Electrochemical Performance Of Lithium-sulfur Batteries Research Articles

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.

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.

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.

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.

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%).

Cobalt/three-dimensional carbon/reduced graphene oxide modified PP separator for Li-S batteries

In this work, a composite of cobalt metal nanoparticles/three-dimensional carbon/reduced graphene oxide (Co-3DC-rGO) was synthesized using corn husk, ZIF-67, and graphene oxide as precursors. The Co-3DC-rGO was used to modify commercial PP to prepare the Co-3DC-rGO/PP separator. The performance of Co-3DC-rGO/PP separator was evaluated in lithium-sulfur batteries (LSBs). SEM images exhibited that a three-dimensional carbon (3DC) skeleton was obtained by corn husk transformation, and the ZIF-67-derived cobalt-decorated carbon dodecahedrons were homogeneously filled into the hierarchical pores of the 3DC structure. TEM images displayed that Co metal nanoparticles can escape from ZIF-67 and scatter on the rGO nano-sheets and 3DC skeleton. Benefitting from the synergy effects of the 3D carbon skeleton, rGO, and Co metal nanoparticles, the electrochemical performance of LSBs with Co-3DC-rGO/PP separator is better than LSBs with commercial PP separator. When tested at 1C, the LSB with Co-3DC-rGO/PP separator presents a high stable reversible capacity of 516.3 mA h g −1 after 500 cycles. This research provides new ideas for the preparation of LSBs separator. • Co/3D carbon/rGO decorated PP separator was prepared and used in Li-S batteries. • Co/3D carbon/rGO can inhibit the shuttle of lithium polysulfides to the Li anode. • Co/3D carbon/rGO accelerate the conversion of lithium polysulfides to Li2S. • The Li-S battery with Co/3DC/rGO/PP separator delivers excellent electrochemical performance.

Preparation of Fe3O4/FexSy heterostructures via electrochemical deposition method and their enhanced electrochemical performance for lithium-sulfur batteries

Lithium-sulfur (Li–S) batteries have received more and more attention due to their excellent theoretical capacity. However, Li–S battery shuttle effect, poor conductivity, slow chemical reaction kinetics and other shortcomings inhibit its development. In this paper, Fe3O4/FexSy heterostructures were prepared by electrochemical deposition method followed by thermal annealing. The samples obtained at 600 °C have high crystallinity and novel morphology, which is conducive to the adsorption and catalytic performance of LiPSs. As cathode host, folded tubular heterostructures can contribute to sulfur loading and improve Li-ion diffusion rate, thus improving sulfur utilization and cycling stability. Combined with these merits, the Li-S batteries showed excellent stability over 500 cycles at 0.5 C, with an average capacity decay of 0.067% per cycle. This work provides an effective method for rapidly preparing multifunctional heterostructures for Li–S batteries.

Porous SiOCN with ultrafine CoSe2 nanoparticle modified separators for promoting polysulfides capture and redox kinetics in lithium-sulfur batteries

Ultrafine CoSe2 nanoparticle/porous silicon oxycarbonitride composite (CoSe2/P-SiOCN) is developed as modifying material of separator to improve the electrochemical performance of lithium-sulfur batteries by promoting polysulfides (LiPSs) trapping and the redox kinetics of sulfur species. CoSe2/P-SiOCN is prepared through pyrolyzing the Schiff-base-SiO2 precursors with Co2+ ions, subsequent selenylation and removal of silica domain. The conventional separator is modified by coating CoSe2/P-SiOCN on the side for facing sulfur cathode. In this structural design, the porous structure of silicon oxycarbonitride (P-SiOCN) functions as polysulfides trapping and support matrix of CoSe2 nanoparticles, while the ultrafine CoSe2 nanoparticles (~7.8 nm) can efficiently promote the redox kinetics of sulfur species. The cell with CoSe2/P-SiOCN modified separator exhibits better cycling and rate performance than that with P-SiOCN modified separator and the pristine separator. At 1 C, the cell delivers a high reversible discharge capacity of 568.1 mAh g−1 after 400 cycles with a low capacity decay of 0.042 % per cycle.

Elaborate interface design of CoS2/Fe7S8/NG heterojunctions modified on a polypropylene separator for efficient lithium-sulfur batteries

Lithium-sulfur (Li-S) batteries are considered as the most promising energy storage system due to their high specific capacity and resource abundance. However, the low electronic conductivity of S8 and Li2S, shuttle effects of lithium polysulfides (LiPSs), and sluggish reaction kinetics hinder the advance of Li-S batteries. The key to addressing the issues mentioned above is to improve the electrical conductivity of the cathode, enhance the absorption of LiPSs and accelerate the conversion. Nevertheless, it is a great challenge to simultaneously optimize them all. In this study, by incorporating the virtues of the large lamellar structure and strong nonpolar adsorption of reducedgrapheneoxide (rGO), strong polar polysulfide adsorption of Fe7S8 and excellent catalytic polysulfide activity of CoS2, a CoS2/Fe7S8 heterostructure grown on the nitrogen-doped rGO (CoS2/Fe7S8/NG) was prepared. Besides, the experiments demonstrate that the designed CoS2/Fe7S8/NG possesses strong absorption and superior catalytic ability towards LiPSs. Subsequently, when applied as a modification layer for the polypropylene (PP) separator, the CoS2/Fe7S8/NG-PP coupled with CNT/S cathode exhibits a high initial discharge capacity (1459 mAh g−1 at 0.1 A g−1), a reversible capacity of 731 mAh g−1 after 100 cycles at 0.2 A g−1, and a robust cycling stability (0.058 % flowing rate at 1.0 A g−1). Moreover, the cathode with a high sulfur mass loading of ∼ 3.8 mg cm−2 can still display a reversible capacity of 663 mAh g−1 after 100 cycles at 0.2 A g−1. More importantly, the study shown herein shed light for the design of heterojunction to improve the electrochemical performance of Li-S batteries.

In situ self-boosting catalytic synthesizing free-standing N, S rich transition metal sulfide/hierarchical CNF-CNT architectures enable high-performance lithium-sulfur batteries

Advanced hierarchical carbon-based hybrid (CH) nanostructures, typical carbon nanotubes/carbon nanofibers hybrid (CNT-CNF), with rationally tailored components, structures, and chemistries, have been widely dedicated to potential energy storage and conversion applications. Here we report a facile methodology for the synthesis of precise hierarchical heteroatom-doped carbon hybrid/transition metal sulfide (CNF-CNT/TMS) nanostructure assemblies with novel in-situ formed TMSs as the catalysts for secondary CNTs growth via a self-boosting catalytic pyrolysis. The key to in situ growth of CNT arrays is obtaining small high active TMS nanocatalysts and sufficient supply of carbon sources originated from polystyrene. Such special structure provides abundant active sites, beneficial for the application of N, S codoped CNF-CNT/TMS hybrid film in the electrochemistry without introducing extra dopants. As a proof-of-concept application, the representative CNF-CNT/Co9S8 hybrid was exploited as a conductive and catalytic membrane reactor in a lithium–sulfur (Li-S) battery. This unique architecture can achieve a maximized synergistic effect of electron/ion transport, polysulfide entrapment and catalytic conversion, which enables significant enhancement in electrochemical performance of Li-S batteries. As anticipated, the Li-S battery exhibits ultrastable long-life (over 1000 cycles, with an ultralow decay of 0.034% per cycle) and high-rate performance (6 C). Moreover, at high loading of sulfur of 3.7 mg cm−2, stable cycling performance with high capacity retention can still be achieved after long-term tests. This work also presents a facile and effective strategy in the development of complex hierarchical carbon nanostructure, which may be a competitive candidate in other energy storage and conversion system.

Manipulating the electrocatalytic activity of sulfur cathode via distinct cobalt sulfides as sulfur host materials in lithium-sulfur batteries

For the better development of lithium-sulfur (Li-S) batteries, it is necessary to fabricate sulfur hosts with cheap, rapid sulfur reaction dynamic and inhibiting the shuttling effect of lithium polysulfides (LiPSs). Herein, four hollow cubic materials with two kinds of nitrogen-doped carbon derived from Prussian blue analogues (PBA) precursor, Co9S8/MnS/NC@NC-400, CoS2/MnS/NC@NC-500, CoS1.097/MnS/NC@NC-600 and CoS1.097/MnS/NC@NC-700, are reported when the vulcanization temperatures are regulated at 400 °C, 500 °C, 600 °C and 700 °C, respectively. Among them, Co9S8/MnS/NC@NC-400, CoS2/MnS/NC@NC-500 and CoS1.097/MnS/NC@NC-600 have the similar hollow cubic structure, which can physically confine the LiPSs's shuttle, however, the Co vacancies of CoS1.097 in the CoS1.097/MnS/NC@NC-600 can promote the rearrangement of surface electrons, which is beneficial to the diffusion of Li+/e−, improving the electrochemical reaction kinetics. As for the CoS1.097/MnS/NC@NC-700 with the same substance but almost collapsed structure, the CoS1.097/MnS/NC@NC-600 can accommodate the volume expansion of sulfur conversion. In the four sulfur-host materials, the CoS1.097/MnS/NC@NC-600 not only displays the outstanding adsorption ability on LiPSs, but also presents the best electrocatalytic activity in the Li2S potentiostatic deposition experiments and active sulfur reduction/oxidation conversion reactions, greatly promoting the electrochemical performances of Li-S batteries. The S@CoS1.097/MnS/NC@NC-600 cathode can deliver 1010.2 mA h g−1 at 0.5 C and maintain 651.1 mA h g−1 after 200 cycles. In addition, the in-situ X-ray diffraction (in-situ XRD) test reveals that the sulfur conversion mechanism is the processes of the α-S8 → Li2S → β-S8 (first cycle), then β-S8 ↔ Li2S during the subsequent cycles. Based on the fundamental understanding of the design and preparation of CoxSy/MnS/NC@NC hosts with the desired adsorption and catalysis functions, the work can provide new insights and reveal the defect-engineering to develop the advanced Li-S batteries.

ZIF-8/Ketjen Black derived ZnO/N/KB composite for separator modification of lithium sulfur batteries

Lithium-sulfur batteries have the advantages of high specific capacity, high energy density, environmental friendliness, and low price, and have become one of the most promising new electrochemical technologies. However, during the charge-discharge reaction, the shuttle effect of the reaction intermediate polysulfides leads to a decrease in battery activity, which limits its development and commercialization. In this paper, we prepared a novel ZnO/N/Ketjen Black (KB) composite to act as a separator barrier to reduce the influence of polysulfides shuttle effects. A simple aqueous solution reaction was used to mix Zn(NO3)2∙6H2O with KB and ZIF-8/KB was obtained by adding dimethylimidazole, and then ZIF-8/KB precursor was carbonized at high temperature in inert gas atmosphere to obtain ZnO/N/KB composite. Finally, the prepared ZnO/N/KB composite was coated on commercial PE separator. Through the dual effects of adsorption and catalysis, the shuttle effect of polysulfides is sufficiently inhibited by the ZnO/N/KB layer, therefore the electrochemical performances of lithium-sulfur batteries are largely improved. The results show that when the sulfur cathode of Li-S battery contains 3 mg cm−2 active material, the specific discharge capacity can be as high as 1284.5 mAh g−1 at 0.05 C. Furthermore, at a higher rate of 0.5 C, the initial discharge specific capacity also reaches 868.3 mAh g−1. After 400 cycles of the charge/discharge process, it still remains 495.9 mAh g−1, which means a capacity retention rate of 57.1%.

Improving the electrochemical performance of lithium-sulfur batteries by interface modification with a bifunctional electrolyte additive

Lithium-sulfur (Li-S) batteries with sulfur cathode and lithium anode are considered as a promising candidate in the next generation of high specific energy secondary batteries. However, the scalable application of Li-S batteries is limited by the unstable electrode/electrolyte interfaces. Herein, a novel bifunctional electrolyte additive, trifluoromethane sulfonamide (TFMSA), is introduced to stabilize both electrode/electrolyte interfaces to enhance the electrochemical performance of Li-S batteries. The TFMSA molecule can not only alleviate the passivation of the sulfur cathode surface, but also suppress Li dendrites growth and prevent the lithium anode from the corrosion of polysulfides. As a result, the assembled Li-S battery with TFMSA-containing electrolyte displays excellent long-cycle performance and rate capability. Moreover, the Li||Li symmetric cell using TFMSA electrolyte can stably cycle for 1200 h at 1 mA cm−2 and 1 mAh cm−2. The Li-S pouch cell with TFMSA also exhibits outstanding performance, indicating the prospect of practical application. This work sheds light on an innovative idea for interface modification via an electrolyte additive to overcome the inherent issues of Li-S batteries.

Correlations between precipitation reactions and electrochemical performance of lithium-sulfur batteries probed by operando scattering techniques

<h2>Summary</h2> A comprehensive description of electrochemical processes in the positive electrode of lithium-sulfur batteries is crucial for the utilization of active material. However, the discharge mechanisms are complicated due to various reactions in multiple phases and the tortuosity of the highly porous carbon matrix. In this work, simultaneous measurements of small-angle and wide-angle scattering and cell resistance are performed on operating lithium-sulfur cells. Results indicate that precipitates grow mostly in number, not in size, and that the structure of the carbon matrix is not affected. The comparison of the small-angle and wide-angle scattering reveals the amorphous discharge products found at a low discharge rate. Further analysis demonstrates the correlation between the diffusion resistance and the compositional change of electrolyte in the mesopores at the end of discharge, which suggests that Li-ion deficiency is the limiting factor for sulfur utilization at a medium discharge rate.

ZnFe2O4 hollow rods enabling accelerated polysulfide conversion for advanced lithium-sulfur batteries

The high energy density and favorable cost-effectiveness make lithium-sulfur (Li-S) batteries one of the most attractive energy storage systems. However, the low sulfur utilization and poor cycle life, resulting from the losses of soluble polysulfide intermediates, and their sluggish redox conversion process, severely impede practical applications of reliable Li-S batteries. Effectively inhibiting the polysulfide diffusion and accelerating their conversion is beneficial to enhance the performance of sulfur cathodes. Herein, a novel carbon-free ZnFe2O4 hollow rod has been developed as an advanced host material to confine polysulfides within the cathode and accelerate the redox conversion during cycling. The soluble polysulfides anchored by the ZnFe2O4 hollow rod structure are shown to be rapidly converted to sulfur and lithium sulfides. Detrimental polysulfide diffusion is therefore effectively inhibited. The redox kinetics of sulfur cathodes has been systematically investigated, revealing that the ZnFe2O4 host can improve the activity for Li2S deposition, facilitate lithium-ion diffusion, and lower the reaction energy barriers for the multistep phase transition of sulfur. As a result, the developed S@ZnFe2O4 composite cathodes exhibit an improved cycling capacity of 1158 mAh g−1. These results demonstrate that the accelerated redox conversion of anchored polysulfides is essential for enhancing the electrochemical performance of Li-S batteries.

Synthesis of Li Conductive Polymer Layer on 3D Structured S Cathode by Photo-Polymerization for Li–S Batteries

The dissolution of lithium polysulfide (Li2Sx, 4 ≤ x ≤ 8, LiPS) during charge/discharge testing is a critical issue hindering the practical application of lithium-sulfur batteries (LSBs). To suppress LiPS dissolution, we propose a facile method to fabricate a Li-ion-conductive polymer layer by photopolymerization. The electrochemical performance of LSBs was investigated by preparing small pouch cells containing a three-dimensional (3D) structured sulfur-based cathode that either was or was not layered with the new polymer. Analysis of the electrolyte in the LSB pouch cell by UV-Vis spectroscopy revealed that a 3D S cathode with polymer layer shows a good discharge capacity of 535 mA h g−1 and a coulombic efficiency (CE) of over 96% after 40 cycles. In comparison, the 3D S cathode without a polymer layer has a poor discharge capacity of 389 mA h g−1 and a CE of over 22% after 40 cycles. The dissolution suppressing ability of our new polymer layer demonstrates promise for the practical application of LSBs.

Identification of the Underlying Processes in Impedance Response of Sulfur/Carbon Composite Cathodes at Different SOC

For lithium-sulfur batteries, porous carbon/sulfur composite cathodes are the primary solution to compensate the non-conductive nature of sulfur. The composition and structure of this class of cathodes are crucial to the electrochemical performance, achieved energy density and the stability of the cell. Electrochemical impedance spectroscopy is employed to investigate and correlate the electrochemical performance of lithium-sulfur batteries to the composition and microstructure of differently fabricated carbon/sulfur composite cathodes. A transmission line model is applied to identify different underlying electrochemical processes appearing in the impedance response of a range of porous carbon/sulfur cathodes. The integration of a lithium ring serving as a counter electrode coupled with advanced wiring has allowed an artifact-free recording of the cathode impedance at different states of charge with the aim to investigate the evolution of impedance during discharge/charge and the kinetics of charge transfer depending on the infiltration method and the utilized carbon host. It is shown that impedance response of this class of cathodes is highly diverse and the plausible underlying processes are discussed in details. To this end, quasi-solid-state and various polysulfide-based charge transfer mechanisms are identified and their time constants are reported.

CeO2 composite metal organic framework is used to construct high-performance lithium-sulfur batteries

The high specific capacity of lithium-sulfur batteries makes them a potential next-generation energy storage device, but the shuttle effect and low electrical conductivity of sulfur hinder their development. In this paper, a one-step hydrothermal method was used to prepare cerium dioxide and metal organic framework composites to improve the electrochemical performance of lithium-sulfur batteries. Polar cerium dioxide has a strong affinity to polysulfides. Experiments further confirmed that CeO2 has a positive effect on the battery performance. UV-Vis tests were performed on the composites. After standing for 1 h, Zn@NPC-CeO2-2 showed the best adsorption of polysulfides. In addition, the specific discharge capacity of the battery was 569.3 mA h/g with 88% capacity retention after 200 cycles at 2 C. The results indicate that the addition of appropriate amounts of rare earth metal oxides can effectively improve the reaction kinetics of the battery and also provide a new idea for improving the performance of lithium-sulfur batteries.

Effect of zinc-based active sites on porous carbon and electrochemical properties in lithium-sulfur batteries

Hierarchically porous carbon (HPC) is considered as one of the promising cathodes of lithium-sulfur batteries (LSBs) owing to its appealing merits. However, it is still preventing the shuttle effect poorly due to the weak interaction between nonpolar carbon and polar polysulfide (LiPS). Herein, we report a facile method to prepare polar hierarchically porous carbon by in-situ loading the different amounts of zinc-based active sites (p-HPC-X) onto the HPC materials. The close combination of a carbon interconnected skeleton and ZnS nanoparticles derived zinc-based active sites forms the strong interaction that suppresses the soluble polysulfide shuttle effect and catalyzes the kinetics redox reactions of lithium polysulfide/sulfide. Moreover, the effect of polar sites introduced in situ on the morphology/porosity of carbon materials is also focused on. Crystallization of zinc-based active sites during the freeze-drying process and self-assembly during carbonization allow the pore size distribution of porous carbon to be customized, which offers fast lithium-ion and electron transport channels. Due to the synergistic effect of the hierarchical pore structure and suitable polarity sites, the p-HPC-X exhibits high capacity and good cycling performance. Especially p-HPC-1, it yields a high reversible capacity of 683.7 mAh g−1 and a low fading rate of 0.041% per cycle over 900 cycles at a current density of 1 C. This work offers novel insights into realizing the effect of in-situ loaded active sites on pore structure and electrochemical performance in LSBs.