Polysulfide Confinement and Highly Efficient Conversion on Hierarchical Mesoporous Carbon Nanosheets for Li–S Batteries

Abstract

performance of Li-S batteries is still restricted by poor conductivity of sulfur itself and discharge products (Li 2 S/Li 2 S 2 ), which makes hard conversion of sulfur. Additionally, the common shuttle effect of intermediate polysulfide also leads to an obvious capacity loss during charging and discharging. [10][11][12][13][14][15] Thus, new strategies are urgent in addressing the problems of high-capacity sulfur cathodes for Li-S batteries.To design high-performance cathode, dispersing nanostructured sulfur particles on carbon materials have attracted particular interests, which not only stabilize sulfur and intermediates but also promotes electron transfer during charging/ discharging. [16][17][18][19][20][21] Although carbon materials have been the first choice to disperse sulfur particles, the nonpolar carbon suffers a weak binding force to the intermediate product, which results in serious shuttle effect and making it difficult to reuse the polysulfide (Scheme 1a). To address these issues, plenty of work has begun to focus on the modification of nonpolar carbon by introducing heteroatoms (B, N, O, or S). [22][23][24][25] This is because the lone pair of electrons on the heteroatoms forms an electrostatic attraction (Li bond) with the polysulfide, which can effectively prevent shuttle of polysulfides. [26,27] However, the high loading and the easy aggregation of sulfur are hard to be balanced, since sulfur are getting easier to migrate and aggregate at surface of carbon supports, which lead to the low concentration loading or limited sulfur utilization. Therefore, sulfur confinement and simultaneous highly efficient conversion remains a challenge for the next-generation Li-S batteries.Herein, we develop honeycomb-like mesoporous carbon nanosheets (MC-NS) with abundant defects and Co-N-C catalytic site as efficient host for simultaneously confinement and efficient conversion of polysulfides, which shows significantly enhanced performance for Li-S batteries cathode (Scheme 1b). The as-prepared MC-NS exhibits an excellent 2D conductive network with a high specific surface area of 335.4 m 2 g −1 and abundant mesopores that provide high storage space and expansion space for sulfur. Moreover, the density functional theory (DFT) calculation and experiments manifest that the presence of abundant defects on the carbon skeleton MC-Ns Lithium-sulfur (Li-S) batteries have attracted increasing attention due to their extremely high theoretical specific capacity and a promising power density. However, practical applications of Li-S batteries are still limited by the relatively low performance, owing to poor conductivity of sulfur itself and discharge products (Li 2 S/Li 2 S 2 ) as well as the shuttle effect of the intermediate polysulfide. Herein, honeycomb-like mesoporous Co, N-doped carbon nanosheets (MC-NS) with a high specific surface area and abundant defects are developed which, simultaneously enable polysulfide confinement and highly efficient conversion. Moreover, density functional theory calculations and experime...

Mesoporous Materials for Electrochemical Energy Storage and Conversion

“…To increase the affinity of carbon hosts on polysulfide species, a class of guest species, such as single atoms, metal clusters, or metal compounds, are introduced into the mesoporous carbon materials, which can provide chemisorption interaction and then can further reduce the shuttle effect of polysulfides. [41,45] The lithium-air batteries (LABs) provide a higher theoretical energy density up to ≈3460 Wh kg −1 compared with other rechargeable energy storage devices. [46,47] The main challenges associated with LABs are to accommodate the deposition insulating Li 2 O 2 particles generated during discharge and to accelerate the formation of original reagents (Li + O 2 ) from the sluggish backreaction during charge.…”

A microporous carbon derived from metal‐organic frameworks for long‐life lithium sulfur batteries

“…Interestingly, only one broad cathodic peak around 2.35 V can be detected in subsequent two cycles; it is reported that the conversion of sulfur to Li 2 S, also called “quasi‐solid‐state” becomes the main reaction mechanism . Comparatively, when the S/MC 3 was tested in ether‐based electrolyte, it is possible to observe three cathodic peaks at 2.30, 2.02, and 1.88 V in the first cycle (Figure B), which presents different electrochemical redox behaviors as compared with other sulfur/carbon or sulfur/metal compounds based cathodes . The peaks at 2.3 and 2.02 V can also be caused by the two‐step reduction reactions of the elemental sulfur with metallic lithium, respectively forming high‐order soluble polysulfides (Li 2 S x , 4 ≤ x ≤ 8) and low‐order polysulfides (Li 2 S x , 2 ≤ x < 4).…”

“…40 Comparatively, when the S/MC 3 was tested in ether-based electrolyte, it is possible to observe three cathodic peaks at 2.30, 2.02, and 1.88 V in the first cycle ( Figure 5B), which presents different electrochemical redox behaviors as compared with other sulfur/carbon or sulfur/metal compounds based cathodes. [16][17][18][19][20] The peaks at 2.3 and 2.02 V can also be caused by the two-step reduction reactions of the elemental sulfur with metallic lithium, respectively forming high-order soluble polysulfides (Li 2 S x , 4 ≤ x ≤ 8) and low-order polysulfides (Li 2 S x , 2 ≤ x < 4). Also, the peak at 1.88 V might be the slow conversion of the low-order polysulfides to insoluble Li 2 S 2 and Li 2 S, 41 which could be attributed to slow lithium ion migration in the inner pore of the MC, as well as limited transformation dynamics from the liquid-phase to insulating solid-phase.…”

“…6,7 However, the insulativity of active sulfur and discharge product (Li 2 S), the solubility of discharge intermediates (Li 2 S x , 4 ≤ x < 8) into organic electrolyte, and the "shuttle effect" by diffusing polysulfides into the negative electrode to produce parasitic reactions, cause serious active sulfur loss from cathode and cycle deterioration of the batteries, greatly hindering the Li-S batteries' development and commercialization. [8][9][10] In order to address these issues, significant progresses have been made in the last decade by immobilizing sulfur within various host materials to form sulfur-based composites, like sulfur/carbon composites, [11][12][13][14][15][16][17][18] sulfur/metallic compounds composites, [19][20][21][22][23][24][25][26] and sulfur/polymer composites. [27][28][29][30][31] It is noted that among the various host materials, microporous carbon is quite distinctive, because esterbased electrolyte was usually used for sulfur/microporous carbon (S/MC) composites based Li-S batteries, while ether-based electrolyte was commonly used in other composites based Li-S battery systems.…”

“…In order to address these issues, significant progresses have been made in the last decade by immobilizing sulfur within various host materials to form sulfur‐based composites, like sulfur/carbon composites, sulfur/metallic compounds composites, and sulfur/polymer composites . It is noted that among the various host materials, microporous carbon is quite distinctive, because ester‐based electrolyte was usually used for sulfur/microporous carbon (S/MC) composites based Li‐S batteries, while ether‐based electrolyte was commonly used in other composites based Li‐S battery systems.…”