Towards stable lithium-sulfur battery cathodes by combining physical and chemical confinement of polysulfides in core-shell structured nitrogen-doped carbons
Graphical abstract
Introduction
The rechargeable lithium-sulfur battery (LSB) is regarded as a promising next-generation electrochemical energy storage device, due to the high theoretical capacity (1675 mAh g−1) of sulfur and high specific energy of the batteries (up to ∼900 Wh/kg, practically achievable maximum level) [1,2], as well as the abundancy, low cost, and eco-friendliness of elemental sulfur [[2], [3], [4], [5], [6]]. One severe challenge hindering its practical application is the formation and dissolution of lithium polysulfide intermediates during cycling, giving rise to the so-called polysulfide shuttle effect. Shuttling can result in irreversible loss of active materials and in parasitic reactions with the anode, leading to capacity decay, poor Coulombic efficiency, self-discharge, and other detrimental effects [[7], [8], [9], [10]].
A wide variety of nanostructured carbonaceous materials were employed as the conductive matrix to host the electrically insulating sulfur in LSB cathodes. In addition to providing electronic conductivity, a major requirement for these host materials is to restrain the soluble polysulfides by adsorption and thus to alleviate the shuttle effects [9]. Among the different nanostructures, microporous carbons (<2 nm) are of particular interest owing to their high adsorption potential towards large polysulfide molecules. However, purely microporous carbons generally suffer from the insufficient sulfur content that can be loaded into the composite due to the low pore volume of the micropores [[11], [12], [13]]. In this regard, some recent studies have developed core-shell architectures to encapsulate sufficient sulfur loadings in a system of larger pores that is encapsulated with a microporous shell. The latter can lead to geometrical confinement of intermediates formed during cycling of the battery due to physical interaction (i.e., weak physical adsorption) [14,15]. Yet, the affinity of the nonpolar carbon surface towards the polar polysulfides into the electrolyte remains limited due to the absence of specific interaction sites. In consequence, pristine carbon shells can only slow down but not fully prevent the shuttle effect [16].
The strength of interaction between polar polysulfides and cathode hosts can be enhanced by employing materials with high polarity. This is known as chemical confinement (i.e., strong chemical adsorption) [[16], [17], [18], [19]]. Many studies have reported the modification of carbonaceous host surfaces with polar adsorbents, such as polymers, metal oxides, or transition-metal disulfides [[20], [21], [22], [23], [24]]. Nevertheless, these materials are most often electrically insulating, and can thus not be used as active sites to directly reduce the adsorbed polysulfides. One class of materials that seems to be particularly promising for strong confinement of polysulfides due to high surface polarity and that provides high electrical conductivity are heteroatom-doped (especially nitrogen-doped) carbons [[25], [26], [27]].
Therefore, it seems to be a promising strategy to combine the physical and chemical confinement in a nitrogen-doped microporous shell with high surface polarity. A nitrogen-doped hierarchical carbon material with core-shell-type particle architecture and a polar microporous shell is developed as a host for LSB cathodes. The desirable electrical conductivity and high specific surface area lead to a high utilization of the active material. The hierarchical porous inner particle with high volume of ordered mesopores provides sufficient space for a high sulfur loading, electrolyte penetration, and volume expansion of sulfur during cycling. The microporous shell with high polarity enables a dual effect of physical and chemical confinement of the polysulfides to improve the cycle life without the use of any metallic species for polysulfide adsorption.
Section snippets
Experimental section
Synthesis of hierarchical porous carbon particle with microporous carbon shell (HC@C) and bare hierarchical porous carbon (HC): For the synthesis of HC@C, ordered mesoporous silica (SBA-15, hydrothermally treated at 130 °C) and ZnCl2 were employed as hard- and salt template, respectively. In a typical procedure, 1.0 g SBA-15 was first impregnated with 4 mL of an aqueous solution of 1.8 g sucrose, 3.6 g ZnCl2, and 0.14 g concentrated sulfuric acid in a Petri dish. The mixture was then heated to
Results and discussion
Nitrogen-doped hierarchical porous carbon particles with microporous carbon shell (NDHC@C) have been prepared by a combined hard-salt-templating approach [28], followed by nitrogen doping at high temperature (Fig. 1). An aqueous solution containing sucrose as the carbon precursor and ZnCl2 as the salt template was firstly infiltrated into the ordered mesoporous silica SBA-15 hard template followed by polymerization of the carbohydrate with ZnCl2 inside. The precursor was soaked into the ordered
Conclusion
Nitrogen-doped hierarchical core-shell engineered carbon particles with an ordered mesoporous carbon core and a polar microporous carbon shell have been synthesized by a combined hard/salt-templating approach, followed by high-temperature treatment with cyanamide. Large specific surface area of 1764 m2 g−1 and high pore volume exceeding 1.5 cm3 g−1 are obtained. When applied as a host for sulfur in LSB cathodes, its high pore volume can help accommodate large ratio of sulfur (72 wt%) inside the
CRediT authorship contribution statement
Runyu Yan: Conceptualization, Investigation, Methodology, Writing - original draft. Martin Oschatz: Conceptualization, Writing - review & editing, Funding acquisition, Supervision, Project administration. Feixiang Wu: Writing - review & editing, Conceptualization, Data curation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was financially supported by the Innovation-Driven Project of Central South University (No.2019CX033). R.Y. acknowledges financial support from China Scholarship Council. M.O. acknowledges financial support provided by a Liebig-Fellowship from the German Chemical Industry Fund.
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