Full Length ArticleMetal oxide heterostructure decorated carbon nanofiber as a novel redox catalyst for high performance Lithium-Sulfur batteries
Graphical abstract
Synopsis. CoFe2O4@SnO2 heterostructure decorated CNFs as functional cathode additives can potentially chemisorb and catalyze the conversion of polysulfides to enhance the performance of lithium sulfur batteries.
Introduction
As one of the next generation energy storage system, lithium-sulfur batteries (LSBs) have great potential to meet the energy demands of high performance electrically driven devices on account of their ultrahigh theoretical capacity (1675 mAh g−1) and energy density (2600 Wh kg−1) based on the multi-electron redox conversion process [1], [2], [3]. Additionally, the low cost and natural abundance of the active material i.e., sulfur also makes the LSBs economically sustainable. Nevertheless, the practical application of LSBs is impeded by the daunting challenges associated with such batteries. These include (1) the low electronic and ionic conductivities of sulfur and its final discharged products (Li2S2/Li2S), (2) high solubility of intermediate lithium polysulfides (LiPSs) in the organic liquid electrolyte and its subsequent shuttling towards the lithium anode during discharging causing rapid capacity decay and low Coulombic efficiency, and (3) the significant volume variation occurring during the cycling process that lead to the instability of the electrode structure [4], [5]. Moreover, the sluggish electrochemical redox kinetics associated with the LiPS conversion is another intractable issue that result in severe rate decay due to inadequate transfer of electrons and Li+ ions [6], [7], [8].
The typical strategies to tackle the aforementioned challenges are the utilization of an appropriate host for sulfur [9], [10], incorporating an interlayer between the sulfur cathode and the separator [11], [12], or the use of functional additives in the sulfur cathode [13], [14] etc. So far, various forms of carbon such as micro/mesoporous carbon [15], activated carbon nanofibers [16], carbon nanotubes [17] etc. have been widely employed in LSBs to physically entrap the LiPSs and improve the electrical conductivity of the sulfur cathode. Among various carbonaceous structures, one-dimensional carbon fibers have been attracting enormous attention due to their highly exposed surfaces and direct pathway for long-range electron conduction. Apart from an excellent choice for interlayers [18] and cathode matrix [19], recently the carbon fibers have shown its efficacy as a reinforcing agent in the sulfur cathode, whose addition help to link the electrode materials through hairline cracks preventing their peeling off and provide an unobstructed electron transfer path [20]. Although such carbon materials can improve the performance of LSBs; however, the weak binding of nonpolar carbon towards polar LiPSs is not sufficient to limit the dissolution of LiPSs. Therefore, to chemisorb the LiPSs, carbonaceous materials are functionalized with polar metal oxides. The transition metal oxides such as SnO2 [21], Co3O4 [22], MnO2 [23], etc. have been investigated to prevent the shuttle effect through strong bonding of metal − oxide (M − O) groups with LiPS anions and also accelerate the redox kinetics of the LiPSs. For instance, Cao et al. reported the fabrication of double-shell SnO2@C hollow nanosphere, which were applied as cathode matrix in LSB [24]. The SnO2@C improved the electronic conductivity of the electrode by the outside carbon shell and suppressed the shuttle effect by confining the LiPSs through S–Sn–O and S–C chemical bonds [24].
Recently, ternary metal oxides (TMOs) with the general formula AB2O4 exhibiting spinel structure have attracted much attention in the field of energy storage and conversion devices due to their superior electrochemical activity [25]. For instance, TMOs such as NiCo2O4 [26], CoFe2O4 [27] etc. have been utilized in sulfur cathode to promote the redox kinetic of LSBs. Furthermore, it is well-documented that the heterostructures constructed by coupling nanocrystals of different bandgaps can facilitate the charge transport and surface reaction kinetics due to the internal electric field at heterointerfaces [28], [29], [30]. Therefore, a combination of metal oxide and TMO is expected to provide a new direction for conquering the notorious shuttle effect of LSBs.
Based on the aforementioned context, herein, a hybrid fiber of CNF decorated with SnO2 and CoFe2O4 heterostructure (CoFe@SnCNF) have been developed and explored as the functional cathode additive. As a proof-of-concept, CoFe@SnCNFs have been introduced into the sulfur cathode to manifest their role towards catalyzing the polysulfide conversion reactions (Fig. 1). The CNF present as the core of the hybrid fibers provides electrically conductive channels, meanwhile, the CoFe2O4@SnO2 heterostructure decorated over them provide a synergistic medium to chemically immobilize and simultaneously expedite the conversion of LiPSs. Unlike the previously reported TMOs employed in LSB cathode such as CoFe2O4 coated carbon fiber as interlayer [27] and NiCo2O4 hollow nanoflower as sulfur host [26], which have inherent drawbacks such as the additional passive weight of the interlayer and complicated fabrication of sulfur hosts, respectively, our strategy of incorporating CoFe@SnCNF as cathode additive is relatively simple and industrially viable. Moreover, in contrast to the electrochemical performance at low areal sulfur loadings (<2 mg cm−1) reported in such works, CoFe@SnCNF/S cathode developed in our work displayed relatively good performances at sulfur loadings above 2 mg cm−1. Benefiting from these structural features, the integrated CoFe@SnCNF/S cathode in the LSB showed a stable cycling performance with a low-capacity fading rate of 0.11 % during 200 cycles at 0.5 C and greatly enhanced rate performance with average capacity of 533.9 mAh g−1 at 2 C-rate. Even at a raised sulfur loading of 5.3 mg cm−2, the cathode was still able to deliver an initial capacity of 442.3 mAh g−1 and retains 78.1 % of its capacity after 80 cycles.
Section snippets
Materials
Polyacrylonitrile (PAN, Mw ~ 150,000 g mol−1), tin(II) 2-ethyl hexanoate (Sn(Oct)2), iron (III) chloride (FeCl3, anhydrous), cobalt(II) chloride hexahydrate (CoCl2·6H2O), sulfur (trace metal basis) and lithium sulfide (Li2S) were procured from Sigma Aldrich. N, N-dimethylformamide (DMF) and ethanol (anhydrous) were procured from Fisher Scientific. All the chemicals were of analytical grade purity and used without further purification. Carbon black (conductive carbon, TIMCAL Graphite & Carbon
Results and discussion
The intriguing structural features of the long-length CNF are highly beneficial for providing an effective platform for the growth of metal oxide nanostructures. Therefore, CNF fabricated via a facile electrospinning technique was decorated with SnO2, followed by CoFe2O4 to prepare CoFe@SnCNF hybrid fibers via a two-step solvothermal-hydrothermal treatment as schematically depicted in Fig. 2(a). Fig. S1 shows the typical FESEM image of CNF representing a rather smooth surface with uniform fiber
Conclusion
In summary, CoFe2O4@SnO2 heterostructure decorated CNF was developed and used as the functional additive in sulfur cathode. The as-prepared CoFe@SnCNF is demonstrated to be an efficient catalyst to immediately transform the LiPSs and can afford a decent adsorption towards the LiPSs. In comparison to the counterparts (SnCNF, CNF and CoFeCNF), the improved electrochemical performance of LSBs with the incorporation of CoFe@SnCNF in the sulfur cathode are believed to originate from the synergy
CRediT authorship contribution statement
Avinash Raulo: Conceptualization, Methodology, Formal analysis, Visualization, Investigation, Validation, Writing – original draft. Sajan Singh: Methodology, Investigation, Validation. Amit Gupta: Resources, Formal analysis, Writing – review & editing. Rajiv Srivastava: Resources, Formal analysis, Writing – review & editing. Bhanu Nandan: Conceptualization, Visualization, Supervision, Validation, Writing – review & editing, Funding acquisition.
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 supported by the Department of Science and Technology, India (Project No. DST/TMD/MES/2k17/73). We thank Materials Research Centre at Malviya National institute of Technology (MNIT) Jaipur for helping with XPS measurements.
Author contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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