Elaborately Designed Micro−Mesoporous Graphitic Carbon Spheres as Efficient Polysulfide Reservoir for Lithium−Sulfur Batteries Jiahui Zheng,†,‡ Guannan Guo,‡ Hanwen Li,‡ Lei Wang,† Biwei Wang,‡ Huijuan Yu,‡ Yancui Yan,† Dong Yang,† and Angang Dong*,‡ †
State Key Laboratory of Molecular Engineering of Polymers and Department of Macromolecular Science, Fudan University, Shanghai 200433, China ‡ Collaborative Innovation Center of Chemistry for Energy Materials, Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, and Department of Chemistry, Fudan University, Shanghai 200433, China S Supporting Information *
ABSTRACT: Hybrid micro−mesoporous graphitic carbon spheres (M-MGCSs) featuring ordered mesoporous graphene-like cores and uniform microporous carbon shells are designed by transformation of self-assembled Fe3O4 nanoparticle supraparticles and are used as an efficient, dual spatially confined sulfur reservoir for lithium−sulfur (Li−S) batteries. Such rationally designed MMGCSs synergistically combine the merits of micro- and mesoporous carbons when used as the sulfur host in Li−S batteries: the core having interconnected spherical mesopores of 9.0 nm provides sufficient space for loading S8 molecules, while the shell having micropores of 0.6 nm can entrap only small S2−4 molecules, which are converted into electrolyte-insoluble polysulfides during discharge, minimizing the outward diffusion of long-chain polysulfides from the core. These advantageous structural features, combined with the highly graphitic nature and mesoscale spherical morphology of MMGCSs, enable Li−S cathodes with greatly improved performance even at high sulfur areal loadings.
A
Confining sulfur in a conductive carbon host has been established to be a very effective strategy for addressing the aforementioned issues.2,10−12 To date, various carbonaceous materials, including graphene,13−16 porous carbons,17−19 and carbon nanotubes,20−23 have been utilized as sulfur reservoir in Li−S batteries. Among them, ordered mesoporous carbons have been demonstrated to be especially promising owing to their high surface area and well-defined porosity,24−27 which enable a high sulfur loading (∼70 wt %) and fast electrolyte diffusion, while also partially mitigating the shuttle effect of polysulfides. Despite the tremendous progress, the cyclability of current Li−S cathodes is far less satisfactory, as the physically trapped polysulfides can still diffuse out from the relatively large mesopores (>2 nm), especially during long cycles.2,3,10,28−30 One promising strategy to address this issue is increasing the chemical adsorption of polysulfides by doping carbon frameworks with heteroatoms such as nitrogen,31 sulfur,32,33
mong the existing myriad of energy storage technologies, lithium−sulfur (Li−S) batteries show great promise as next-generation energy storage devices owing to their high theoretical energy density of 2600 Wh kg−1,1 which is about an order of magnitude greater than that of current lithium-ion batteries.2,3 In addition, sulfur is naturally abundant, inexpensive, and environmentally benign.4 However, the commercialization of Li−S batteries is hindered, primarily because of the fast capacity decay arising from the dissolution of long-chain polysulfides (Li2Sx, 4 < x ≤ 8) in the course of cycling.1,5 These long-chain polysulfides can reach the surface of the lithium anode by diffusion and are then reduced to either Li2S or short-chain species.6,7 The short-chain polysulfides obtained can move back to the cathode and are oxidized to long-chain species, resulting in a polysulfide shuttle effect, which can dramatically reduce the utilization of active materials and lead to a low Coulombic efficiency.7 Additionally, the insulating nature of sulfur (5 × 10−18 S m−1 in conductivity) and its large volumetric expansion (∼80%) during discharge can greatly restrict the efficient utilization of active materials,8,9 leading to the unsatisfactory reversible capacity. © 2017 American Chemical Society
Received: March 15, 2017 Accepted: April 18, 2017 Published: April 18, 2017 1105
DOI: 10.1021/acsenergylett.7b00230 ACS Energy Lett. 2017, 2, 1105−1114
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http://pubs.acs.org/journal/aelccp
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ACS Energy Letters
Figure 1. (a) Schematic illustration of the synthesis of M-MGCSs and S@M-MGCSs. (b) HRSEM image of carbon-coated Fe3O4 NP supraparticles. TEM images of (c) PPy-coated Fe3O4 NP supraparticles, (d) PPy-coated mesoporous carbon spheres, (e) M-MGCSs, and (f) S@M-MGCSs. (g) STEM image of S@M-MGCSs.
oxygen,34 and boron.35 Nonetheless, traditional mesoporous carbons, prepared by either hard- or soft-templating methods, typically possess a low graphitization degree,36−38 which is undesirable for fast electron transport and efficient utilization of active materials. The ability to tailor the porosity, wall crystallinity, and mesoscale morphology of mesoporous carbons is thus essential to improve the comprehensive performance of Li−S batteries.12,36,38,39 In terms of porosity engineering, previous studies have revealed that microporous carbons, especially those with pore sizes below 0.69 nm,7,40,41 can accommodate S2−4 molecules as opposed to conventional S8 molecules entrapped in mesopores42 because of the space confinement effect. Notably, these small S2−4 molecules will be converted into short-chain, electrolyte-insoluble polysulfides (Li2Sx, 2 ≤ x ≤ 4) during discharge,25,43 thus significantly improving the capacity retention of Li−S cathodes. Although microporous carbon itself is not an ideal host because of the limited sulfur loading capacity (
