.

[0011] FIGS. 7A-7B shows (a) Effect of E/S weight ratio on the cycling performance and (b) coulombic efficiency of S-GO@C5H according to an example of the invention.

[0012] FIG. 8 shows cycle stability of thick sulfur-carbon electrode with 3.5 mg of sulfur per cm² at 0.1 C charge-discharge rate according to an example of the invention.

[0013] FIG. 9 shows surface area, porevolume, theoretical and actural sulfur loading data of the GO@C samples according to an example of the invention.

[0014] FIGS. 10A-10J show TEM images of sample GO@C2H, GO@C3H, GO@C4H,

GO@C5H, and GO@C6H before and after sulfur loading according to an example of the invention.

[0015] FIGS. 1 lA-1 IE show 1st and 50^th charge/discharge curve of S-GO@C2H, S-

GO@C3H, S-GO@C4H, S-GO@C5H, and S-GO@C6H according to an example of the invention.

[0016] FIG. 12 shows molecular weight and ion concentration data of different binders according to an example of the invention.

[0017] FIGS. 13A-13D show 1st and 50^th charge/discharge curve of S-GO@C5H with different binders (a) PVP, (b) Dl 1 and (c) PDADMAC; (d) CV curves of S-

GO@C5H with different binders at 0.1 mV/s according to an example of the invention.

[0018] FIG. 14 shows a battery according to an example of the invention.

[0019] FIG. 15 shows a method of forming a material according to an example of the invention. Detailed Description

[0020] In the following detailed description, reference is made to the

accompanying drawings which form a part hereof, and in which is shown, by way of illustration, specific embodiments in which the invention may be practiced. In the drawings, like numerals describe substantially similar components throughout the several views. These embodiments are described in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments may be utilized and structural, or logical changes, etc. may be made without departing from the scope of the present invention.

[0021] Lithium-sulfur (Li-S) battery is one of the most attractive electrochemical energy storage technologies due to its high theoretical specific energy of 2400 Wh kg. Despite great efforts have been made to solve various problems of sulfur cathode, Li-S batteries are still suffering from poor cyclability due to polysulfide shuttling effect and low practical capacity. Herein, we proposed a multifaceted approach to improve the cycle stability and areal loading of the sulfur cathode through carbon host microstructure refinement, introducing new positively charged binders to trap polysulfides through strong electrostatic interaction, and optimizing the E/S weight ratio to constrain the dissolution of polysulfides. The sulfur cathode produced from this approach demonstrated systematic improvement. Moreover, thick electrode with high sulfur loading also exhibits good areal capacity with stable cycling performance.

[0022] In the past three decades, rechargeable lithium-sulfur (Li-S) battery

technology has been intensively investigated for its high specific energy, which is projected to be two to three times higher than that of the state-of-the-art Li-ion batteries. However, the commercialization of Li-S batteries is still hindered by several key challenges from both Li anode and sulfur cathode. Metallic Li is known to have inferior deposition-stripping efficiency in liquid electrolytes, therefore significant excess of Li is required for prolonged cycling thus reducing the overall cell specific energy significantly. Li dendrite is another challenge for safety concerns although in situ passivation by reacting to polysulfides could alleviate the Li dendrite formation in Li-S batteries. On the other hand, sulfur cathode suffers from low Li-S reaction round trip efficiency and fast capacity loss due to the dissolution of intermediate polysulfides generated during charge- discharge process (known as polysulfide shuttle phenomenon). Furthermore, to achieve high specific energy at the full cell level it is essential to both improve cathode areal capacity (high sulfur loading and high specific capacity) and lower the weight ratio of the electrolyte in the full cell.

[0023] Innovative mechanisms and strategies of Li-S batteries have been

proposed in recent years including elimination of polysulfides via sulfur sub- nano confinement and utilization of solid-state ceramic or glass electrolytes. Nevertheless, at current developmental stage the most promising Li-S cell configuration in terms of overall capacity, cost and manufacturability is still the conventional one composed of Li metal anode and sulfur-carbon composite cathode in electrolytes mainly consisting of ethereal solvents. In this disclosure, we focus on the cathode by elucidating a multifaceted approach to design a sulfur-carbon composite cathode with properties approaching industrial relevant standards by considering aspects of carbon host structure design, selection of binders, electrolyte/sulfur (E/S) mass ratio, polysulfide absorbent additive and high areal sulfur loading.

[0024] To address those aforementioned problems, various methods are shown, including the design of novel cathode sulfur host, new electrolytes, binders, separator modification, and lithium metal anodes protection. Among them, there are two basic principles to suppress the dissolution and migration of long -chain polysulfides: The first one is physically confinement of polysulfide dissolution using barrier materials. And the most popular way to encapsulate sulfur is the use of hybrid carbon structures, such as carbon nanotubes, carbon fibers, graphene, and various porous carbon, which give rise to advanced electrodes with improved electrical conductivity and cycle performance. Typically a mesoporous carbon framework can provide essential electrical contact to the insulating sulfur and precisely constrain sulfur within its channels. Also, confined metastable small sulfur molecules in the conductive microporous carbon matrix can totally avoid the unfavorable transition of polysulfides. However, further investigations also shows that larger mesoporous (2-50 nm) carbon cannot fully eliminate the loss of active materials, meanwhile, those sulfur host with ultra-small pore size usually have low pore volume and low sulfur loading, which may significantly affect the gross capacity of the electrode and the energy density of the whole cell. The second methodology is to trap the soluble species through chemical interactions with the cathode surface by heteroatom doping or surface coating (on electrode or separator). It is demonstrated that this hetero-atom or coating layers with electron-donating groups can promote chemical bonding between carbon scaffold and sulfur chains during the sulfur loading process, and further can enhance the adsorption of polysulfides intermediates. For example, decorated graphene sheets with rich nitrogen-containing active sites, may be formed which induce strong polysulfide adsorption ability, and a high capacity of ~ 1000 mAh/g for over 100 cycles. The electrochemical behavior between lithium polysulfides and polymer-electrolyte coating layers shows the lithium bonds between lithium polysulfides and the electron-donating groups. Also, sulfur cathodes based on metal oxide (e.g. TiC , MnC ) nanostructures can suppress the shuttle-effect and enable controlled lithium sulfide deposition. Therefore, carbon-sulfur composites are promising cathodes for Li-S batteries.

[0025] To fully realize the industrially relative cathode, electrodes with high sulfur loading and high capacity are desired. However, because of the addition of conductive carbon and binders, some materials have low sulfur content in the composite or on the electrode, resulting in low energy density and areal capacity. Moreover, these approaches by adding metal oxides/sulfides or carbon interlayers onto sulfur cathodes also scarify the overall capacity to some extent due to the added mass. Hence, it is beneficial to balance these competing parameters in the design of high-performance and well-optimized cathode materials. Recently, binders have been demonstrated very important to retain the stable void structure for long charge/discharge cycling. It appears that many water soluble (e. g. PEO, PVP, SBR) or insoluble (e. g. PVDF) binders have been widely used, due to their low cost, resistance and high physical and electrochemical stability. However, they would be more appealing if they can suppress the solubility of lithium polysulfides, while few people have paid attention to those functionalized binders.

[0026] Herein, we present the synthesis of porous carbon sheet with controllable high surface area and volume by using graphene oxide as a shape -directing template for sulfur cathode. Aiming to stabilize the polysulfides in the Li-S cathode, we systematically discussed the balance of pore size and volume, introduced a new cationic polymer (PDADMAC) as the binder, which can trap polysulfide intermediates via electrostatic interaction, and compared the influence of electrolyte/sulfur (E/S) ratio. Overall, the proper pore size, positively charged binder and lower E/S ratio result in improved specific capacity by ~ 25%.

EXPERIMENTAL SECTION

[0027] Preparation of sulfur-graphene oxide activated carbon (S-GO@C) active materials: The porous carbon sheets GO@C was synthesized using a templating approach through polymerization of melamine-resorcinol- formaldehyde (MRF) resin onto few-layer GO sheets, followed by carbonization and activation to tune the surface area and pore size. In a typical synthesis, 2.86 g resorcinol and 4.22 g formaldehyde (from 37 wt. % aqueous solution) were co- dissolved in 30 mL deionized water (solution A), 3.28 g melamine and 6.32 g formaldehyde were co-dissolved in 30 mL deionized water (solution B) at 80 °C. Then certain amount of the aqueous dispersion of GO (4 mg mL^"1) was added into the mixed solution of A and B under ultrasonication treatment for 10 min. The mixture was then stirred at 80 °C for 24 h. The schematic MRF

polymerization reaction is shown in the Supporting Information. The obtained MRF-coated GO sheets were filtered and dried at 70 °C overnight. The carbonization was carried out at 1000 °C in flowing Ar for 3 h with a heating ramp of 3 °C min^"1 to obtain the GO@C sheets.

[0028] The CO2 activation was performed by placing 0.1 g of GO@C composite in the tube furnace under flowing argon and being heated to 900 °C with a heating rate of 5 °C min^"1. After the targeted temperature was reached, the activating gas CO2 was introduced for certain hours. At the end of activation, CO2 flow was shut off and the activated GO@C was allowed to cool down to room temperature in Ar environment. To prepare the S-GO@C composites, sulfur and GO@C were thoroughly mixed with certain weight ratios, and the mixtures were heated at 155 °C for 10 h in Ar environment.

[0029] Polysulfide Adsorption Experiment: 2 mM L12S8 solution was prepared by reaction of designated amount of sulfur to L12S in anhydrous EC/DEC (1 : 1, v/v) at room temperature (according to reaction 7S + L12S— » L12S8. The mixture was stirred overnight in the glovebox for complete conversion. GO@C carbon host activated for 5 h (GO@C5H) is mixed with binder PVP and PDADMAC, respectively, in a mass ratio of 2.5: 1 followed drying to form the adsorbents. 20 mg of the obtained adsorbents were added to 5 mL of 2 mM L12S8 solution for 2 min for adsorption immediately followed by filtration for photographing and UV-Vis spectroscopy.

[0030] Cathode Mass Change Measurement by EQCM: EQCM

measurements of operando mass change of sulfur-carbon cathodes were undertaken using Gamry eQCM 10M™ Quartz Crystal Microbalance and 10 MHz Au-coated Quartz Crystal with an electrode area of 0.205 cm². The mass change of the electrode during the discharge can be calculated by using

Sauerbrey equation:

[0031] Where Afis the measured resonant frequency (Hz),/is the intrinsic

crystal frequency, Am is the mass change, p_q is the density of quartz, μ is the shear modulus and A is the electrode area. In this system, the mass sensitivity factor ( ) is 226 Hz cm²/ g, which is supported by Gamry instrument.

[0032] Electrochemical Test. The slurry was prepared by mixing 80 wt% active material, 10 wt% carbon black (super C65) and 10 wt% binder dissolved in deionized water. Then it was coated onto a carbon-coated aluminum foil with a doctor blade and dried at 50 °C overnight. The electrolyte was 1M

bis(trifluoromethane) sulfonamide lithium salt (LiTFSI) dissolved in a mixture of PYR14TFSI, 1, 2-dioxolane (DOL) and dimethoxymethane (DME) (2: 1 : 1 by volume) with 1.5 wt% of L1NO3. For high sulfur loading electrode, the slurry was doctor-blade coated onto graphite paper (Spectracarb 2050A-0550). A Celgard membrane 2400 was selected as the separator. The charge-discharge performances were performed on an Arbin battery test station at 0.1C, and the CV analysis was conducted on a Gamry Interface 1000 with a scan rate of 0.1 mV s-¹.

[0033] Materials Characterization. The morphologies of the nanocomposites were observed under scanning electron microscopy (FEI NNS450). The microstructures were observed under transmission electron microscopy (FEI Tecnai 12) operated at 120 kV. Nitrogen adsorption/desorption was performed using an ASAP 2020 instrument at 77K. The specific surface area and the pore size distribution were calculated using the Brunauer-Emmett-Teller (BET) and Non-local density functional theory (NL-DFT) methods, respectively. Thermal gravimetric analysis (TGA) was conducted on a TG instrument (Q500) under nitrogen protection at a heating rate of 10 °C/min from room temperature to 600 °C. The XPS spectra of lithiated porous carbon with different binders were collected on AXIS Supra using monochromatic Al K radiation (280 W). UV-Vis spectroscopy analysis was done with UV-visible spectrophotometer (Horiba Aqualog).

RESULTS AND DISCUSSION

[0034] The porous carbon sheets (GO@C) with sandwich structure were

prepared by in situ chemical coating of resorcinol-formaldehyde-melamine (RFM) resin onto Graphene Oxide (GO) sheets. It was reported that RFM resin was easy to form microspheres, however, in the presence of GO, sandwich structure was obtained and well replicated the shape of GO. Key to the success of this synthesis approach lie in the surface chemistry of GO, which has a number of hydroxyl groups, is easy to adsorb those hydrophilic groups of RFM oligomer, and promote uniform polymerization on both sides. Thus, after carbonization, the GO@C sandwich composites with controlled surface coating could be obtained (Fig. la).

[0035] As shown in Fig lb and lc, although direct carbonization of this

sandwich composites produced GO@C with rich micropores smaller than 1 nm, higher surface area and larger pore volume are generally necessary for carbon materials in many applications. Hence, we further introduce CO2 activation as a mild process to activate the carbonized materials. In this work, the GO@C composites were activated by CO2 at 900 °C for several hours (2-6 h), and the surface area and pore volume are significantly increased (as shown in Fig. 9), surface area increased from 500 to 2775 m²/g, pore volume increased from 0.29 to 1.53 cm³/g). The N2 adsorption-de sorption isotherms and pore size distribution of the activated GO@C composites are shown in Fig lb and lc. All five samples show an initial rapid increase in low relative pressure (<0.05), suggesting the presence of abundant micropores. The pore size distribution calculated by density functional theory (DFT) show that all the activated GO@C samples have relatively micropores <1 nm, while, on the same time, certain portion of mesopore around 2-4 nm also increased along with the activation time.

[0036] For those carbon materials serve as sulfur host, they need to meet the requirements of high sulfur loading and effectively polysulfides restrain simultaneously. All the activated porous carbon was mixed with sulfur through heat melt-diffusion method at 155 °C. The filling of carbon pores with sulfur is verified by the transmission electron microscopy (TEM) images (Fig. lOa-j). No obvious bulk sulfur was seen from the comparison of the images before and after sulfur loading. Comparing the TGA curves of the S-GO@C composites in Fig. Id, the sulfur contents in GO@C6H has the highest sulfur content of 76.5%, while GO@C2H has the lowest sulfur content of 60%.

[0037] To reveal the electrochemical performance of the GO@C/S composites with different porosity, the batteries were carried with PVP binder using coin cells, and the capacity were calculated based on sulfur/carbon composites. As can be seen from the cycling plots in Fig. 2, S-GO@C6H with highest surface area and pore volume gave the highest initial discharge capacity, while S- GO@C2H with lowest surface area and pore volume show the lowest capacities. However, capacity after 50 cycles showed that, S-GO@C5H has the highest discharge capacity. Comparing the detailed discharge/discharge curves of these samples (Fig. 1 la-e), they all show two distinct discharge plateaus around 2.35 V and 2.1 V, which can be attributed to the formation of long-chain polysulfides (L12SX, 4< x <8) and further reduction products

respectively. As a result, considering the high sulfur loading and pore structure, sample S- GO@C5H show the best over-all capacity and cycle stability.

[0038] The performance and cycle life of Li-S batteries are often limited by the dissolution of polysulfides. Despite the confinement of porous structure, binders also play a very important role in improving the cycling life, which is supposed to retain stable void structure formed during the charge/discharge process, as well as to be chemically stable against the reduction intermediates/products. It appears that many elastomeric materials have been widely used as electrode binders, such as PVP, PVDF, PEO, SBR, and are demonstrated favorable to improve battery performance to some extent. Here, we introduce a positively charged binder for the sulfur cathode of Li-S battery, and compared the battery performance of these three binders: (1) Polyvinylpyrrolidone (PVP), which can stabilize the discharge products and improve the cycling performance; (2) Polyquaternium (D l 1), a water soluble polyelectrolyte with moderate positive ion concentration (Fig. 12); (3) Poly(diallyldimethyl ammonium chloride) (PDADMAC), a water soluble polyelectrolyte with highest positive ion concentration (almost 10 times higher than D l 1). To avoid the pitting corrosion of Al current collector, an ion exchange reaction was performed by silver triflate to remove the chlorine ion (CI^"). As shown in Fig. 3b and 3c, the electrochemical performance of S-GOO@C5H with different binders are quite different. It is shown that batteries with PVP and D 11 binders have similar initial discharge capacity around 900 mAh/g, while the cycle stability of D 11 is much better, which may due to the electrostatic interaction between positively charged binder and negatively charge polysulfides. Notably, ascribed to the much higher positive ion concentration, battery with PDADMAC binder exhibit highest capacity than PVP and D 11.

[0039] The adsorption ability experiment was performed to evaluate the

interaction on lithium polysulfides with two different binders PVP and

PDADMAC. Typically, pure L12S8 in DOL/DME solution with the concentration 2 mM was investigated using ultraviolet-visible (UV-vis) spectroscopy before and after exposure to S-GO@C5H composites with different binders. As shown in Fig. 4a, the main difference happened in the region around 350 cm^"1 and 430 cm^"1. The fresh L12S8 solution with dark yellow color shows strongest absorbance, and upon exposed to both S-GO@C5H with different binders, they show much lower absorbance intensity with PVP binder, and finally disappeared with PDADMAC binder, indicating strong adsorption between polysulfides and positively charged PDADMAC. The positively charged PDADMAC can thus effectively trap soluble polysulfides during cycling, and mitigating loss of active materials.

[0040] To further confirm the strong adsorption between polysulfides and

positively charged binder, in situ EQCM experiments were used to investigate the mass changes on the cathode for a Li-S battery during the charge-discharge process. The use of EQCM to study ion adsorption in porous carbons was pioneered by Levi and Aurbach, which can measure the mass changes by monitoring the resonant frequencies of an oscillating quartz crystal. The change of frequency of the quartz crystal is proportional to the weight change on the crystal via the Sauerbrey equation. Fig. 5a shows the CV scan of the S- GO@C5H electrodes with PVP and PDADMAC as binders (top), respectively, and the simultaneous mass changes during the cathodic scan (buttom). Electrode with PDADMAC binder show mass increase for the first cathodic peak around 2.3 V, which represents the formation of high order polysulfides. Following a slightly mass loss, the mass continuously increased due to the formation of solid products L12S/L12S2. On the other hand, the electrode with PVP binder shows continuous mass lose, which means more polysulfides dissolved into the electrolyte. In addition to CV, we also performed chronopotentiometry on the S- GO@C5H electrodes with PVP and PDADMAC as binders. Fig. 5b shows their galvanostatic discharge curves and the simultaneous mass changes during the discharge process, the significantly more mass increase with the PDADMAC binder confirmed that positively charged binder does help the attraction of polysulfides anion.

[0041] We further analyzed the composition of the final products from the

lithiation of S-GO@C5H with binder PVP and PDSDMAC, and the XPS analyses of S 2p spectra on these lithiated and un-lithiated electrodes are shown in Fig. 6a-d. To avoid contamination, the lithiated samples were prepared in an argon-filled glovebox, and the XPS samples were inserted into the vacuum chamber via a load lock within a glovebox. As shown in Fig. 6a and 6c, samples with PVP binder only show pure sulfur peak at 164.0 eV (S 2pm) and 165.2 eV (S 2p3 2) before lithiation, while the peak at 170.0 eV appeared on sample with PDADMAC binder can be attributed to the sulfur oxide species from

trifluoromethanesulfonate anion species^62"64. After the fully discharge process, both of the two samples show new peak at 162.0 eV, which can be assigned as lithium sulfide (L12S)⁶⁵. No other polysulfide species were observed since they can spontaneously disproportionate to sulfur and L12S upon drying^66"67. Despite these different sulfur species, the percentage of pure sulfur and L12S in the fully lithiated samples with different binders are distinctly different as indicated by the peak area. Only 37.7% of the lithiated sulfur in S-GO@C5H with PDADMAC binder still remains as elemental sulfur, and the content of L12S is 62.3%. On the contrary, 51.4% of the lithiated sulfur in S-GO@C5H with PVP binder still remains as elemental sulfur, and the content of L12S is 48.6%. The much higher L12S content in the cathode with PDADMAC binder clearly indicates superior sulfur utilization, which is also consistent with the stronger adsorption and higher sulfur-based capacity of S-GO@C5H with PDADMAC binder.

[0042] In addition to the aforementioned methods of micropore confinement and chemistry adsorption, the electrolyte/sulfur (E/S) ratio relates a lot to the dissolution of polysulfides, which may cause redox shuttle reactions between the two electrodes⁶⁸. In this work, we studied the optimized E/S ratio by measuring the cycling stability of Li-S batteries. As shown in Fig. 7a-b, batteries with excess amount of electrolyte (E/S = 78 g/gsuifur) show slightly higher initial capacity, however, the capacity decreases quickly with cycle number, which means more electrolyte may promote the dissolution of polysulfides. When lower the E/S ratio to 60/1 and 24/1, batteries show better specific capacity and capacity retention. By further decreasing the E/S ratio to 12/1, capacity after 50 cycles show great improvement around 25%. This result confirmed the fact that higher solubility drives the shuttle phenomenon, leading to lower discharge capacity.

[0043] In order to achieve batteries with specific capacity, electrodes with high sulfur areal loading is critical. Here, thick electrodes were demonstrated on porous carbon paper current collector with E/S ratio of 12. As shown in Fig. 8, the prepared thick electrode with 3.5 mg of sulfur per cm² can deliver > 3 mAh cm^"2 for over 40 cycles.

CONCLUSION

[0044] Targeting high-energy-density Li-S batteries, comprehensive approaches have been demonstrated to improve the sequestration of polysulfides. (1) Porous carbon GO@C5H with small pore size distribution, as well as large pore volume are well produced to meet the requirements of high sulfur loading and physical confinement on the same time. (2) A new water soluble binder PDADMAC successfully decorated carbon backbone with plenty of positive charge, which showed strong adsorption to negatively charge polysulfide anions, UV-vis adsorption, EQCM and XPS characterization all favored this statistic interaction. (3) E/S weight ratio are well optimized to suppress the solubility of polysulfides, smaller E/S weight ratio can improve the 50th cycle capacity by 25%. (4) Thick sulfur-carbon electrodes demonstrated very high areal capacity with minimized E/S ratio.

[0045] Figure 14 shows an example of a battery 1400 according to an

embodiment of the invention. The battery 1400 is shown including an anode 1410 and a cathode 1412. An electrolyte 1414 is shown between the anode 1410 and the cathode 1412. In one example, the battery 1400 is a lithium-ion battery. In one example, the anode 1410 is formed from a carbon-sulfur based electrode as described in examples above. In one example, although the invention is not so limited, the battery 1400 is formed to comply with a 2032 coin type form factor.

[0046] Figure 15 shows an example method of forming an electrode according to an embodiment of the invention. In operation 1502 a resin is polymerized onto graphene oxide sheets to form resin coated graphene oxide sheets. In operation 1504, the resin coated graphene oxide sheets are carbonized to form a carbonized structure. In operation 1506, the carbonized structure is activated to form a porous carbon matrix. In operation 1508, sulfur and a positively charged polymeric binder are mixed with the porous carbon matrix.

[0047] To better illustrate the method and device disclosed herein, a non-limiting list of embodiments is provided here:

[0048] Example 1 includes a battery. The battery includes a first electrode, a first electrode, including a porous carbon matrix having a number of pores, sulfur within the number of pores, and a positively charged polymeric binder mixed with the porous carbon matrix and the sulfur. The battery includes a second electrode and an electrolyte in contact with both the first electrode and the second electrode.

[0049] Example 2 includes the battery of example 1, wherein the porous carbon matrix includes graphene oxide sheets coated with resorcinal-formaldehyde- melamine (RFM) resin wherein the coated graphene oxide sheets have been carbonized.

[0050] Example 3 includes the battery of any one of examples 1-2, wherein the porous carbon matrix includes a surface area greater than 2000 M²/g.

[0051] Example 4 includes the battery of any one of examples 1-3, wherein the porous carbon matrix includes a surface area between 2200 and 2600 M²/g.

[0052] Example 5 includes the battery of any one of examples 1-4, wherein the porous carbon matrix includes a first distribution of pores at a size less than one nm, and a second distribution of pores at a size between 2 and 4 nm.

[0053] Example 6 includes the battery of any one of examples 1-5, wherein the a positively charged polymeric binder includes poly (diallyldimethyl ammonium chloride) (PDADMAC).

[0054] Example 7 includes the battery of any one of examples 1-6, wherein the a positively charged polymeric binder includes polyquaternium. [0055] Example 8 includes the battery of any one of examples 1-7, wherein the second electrode includes lithium metal.

[0056] Example 9 includes the battery of any one of examples 1-8, wherein a ratio of electrolyte to sulfur (E/S) is 12 to 1.

[0057] Example 10 includes a method of forming a battery electrode. The

method includes polymerizing a resin onto graphene oxide sheets to form resin coated graphene oxide sheets, carbonizing the resin coated graphene oxide sheets to form a carbonized structure, activating the carbonized structure to form a porous carbon matrix, and mixing sulfur and a positively charged polymeric binder with the porous carbon matrix.

[0058] Example 11 includes the method of example 10, wherein polymerizing a resin onto graphene oxide sheets includes polymerizing resorcinal- formaldehyde-melamine (RFM) resin.

[0059] Example 12 includes the method of any one of examples 10-11, wherein carbonizing includes heating at 1000 °C in argon.

[0060] Example 13 includes the method of any one of examples 10-12, wherein activating includes introducing carbon dioxide (CO2) gas at a temperature of 900

°C.

[0061] Example 14 includes the method of any one of examples 10-13, wherein activating includes introducing carbon dioxide (CO2) gas for five hours.

[0062] Example 15 includes the method of any one of examples 10-14, wherein mixing sulfur and a positively charged polymeric binder includes mixing sulfur and poly (diallyldimethyl ammonium chloride) (PDADMAC).

[0063] The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as "examples." Such examples can include elements in addition to those shown or described.

However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. [0064] In this document, the terms "a" or "an" are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of "at least one" or "one or more." In this document, the term "or" is used to refer to a nonexclusive or, such that "A or B" includes "A but not B," "B but not A," and "A and B," unless otherwise indicated. In this document, the terms "including" and "in which" are used as the plain-English equivalents of the respective terms "comprising" and "wherein." Also, in the following claims, the terms "including" and "comprising" are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms "first," "second," and "third," etc. are used merely as labels, and are not intended to impose numerical requirements on their objects.

[0065] The above description is intended to be illustrative, and not restrictive.

For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

What is claimed is:

1. A battery, comprising:

a first electrode, including:

a porous carbon matrix having a number of pores; sulfur within the number of pores;

a positively charged polymeric binder mixed with the porous carbon matrix and the sulfur;

a second electrode; and

an electrolyte in contact with both the first electrode and the second electrode.

2. The battery of claim 1, wherein the porous carbon matrix includes graphene oxide sheets coated with resorcinal-formaldehyde-melamine (RFM) resin wherein the coated graphene oxide sheets have been carbonized.

3. The battery of claim 1, wherein the porous carbon matrix includes a surface area greater than 2000 M²/g.

4. The battery of claim 1, wherein the porous carbon matrix includes a surface area between 2200 and 2600 M²/g.

5. The battery of claim 1, wherein the porous carbon matrix includes a first distribution of pores at a size less than one nm, and a second distribution of pores at a size between 2 and 4 nm.

6. The battery of claim 1, wherein the a positively charged polymeric binder includes poly (diallyldimethyl ammonium chloride) (PDADMAC).

7. The battery of claim 1, wherein the a positively charged polymeric binder includes polyquaternium.

8. The battery of claim 1, wherein the second electrode includes lithium metal.

9. The battery of claim 1, wherein a ratio of electrolyte to sulfur (E/S) is 12 to 1.

10. A method of forming a battery electrode, comprising:

polymerizing a resin onto graphene oxide sheets to form resin coated graphene oxide sheets;

carbonizing the resin coated graphene oxide sheets to form a carbonized structure;

activating the carbonized structure to form a porous carbon matrix; and mixing sulfur and a positively charged polymeric binder with the porous carbon matrix.

11. The method of claim 10, wherein polymerizing a resin onto graphene oxide sheets includes polymerizing resorcinal-formaldehyde-melamine (RFM) resin.

12. The method of claim 10, wherein carbonizing includes heating at 1000 °C in argon.

13. The method of claim 10, wherein activating includes introducing carbon dioxide (CO2) gas at a temperature of 900 °C.

14. The method of claim 13, wherein activating includes introducing carbon dioxide (CO2) gas for five hours.

15. The method of claim 10, wherein mixing sulfur and a positively charged polymeric binder includes mixing sulfur and poly (diallyldimethyl ammonium chloride) (PDADMAC).