Performance Of Lithium-sulfur Batteries Research Articles

Lithium Sulfide: Magnesothermal Synthesis and Battery Applications.

As a critical material for emerging lithium-sulfur batteries and sulfide-electrolyte-based all-solid-state batteries, lithium sulfide (Li2S) has great application prospects in the field of energy storage and conversion. However, commercial Li2S is expensive and is produced via a carbon-emissive and time-consuming method of reducing lithium sulfate with carbon materials at high temperatures. Herein we report a novel method of synthesizing Li2S by thermally reducing lithium sulfate with the first non-carbon-based reductant Mg. Compared with the commercial carbothermal method, our magnesothermal technique has multiple advantages, such as completion in minutes, operation at lower temperatures, emission of zero amount of greenhouse-gases, and a valuable byproduct MgO. Moreover, the prepared Li2S product demonstrates excellent cathode performance in lithium-sulfur batteries, in terms of cycling stability, activation voltage, and rate capability. Thus, this innovative method opens a new direction for the research of Li2S and has great potential for practical applications.

CO2-derived free-standing carbon interlayer embedded with molecular catalysts for improving redox performance in Li-S batteries

The use of a free-standing carbon interlayer is a promising approach for the development of lithium-sulfur (Li-S) batteries because it suppresses the shuttle phenomenon and provides outstanding flexible characteristics. However, the thickness required to maintain the unique properties of the free-standing interlayer inevitably inhibits the transport of Li ions, causing sluggish redox kinetics. This work tackles the critical problem of the interlayer by synthesizing a composite in which Fe-based molecular catalysts are atomically incorporated into carbon nanofibers with superior pore characteristics realized by CO2 treatment. The templates self-generated during CO2 annealing provide high porosity and surface area, leading to effective Li-ion diffusion, and homogeneous distribution of the catalytic sites in the form of Fe-N2 to the free-standing paper. The Fe-N2 configuration thermodynamically aids in overcoming the energy barrier of the rate-limiting step of Li2S4 to Li2S conversion while minimizing the shuttle phenomenon. Based on the effective Li-ion transport by improved pore properties, and the superior redox reaction ability of Fe-N2, the assembled cell maintains a coulombic efficiency of 95% up to 1600 cycles at 3.0 C. In addition, a maximum areal capacity of 2.9 mAh cm−2 is delivered for a high loading electrode with 4.2 mg cm−2 at 45 °C.

CoS2-NC@CNTs hierarchical nanostructures for efficient polysulfide regulation in lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries have received extensive attention in electrochemical energy storage systems due to their high theoretical energy density as well as natural abundance of sulfur and low cost. However, Li-S batteries exhibit rapid capacity decay and inferior rate performance due to polysulfides (LiPSs) shuttling and sluggish redox kinetics. Therefore, it is crucial that LiPSs are anchored and their conversion reactions are catalyzed in order to improve the performance of Li-S batteries. In this work, ZIF-67-derived carbon polyhedra decorated with ultrafine CoS2 nanoparticles are interconnected by carbon nanotubes (CNTs). The uniformly distributed CoS2 nanoparticles can immobilize LiPSs and catalyze their conversion of solid-to-liquid-to-solid reactions, which enhances the redox kinetics. Furthermore, the three-dimensional conductive network consisting of ZIF-67-derived carbon polyhedra and CNTs facilitates fast electron transport and thus leads to excellent rate and cycling performance. The cells were assembled using CoS2-NC@CNTs-pp and show a high initial discharge capacity of 1408.5 mAh g−1 at 0.1 C and 1186.7 mAh g−1 at 1 C, and they remain outstanding cycling stability for over 500 cycles. Even at a high C rate of 5 C, it still can deliver a high discharge capacity of 832.6 mAh g−1. This work provides an efficient route to accelerate redox conversion and suppress the shuttle effect by designing a multifunctional separator.

A Facile Immobilization Strategy for Soluble Phosphazene to Actualize Stable and Safe Lithium-Sulfur Batteries.

Lithium-sulfur batteries (LSBs) have attracted extensive attention owing to their high energy density and abundant sulfur resources. However, LSBs are still restricted by the unsatisfactory electrochemical performance resulting from the shuttle effect of lithium polysulfide (LiPSs), and the potential fire hazard caused by inflammable ether electrolytes and polyolefin separators. Herein, a facile immobilization strategy for hexachlorocyclotriphosphazene (HCCP) is creatively applied to address the above issues simultaneously. Insoluble HCCP cross-linked microspheres (H-CMP) are firstly obtained at ambient temperature using tannic acid (TA) as a cross-linking agent and then a multifunctional separator coating is constructed based on H-CMP. The released phosphorus-related radicals from H-CMP in wide temperatures effectively prevent the combustion of electrolytes and separators, and hence improve the fire safety of the Li-S pouch cell. Furthermore, H-CMP availably chemisorbs LiPSs to interdict the shuttle effect, thereby dramatically improving the electrochemical performance of LSBs. The effectiveness of this strategy is also verified in high sulfur loading (6.38mg cm-2 ), high temperature (50 °C), and Li-S pouch cells. More importantly, H-CMP exhibits sufficient stability for Li metal and suppression of Li dendrites. This facile immobilization strategy for multifunctional phosphazenes provides a competitive option for the large-scale fabrication of high-safety and high-performance LSBs.

Pyrrole derivatives as interlayer modifier of Li-S batteries: Modulation of electrochemical performance by molecular perturbation

The electrochemical performance of lithium-sulfur (Li-S) batteries is strongly hampered by the shuttle effect and slow redox kinetics of lithium polysulfides (LiPSs). Surface modified interlayer of a separator of Li-S batteries is demonstrated to be an effective strategy to overcome this problem. Herein, cobalt nanoparticles confined in nitrogen co-doped porous carbon framework (Co-CN) were developed from pyrolysis of ZIF-67 and used as interlayer of PP separator for Li-S batteries, and were functionalized by four pyrrole derivatives, 1-phenylpyrrole, 1-methyl pyrrole, 1-(p-toluenesulfonyl) pyrrole, and 1-pyrrole, respectively, which were screened in terms of the electron-withdrawing/donating ability of the substituent groups on the pyrrolic nitrogen. The impact of the molecular structure of pyrrole derivatives on the interaction with LiPSs and the electrochemical performance of Li-S batteries were explored by nuclear magnetic resonance and theoretical calculation. It is uncovered that 1-phenylpyrrole shows the highest enhancement of redox kinetics of LiPSs, attributing to the optimal interaction with Co nanoparticles and LiPSs. Therefore, 1-phenylpyrrole modified Co-CN interlayer enables the best electrochemical performance for the Li-S batteries, delivering a specific capacity of 562 mAh g−1 at 5 C and a capacity of 538, 526, and 449 mAh g−1 after 500 cycles at 1, 2, and 3 C, respectively. At a high sulfur loading of 5.5 mg cm−2, it achieves a capacity of 440 mAh g−1 after 500 cycles at 1 C. This work reveals the interaction mechanism among LiPSs, Co nanoparticles and the molecular modifiers in improving the electrochemical performance of Li-S batteries.

Constructing interlayers with micro-nano particles to enhance the fire safety and polysulfide adsorption ability of polypropylene separators for lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are widely accepted representatives of next-generation energy storage with high energy density. However, the high flammability of polypropylene (PP) separators and ether electrolytes deteriorates the safety of LSBs. Moreover, the severe shuttle effect of lithium polysulfides (LiPSs) induces the capacity degradation and poor cycle stability of LSBs. In this work, we unprecedentedly synthesize multifunctional micro-nano-sized particles (ACNP) that are constructed on PP separators to improve the safety and electrochemical performance of LSBs. The ACNP is obtained by the electrostatic assembly of ammonium polyphosphate and chitosan. The presence of ACNP decreases the burning process and improves the thermal stability of separators and electrolytes. Besides, the temperature of pouch cells during combustion is decreased after the addition of ACNP or ACNP@super-P(SP) interlayers. Moreover, the abundant N-containing and O-containing groups in the ACNP layer effectively inhibit the shuttle effect of LiPSs via chemical adsorption, increase the affinity to electrolytes, and facilitate the Li+ diffusion and redox kinetics. The SP enhances the conductivity of the interlayer, thus enhancing the capacity and cycle stability of LSBs. It is expected that this work provides a universal strategy to enhance the safety of separators as well as the batteries.

Polyglycerol-functionalized microporous carbon/sulfur cathode for Li-S battery

Lithium–sulfur (Li-S) batteries are promising next-generation rechargeable devices that have low cost and high energy densities. However, the commercialization of Li-S batteries is challenging because lithium polysulfide (Li2Sn), which forms during the charge–discharge process, dissolves in the electrolyte. To suppress Li2Sn dissolution, we propose a method of modifying microporous carbon with polyglycerol, in which only microporous carbon materials and monomers are mixed and heated. Using a sulfur cathode with polyglycerol-treated microporous carbon as a host material for the sulfur, the capacity retention (discharge capacity at the 50th cycle divided by the discharge capacity at the 2nd cycle) improved from 57.6 to 75.2%. We also used Fourier-transform infrared spectroscopy to reveal that polyglycerol treatment suppresses the dissolution of Li2Sn from the cathode. Finally, polyglycerol treatment is shown to improve the cycle performance of Li-S batteries.

Bifunctional Al2O3/polyacrylonitrile membrane to suppress the growth of lithium dendrites and shuttling of polysulfides in lithium-sulfur batteries

Interfacial engineering plays an important role in dynamically stabilizing the electrochemical performance of lithium-sulfur batteries (LSBs). Constructing a robust interface, e.g., using a multifunctional membrane, benefits not only for the suppression of lithium polysulfides (LiPSs) shuttling, but also for the inhibition of uncontrollable Li dendrite growth. Herein, a bifunctional membrane is fabricated with the Al2O3 nanoparticle-embedded polyacrylonitrile nanofibers (Al2O3/PAN) in a facile way as separator, which are found effective in blocking the growth of Li dendrite and the shuttling of LiPSs in LSBs. The exposed Al2O3 nanoparticles work in a responsible way to physiochemically capture the LiPSs and simultaneously let the Li ions shuttle through the PAN nanofiber network as the simulation indicated. The 3D cross-linked membrane can also block the oriented Li deposition and growth to make the symmetric Li||Li cell cycle over 1500 h without short circuit at 0.5 mA cm−2. Moreover, the LSB exhibits a reversible capacity of 2.08 mAh cm−2 at 500 mA g−1 under the sulfur loading of 3.6 mg cm−2. These results provide a doable and economy way to fabricate the robust membrane to deal effectively with the tough issues, which sheds new light on the design and optimization of high-energy-density sulfur prevailed rechargeable batteries.

Polar Co9S8 anchored on Pyrrole-Modified graphene with in situ growth of CNTs as multifunctional Self-Supporting medium for efficient Lithium-Sulfur batteries

Lithium-sulfur batteries (LSBs) have stimulated burgeoning interest in both the academic and industrial communities due to their ultrahigh energy density and high cost-effectiveness. However, the real-world application of LSBs has long been plagued by the shuttling effect and sluggish conversion kinetics of soluble lithium polysulfides (LiPSn). Herein, we have designed and synthesized creative metallic and polar Co9S8 nanoparticles with in situ growth of CNTs anchored on pyrrole-modified graphene (Co9S8/CNTs-Gr) via a facile pyrolysis method, which further demonstrates superior electrochemical performance for the fixation of sulfur and effective activation of LiPSn redox conversion. The in situ growth of CNTs and introduction of pyrrole-modified graphene are beneficial for electron migration, thus significantly boost the rate performance of LSBs. Concurrently, the Co9S8 electrocatalyst shows high catalytic activity to reinforce the polysulfide conversion. Benefiting from this exquisite nanoarchitecture design, LSBs with a S@Co9S8/CNTs-Gr cathode exhibit outstanding performance, delivering a high reversible specific capacity of 950 mA h g−1 at 1C with a decay rate of only 0.01 % per cycle. Moreover, the S@Co9S8/CNTs-Gr electrode still achieves good cycling stability with a high sulfur loading of 7.2 mg cm−2, and the specific area capacity reaches as high as ∼ 6 mA h cm−2. This work provides a delicate design to construct Co9S8-based media for high-performance LSBs and can also promote one’s understanding of the advantages of the process of adsorption and redox conversion of LiPSn.

Quasi-Solid-State Lithium-Sulfur Batteries Assembled by Composite Polymer Electrolyte and Nitrogen Doped Porous Carbon Fiber Composite Cathode

Solid-state lithium sulfur batteries are becoming a breakthrough technology for energy storage systems due to their low cost of sulfur, high energy density and high level of safety. However, its commercial application has been limited by the poor ionic conductivity and sulfur shuttle effect. In this paper, a nitrogen-doped porous carbon fiber (NPCNF) active material was prepared by template method as a sulfur-host of the positive sulfur electrode. The morphology was nano fiber-like and enabled high sulfur content (62.9 wt%). A solid electrolyte membrane (PVDF/LiClO4/LATP) containing polyvinylidene fluoride (PVDF) and lithium aluminum titanium phosphate (Li1.3Al0.3Ti1.7(PO4)3) was prepared by pouring and the thermosetting method. The ionic conductivity of PVDF/LiClO4/LATP was 8.07 × 10−5 S cm−1 at 25 °C. The assembled battery showed good electrochemical performance. At 25 °C and 0.5 C, the first discharge specific capacity was 620.52 mAh g−1. After 500 cycles, the capacity decay rate of each cycle was only 0.139%. The synergistic effect between the composite solid electrolyte and the nitrogen-doped porous carbon fiber composite sulfur anode studied in this paper may reveal new approaches for improving the cycling performance of a solid-state lithium-sulfur battery.

High-Performance Flexible Sulfur Cathodes with Robust Electrode Skeletons Built by a Hierarchical Self-Assembling Slurry.

The electrochemical performance of lithium–sulfur batteries is fundamentally determined by the structural and mechanical stability of their composite sulfur cathodes. However, the development of cost‐effective strategies for realizing robust hierarchical composite electrode structures remains highly challenging due to uncontrollable interactions among the components. The present work addresses this issue by proposing a type of self‐assembling electrode slurry based on a well‐designed two‐component (polyacrylonitrile and polyvinylpyrrolidone) polar binder system with carbon nanotubes that forms hierarchical porous structures via optimized water‐vapor‐induced phase separation. The electrode skeleton is a highly robust and flexible electron‐conductive network, and the porous structure provides hierarchical ion‐transport channels with strong polysulfide trapping capability. Composite sulfur cathodes prepared with a sulfur loading of 4.53 mg cm−2 realize a very stable specific capacity of 485 mAh g−1 at a current density of 3.74 mA cm−2 after 1000 cycles. Meanwhile, a composite sulfur cathode with a high sulfur loading of 14.5 mg cm−2 in a lithium–sulfur pouch cell provides good flexibility and delivers a high capacity of 600 mAh g−1 at a current density of 0.72 mA cm−2 for 78 cycles.

Bamboo-shaped Co@NCNTs as superior sulfur host for Li-S batteries

Functional and active carbon material is of great concern in facilitating the performance of lithium-sulfur (Li-S) battery. In this work, we develop a bamboo-like nitrogen-doped carbon nanotubes (Co@NCNTs-2) with low cobalt metal content, which acts as not only the host matrix for sulfur loading but also an efficient catalyst for the adsorption and conversion of lithium polysulfides (LiPSs). The characterization results show that the Co particles are mainly encapsulated inside the Co@NCNTs-2, and the N species are mainly distributed on the outer surface of Co@NCNTs-2. The Co particles are the growth sites of carbon nanotubes and are wrapped by carbon to form Co@NCNTs-2. It is confirmed that the uniform graphitized structure contributes to charge transfer, while the enriched N species assist the trap and catalytic conversion of the polysulfides, thus helping to restrain the shuttle effect. As a result, the resultant Co@NCNTs-2-S cathode presents high reversible capacity at different current densities and exhibits superior cycling stability for 300 cycles at 0.5 C.

Active Sulfur-Host Material VS4 with Surface Defect Engineering: Intercalation-Conversion Hybrid Cathode Boosting Electrochemical Performance of Li-S Batteries.

Transition-metal sulfides as late-model electrocatalysts usually remain inactive in lithium-sulfur (Li-S) batteries in spite of their advantages to accelerate the rapid conversion of lithium polysulfides (LiPSs). Herein, a series of cobalt-doped vanadium tetrasulfide/reduced graphene oxide (x%Co-VS4/rGO) composites with an ultrathin layered structure as an active sulfur-host material are prepared by a one-pot hydrothermal method. The well-designed two-dimensional ultrathin 3%Co-VS4/rGO with heteroatom architecture defects (defect of Co-doping and defect of S-vacancies) can significantly improve the adsorption ability on LiPSs, the electrocatalytic activity in the Li2S potentiostatic deposition, and the active sulfur reduction/oxidation conversion reactions and greatly boost the electrochemical performances of Li-S batteries. On the one hand, the ultrathin 3%Co-VS4/rGO possesses good conductivity inheriting from rGO which contributes to the capacity of internal redox reactions on lithiation from VS4. On the other hand, the hybrid architectures provide strong adsorption and excellent electrocatalytic ability on LiPSs, which benefit from the surface defects caused by heteroatom doping. The S@3%Co-VS4/rGO cathode displays a high specific capacity of 1332.6 mA h g-1 at 0.2 C and a low-capacity decay of only 0.05% per cycle over 1000 cycles at 3 C with a primary capacity of 633.1 mA h g-1. Furthermore, when the sulfur loading (single-side coating) reaches 4.48 mg cm-2, it still can deliver 756.2 mA h g-1 after the 100th cycle at 0.2 C with 89.5% capacity retention. In addition, the in situ X-ray diffraction test reveals that the sulfur conversion mechanism is the processes of α-S8 → Li2S → β-S8 (first cycle) and then β-S8 ↔ Li2S during the subsequent cycles. The designing strategy with heteroatom doping and self-intercalation capacity adopted in this work would provide novel inspiration for fabricating advanced sulfur-host materials to achieve excellent electrochemical capability in Li-S batteries.

Ultra-Fast Synthesis of Single Atoms for All-Solid-State Lithium-Sulfur Batteries

All-solid-state lithium-sulfur (Li-S) battery has attracted the most attention in the past decades due to its high theoretical energy density (2600 Wh kg-1), high safety, low cost, and environmentally friendly feature. However, several issues need to be addressed before commercializing all-solid-state Li-S battery. The insulating nature of charged product sulfur and discharged product lithium sulfide results in low active material utilization. The dissolution of polysulfides intermediate and shuttling effect leads to fast capacity decay. Incorporating catalysts into the cathode or separator has proven to be very effective for promoting the sluggish Li-S reaction kinetics, suppress the shuttling effect of polysulfides, thus achieving superior cycling performance of Li-S battery using the liquid electrolyte. Single atom catalyst with the maximum atom utilization, unique coordination environment attracted particular attention. Whether single atom sites are feasible in the solid-state electrolyte is still unexplored. Herein, we synthesized single atom catalysts using ultra-fast approach. High-angle annular dark-field aberration corrected scanning transmission electron microscopy image and extended X-ray absorption fine spectroscopy verified the presence of single atoms on the support. The optimized absorption sites of single atoms were further studied with density functional theories. Using polyethylene oxide and lithium bis(trifluoromethanesulfonyl)imide infiltrated polyimide as the solid polymer electrolyte, we evaluated the cycling performance of all-solid-state Li-S battery. Single atoms incorporated sulfur cathodes exhibits higher cycling performance than the bare sulfur cathodes due to the significantly enhance lithium-sulfur redox reaction kinetics. The fast Li-S redox reaction kinetics were also revealed by calculating the energy barrier from sulfur to lithium sulfide using density functional theories. The formation of lithium sulfide from lithium disulfide was found to be the rate-limiting step for discharging sulfur. With the help of single atoms, the energy barrier of the rate-limiting step is significantly reduced. Our work provides new insights to improve the capacity and cycling life of all-solid-state Li-S batteries for commercializing Li-S technology.

Low-Temperature Calcination of Metal-Organic Frameworks(MOFs) to Derive the High Entropy Stabilized Oxide for High Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) battery owing to high energy density and theoretical capacity is anticipated as a high-performance rechargeable power source for flexible electronic devices and electric vehicles. However, the rapid capacity fade, low Coulombic efficiency, and significant self-discharge capacity loss due to the polysulfides shuttling are the major constraints of its real-world applications. To conquer these challenges, despite the physical encapsulation of polysulfides, chemical interactions between shuttle effect-suppressive sulfur host materials and soluble lithium polysulfides have recently been emphasized. Herein, a novel approach to synthesize the high entropic stabilized oxide (HEO) at low temperature is developed from the self-sacrificing template of metal-organic frameworks (MOFs) to chemically anchor the lithium polysulfides. As-synthesized HEO850 at 850 ºC (transition temperature) exhibited a single-phase rocksalt crystalline structure with homogenous dispersion of Ni, Mg, Cu, Co, and Zn and reversible entropic phase stabilization in the certain temperature range (750-850 ºC). When employed as a chemical anchor to lithium polysulfides (LIPSs) and compared the electrochemical performance of Li-S cells with medium configurational entropic oxide (MEOs) (HEOs-one metal cation), low entropy oxide (LEOs) (HEOs-three metal cations), and routine sulfur ketjen black (S/KB) cells, it revealed a competitive reversible capacity, excellent cycling stability and a low capacity decay rate by immobilizing the LIPSs and facilitating the redox reaction in the cathode. The cells with HEO850/KB/S cathode delivered a higher initial specific discharge capacity of 1244.1 mAh g-1 than those of MEO850/S/KB (979.625 mAh g-1), LEO850/S/KB (908 mAh g-1), and S/KB (966 mAh g-1). After 800 cycles of continuous charging and discharging at 0.5 C, the capacity of 784.1 mAh g-1 with outstanding Coulombic efficiency of 99.66 % is still retained demonstrating excellent cycling stability with a negligible capacity fade rate of 0.04% per cycle. This outstanding performance could be attributed to the synergistic contribution and exposure of numerous active sites of randomly dispersed elements in the HEO850. This study not only highlights the extraordinary electrochemical performance of Li-S battery with efficient immobilization of LIPSs but also provide a novel strategy for the synthesis of HEOs at lower calcination temperature for various energy conversion and storage applications. After getting confidence with these prilimary results, Ti-based HEOs owing to both high ionic and electric conductivities will be investigated at coin and pouch cell level as future research direction.

Hybridization of Mesoporous Carbon and Iron Oxide for Better Mitigation of Polysulfide Shuttling in Li-S Batteries

With a higher theoretical capacity, lithium-sulfur (Li-S) batteries have been considered as promising candidates for next-generation batteries. Due to the non-conducting nature of sulfur, however, lithium-sulfur batteries tend to exhibit poor performance at a high current rate (C-rate).Here we demonstrate that Fe3O4, synthesized from precursor iron (III) acetylacetonate (AAI), and mesoporous carbon material, Ketjen Black (KB), can be synergistically combined to enhance the electrochemical performance of lithium-sulfur batteries substantially. Instead of adding commercial magnetite particles into Li-S cathodes, iron oxides are synthesized and imbedded into KB from a precursor, AAI, through thermal treatments in air and Ar. The sulfur, then, is incorporated into iron oxides/KB hybrids by melting at an elevated temperature. With air-controlled electrospray as the processing method, sulfur embedded iron oxides/KB, poly(acrylic acid), and rGO sheets are mixed and directly deposited onto the carbon-coated aluminum collector, to employ as the Li-S battery cathode. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical impedance spectroscopy (EIS) studies confirm that Fe3O4 and iron carbide (Fe3C) are synthesized from the precursor, AAI, embedded in mesoporous carbon materials, and reducing the charge transfer resistance of batteries. The rate-capability tests show that systems with KB/Fe3O4 can achieve enhanced performance compared to batteries without iron oxides, especially at high C-rates. With 14 wt % of solid materials as iron oxides and iron carbide, batteries exhibit 750 mAh/g at 2C discharge/charge rates, which is 83% higher compared to systems without iron oxides. However, incorporated via 250 °C for 0.5 hr in air and 850 °C for 2 hr in Ar, the composite of KB/Fe3O4/Fe3C induces irreversible side reactions during the initial charging process, which causes a huge difference in capacity between the first and the second cycle. To achieve an optimal status with improved rate capability and acceptable initial charging time, we modify the thermal treatments and thus increase the proportion of Fe3O4 in the mixture. According to the postmortem analysis, cathodes with iron oxides can interact with soluble polysulfides strongly and alleviate the polysulfide shuttle effect significantly, compared with those without Fe3O4. Such enhanced rate capability by KB/Fe3O4 (synthesized) over KB only or KB/Fe3O4 (commercial) systems suggests that the incorporation of iron oxides can play an important role in improving the electrical conductivity of cathode and mitigating polysulfide shuttle effect. Figure 1

Incorporation of Functionalized Graphene and Its Derivates into Electrolyte: A Facile Approach to Improve the Electrochemical Performance of Lithium-Sulfur Batteries

The impending global energy crisis and the severe environmental problems in the recent years has bolstered the research for sustainable electrochemical energy storage. With the lithium-ion technology reaching its theoretical limit, lithium sulfur (Li-S) batteries have garnered extensive attention owing to their ultrahigh theoretical capacity and the low-cost, Earth abundant and environmentally friendly nature of the sulfur cathode. Despite all the advantages, the development of Li-S technology is hindered by several inherent challenges namely, the large capacity drain due to the dissolution of the intermediate polysulfides in the electrolyte leading to polysulfide shuttling and low conductivity in the cell.To overcome these issues, graphene and its derivatives have been extensively explored to mitigate polysulfide shuttling and enhance conductivity in the cell. However, at present, most of this research is limited to cathode modification which despite improving capacity retention is unable to curb the deleterious impact of polysulfide shuttling once the polysulfides dissolve in the electrolyte.In this work, we have probed the impact of implementing graphene and its derivatives as additives to the Li-S electrolyte. We have extensively characterized various commercially available rGO and graphene samples to determine their morphology, functionalization, and other key ancillary properties such as defects and ionic resistivity and have studied their impact towards the electrochemical performance of the electrolyte-modified Li-S cell. Results showed a marked improvement in initial capacity of 1323 mAh/gs and a capacity retention of 83% at 0.2 C over 105 cycles with single layer graphene additive. Further, it was shown that increasing functionalization led to improved mitigation of polysulfide shuttling and about 20% higher capacity retention (than reference) over 105 cycles. The impact of different functional groups on graphene was also probed to compare their polysulfide shuttling mitigation capacity in the electrolyte.Overall, we successfully achieved superior electrochemical performance of the Li-S cell simply by addition of graphene to the electrolyte. It was successfully shown that functionalized graphene dispersed in the electrolyte can enhance polysulfide encapsulation and improve the ionic conductivity in the cell.

Modified separator engineering with 2D ultrathin Ni3B@rGO: Extraordinary electrochemical performance of the lithium-sulfur battery with enormous-sulfur-content cathode in low electrolyte/sulfur ratio

Compared to other batteries, a next-generation lithium-sulfur (Li-S) battery's specific capacity and energy density are relatively high. However, the actual electrochemical performance (particularly for the cell with a high cathode sulfur content and a lean electrolyte/sulfur ratio) falls well short of practical usage requirements. The ultrathin Ni 3 B@rGO-PP is designed in this study as a unique modified separator for Li-S cells. Because of its distinct electronic structure, the modified separator shows outstanding adsorption ability toward polysulfide through the boron element's active sites. Additionally, it has excellent electrocatalytic activity, significantly enhancing the performance of Li-S batteries. After the 100th cycle, the cell with 80 wt% sulfur content can retain 819 mA h g −1 under 0.2 C. Also, when the electrolyte/sulfur ratio is reduced to 15 µL mg −1 and 5 µL mg −1 , respectively, considerable capacity retention of 736.3 mA h g −1 and 615 mA h g −1 is achieved after the 100th cycle at 0.2 C. What's more, for the high sulfur loading (single-side up to 4.5 mg cm −2 ) cathode, the cell obtains a retention capacity of 578 mA h g −1 after the 200th cycle under 0.2 C. The surface engineering strategy with the Ni 3 B@rGO modified coating adopted in this work would open an efficient route to acquire the Li-S battery's outstanding electrochemical capability in the practical application situation. The boron element's active cites provide a unique electronic structure to the Ni 3 B@rGO coating layer, which effectively enhance the adsorption ability toward polysulfide, and further provide excellent electrocatalytic activity during the charging/discharging process, greatly improving the electrochemical performance of Li-S batteries. Thus, the Ni 3 B@rGO-PP exhibits long cycling stability even at a low E/S ratio, fast Li + transportation, and high ionic conductivity. • The Ni 3 B@rGO nanohybrid developed as a novel modified coating on the celgard separator in Li-S batteries. • The Ni3B@rGO-PP show outstanding adsorption ability to LiPSs through B active cites and enhances the cell conductivity. • The Ni 3 B@rGO-PP exhibits excellent anti-self-discharge, high ionic conductivity, and fast Li + transportation. • The Ni3B@rGO-PP presents promising electrochemical performances with a high-sulfur-content cathode (up to 83 wt%).

Surface strain-enhanced MoS2 as a high-performance cathode catalyst for lithium–sulfur batteries

Lithium–sulfur batteries (LSBs) are one of the main candidates for the next generation of energy storage systems. To improve the performance of LSBs, we herein propose the use of strained MoS2 (s-MoS2) as a catalytically active sulfur host. The introduction of strain in the MoS2 surface, which alters its atomic positions and expands the S–Mo–S angle, shifts the d-band center closer to the Fermi level and provides the surface with abundant and highly active catalytic sites; these enhance the catalyst's ability to adsorb lithium polysulfides (LiPS), accelerating its catalytic conversion and promoting lithium-ion transferability. Strain is generated through the synthesis of core–shell nanoparticles, using different metal sulfides as strain-inducing cores. s-MoS2 nanoparticles are supported on carbon nanofibers (CNF/s-MoS2), and the resulting electrodes are characterized by capacities of 1290 and 657 ​mAh g−1 ​at 0.2 and 5 ​C, respectively, with a 0.05% capacity decay rate per cycle at 8 ​C during 700 cycles. Overall, this work not only provides an ingenious and effective strategy to regulate LiPS adsorption and conversion through strain engineering, but also indicates a path toward the application of strain engineering in other energy storage and conversion fields.

A biomass-based cathode for long-life lithium-sulfur batteries

With the advantages of high conductivity and low cost, porous carbons have been considered as the most attractive host materials of sulfur cathodes in lithium-sulfur batteries (LSBs). However, LSBs always suffer short cycle life due to the “shuttle effect” of lithium polysulfide species (polysulfides), which are intermediate products during the charge/discharge processes. The weak interaction between carbon and polysulfides results in the dissolution of polysulfides from the cathodes, loss of active material of sulfur and eventually fast capacity fading. To overcome these drawbacks, we employed a biomass-derived carbon as the host material in sulfur cathodes. Results from X-ray diffraction (XRD), scanning electron microscopy (SEM) and nitrogen sorption reveals that this biomass-derived product is amorphous carbon and is composed of both large (>10 nm) and small (<5 nm) pores at an appropriate ratio. Using as hosts of cathodes in LSBs, the biomass-derived carbons deliver a high reversible capacity of >800 mAh/g and retain >80% of initial capacity after 200 cycles. Especially, the activated carbons exhibited an unprecedently high durability with 80% capacity retention after 400 cycles. The promising LSB performance can be ascribed to the unique porous architecture of biomass-derived carbons. The small pores in biomass-derived carbons provide more sites to anchor sulfur and polysulfides, while large pores provide channels for fast transport of ions. This is corroborated by the results from the electrochemical impedance spectroscopy (EIS), the thermogravimetric analysis (TGA) and absorption measurements.