Building MoSe2-Mo2C incorporated hollow fluorinated carbon fibers for Li-S batteries
Graphical abstract
Introduction
Up to present, metal-sulfur batteries including Li-S, Na-S and Mg-S batteries have developed to be advanced energy storage devices because of their high specific energies [[1], [2], [3], [4], [5], [6], [7], [8]]. The Li-S battery has attracted increasing attention due to superior energy density (2600 W h kg−1), excellent specific capacity (1673 mA h g−1), low-cost, environmental mildness and long cycles [[9], [10], [11], [12], [13], [14], [15]]. However, the Li-S battery is seriously confined by high irreversible capacity losses due to the instinctive insulating feature of sulfur and sluggish chemical kinetics, causing rapid capacity fade [[16], [17], [18], [19]]. The extremely slow chemical dynamics gives arise to an incomplete reduction of LiPSs and hence the low yield of Li2S. The fatal problems cover the formation of various LiPSs (Li2Sn, 2 < n ≤ 8) during the sulfur conversion in charge and discharge process [[20], [21], [22], [23]].
The shuttle effect due to the dissolution of LiPSs into the electrolyte during cycling results in the rapid capacity fade and the low utilization of sulfur. Additionally, the low conductivity of S, the huge volume variations of S and the growth of lithium dendrites dramatically influence the rate and cycle performance of Li-S batteries. An effective S-host with a rational morphology, composition and structure can effectively eliminate the shuttle effect, the volume expansion problem, and lead excellent cycle and rate performances, which are significant important and the prerequisite conditions for the further optimization and the realization of large-scale application of the Li-S battery.
Significant efforts have been performed for designing artificial S host materials. The conductive carbon matrixes, such as carbon materials were introduced to enhance the electron transport capability [[24], [25], [26], [27]]. The blocking layer, such as functional interlayers and separators, was built to anchor the LiPSs and restrain the LiPSs shuttling effect. The electrocatalysts, such as 2D transition metal sulfides/selenides/oxides were combined to promote the chemical conversion kinetics of LiPSs [[28], [29], [30], [31], [32], [33], [34]]. Besides, the combination of multidimensional composites is beneficial for the mitigation of volume variations during cycling, the efficient absorption of S species, and promoting the conversion of LiPSs.
Carbon materials are highly electrically conductive, and hence enable the insulating sulfur to achieve the high specific capacity. The porous carbon material can efficiently load maximal S to achieve a high energy density. However, carbon materials with non-polar or weak-polar possess weak interactions toward polar LiPSs, generally resulting in the shuttle effect of LiPSs be poorly hampered after a long cycle [35]. Additionally, the volumetric energy density of Li-S batteries based on carbon materials is relatively constrained due to the distinct lightweight property [36].
Lithiophilic and sulfophilic sulfur hosts with a high volumetric energy density can chemically interact with polar LiPSs, however, compared with carbon materials, their intrinsic lower conductivity compromises the poor rate capability and low capacity of sulfur [[37], [38], [39]]. Based on the individual advantages of carbon and transitional metal based composites, these carbon skeletons can be modified with adjustable heteroatoms or metal (composite) nanoparticles that can chemically or electrostatically anchor the polysulfide intermediates and accelerate the conversion kinetics of Li-S batteries. The incorporation of the physical and chemical barrier for the dissolution of LiPSs, is highly desirable for expediting the chemical kinetics and repressing the shuttle effect. Noticeably, the extremely slow kinetics especially the conversion of short-chain Li polysulfide (Li2S) to long-chain polysulfides causes the low coulombic efficiency or poor cycling performance. The polar catalysts can accelerate such conversion reaction. Introducing transition metal-based catalysts is a promising way due to their low cost and feasible availability. Remarkable catalytic performance over a series of electrochemical reduction reactions were obtained [32].
It is noted that many catalysts investigated are metals (Fe and Ni), transition metal oxides (MoO3 etc.) and sulfides (MoS2, CoS2, and WS2) with strong LiPSs anchor capability [40]. However, many electrocatalysts show pure structure stability during prolonged cycling, which are thermodynamically unstable when rendered in the Li-S battery. By contrast, carbonaceous materials are more stable but are less catalytic because of the weak interaction between nonpolar carbon and polar LiPSs. Transition metal and fluorine co-doped carbon materials (FC) are expected to present effective stable electrochemical performance and superior interaction with LiPSs [41]. However, the lithiophilic and sulfophilic capability of transition metals are limited to suppress the solvation effect, which can aggravate the shuttle effect in some degree. Additionally, the poor electron structure coupled with the less lithiophilic and sulfophilic sites is detrimental to the chemical adsorption and catalytic capability to the conversion of LiPSs.
Considering the multi-step of the conversions of sulfur in both long-chain polysulfides (Li2Sn, 4≤ n ≤ 8) and short-chain polysulfides (Li2Sn, n < 4), the energy barriers in solid-liquid-solid phase transformations of insoluble short-chain LiPSs and soluble long-chain LiPSs, a single catalyst is deficient to accelerate all the conversion of LiPSs involved phase transformations (S8 (solid) ↔ Li2S4 (liquid) ↔ Li2S (solid)) [[42], [43], [44], [45], [46], [47], [48]]. The synergistic advantages of multi-functional catalysts including exceptional ion and charge transportation capability as well as adjusted electronic structure, entrust outperformed structural merits over the individual component.
In this work, an artificial route to preparing a hybrid of 0D/1D/2D MoSe2@FC@Mo2C with transition metal MoSe2 and metallic Mo2C encapsulated into hollow FC was evaluated for Li-S batteries. The unique heterostructure entrusts a large surface area, rich exposed lithiophilic and sulfophilic active sites, high catalytic conversion of LiPSs, and fast electron/ion transportation. The incorporation of multi-dimensions releases mechanical stress-strain, accommodates the volume change during Li plating/stripping. The open nanoscale vertical structure helps to decrease the current density and stabilizes the SEI membrane formation. The stepwise multi-electrocatalysis was realized, polar FC with wrong electronegativity promotes the conversion of short chain (S8 ↔ Li2S4), while the polar MoSe2 and Mo2C with enriched oxygen vacancies accelerate the long chain conversion (Li2S4 ↔ Li2S), the electrochemical conversion from long-chain, leading fast kinetics behaviors. In the absence of MoSe2@FC@Mo2C, the LiPSs cannot be well immoblized and seriously dissolute in the organic electrolyte, resulting in the low battery working efficiency and poor cycle stability. The in-situ formed SEI can not be finely maintained and the Li metal is gradually etched, leading to the growth of Li dendrites and potential safe hazards. Additionally, the slow LiPSs conversion efficiency causes the sluggish kinetic behaviors. In the presentence of MoSe2@FC@Mo2C, the strong physisorption, chemisorption as well as the outstanding stepwise catalysis effects reduce the dissolution of LiPSs and greatly accelerate the LiPSs conversion efficiency. The uniform electron distribution leads to the fine growth of SEI, enabling the prominent structure stability and protecting the Li metal from the corrosion of LiPSs.
The as-synthesized S based composite entrusts Li-S batteries the considerable low LiPSs diffusion ability and fast conversion capability. Additionally, the charge storage mechanism of this system was carefully elucidated as the synergistic of reversible Li+ insertion/exfoliation with S and MoSe2, in which Mo2C contributes greatly to the optimized interface resistance and shorted active process. MoSe2, Mo2C and FC substantially catalyze cleavage of Li-S bond in Li2S and efficiently fasten the migration of Li+. As expected, the fabricated MoSe2@FC@Mo2C/S as a sulfur host displays exceptional rate and cycling stability. The material and methodology proposed in this work are expected to guide the construction of advanced multifunctional materials for Li-S batteries.
Section snippets
Results and discussion
The preparation progress of MoSe2@FC@Mo2C/S was schematically described in Fig. 1. The structure information of the synthesized MoSe2@FC@Mo2C/S was uncovered by its corresponding XRD patterns (Fig. 2 and Fig. S1a). The peaks concentrated at 13.7°, 31.4°, 34.3°, 37.9°, and 55.9° correspond to the crystal planes of hexagonal MoSe2 (PDF#29-0914) (002), (100), (102), (103) and (110), respectively. The peaks of 34.4°, 38.0°, 39.4°, 52.1° and 61.5° are derived from (100), (002), (101), (102) and
Conclusions
In summary, the hierarchical polar MoSe2@FC@Mo2C/S was elegantly fabricated for physisorption and chemisorption carrier coupled with LiPSs conversion catalysts in Li-S batteries. FC, MoSe2, and Mo2C present considerable selectivity differences in the catalysis of short-chain and long-chain LiPSs, corroborated by the CV profiles and DFT simulations. The efficient stepwise electrocatalysis strategy, and the optimized interface resistance bring the accelerated LiPSs conversion efficiency and
CRediT authorship contribution statement
Yaoyao Xiao: Conceptualization, Data curation, Supervision, Software. Yuting Liu: Investigation, Methodology. Guohui Qin: Writing - original draft, Writing - review & editing. Pinyu Han: Formal analysis. Xinyu Guo: Methodology. Shixun Cao: Visualization. Fusheng Liu: Project administration, Resources.
Declaration of competing interest
We would like to submit an enclosed manuscript entitled “Building MoSe2-Mo2C Incorporated Hollow Fluorinated Carbon Fibers for Li-S Batteries”, which we wish to be considered for publication in “Composites Part B: engineering”. No conflict of interest exits in the submission of this manuscript and the manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described is an original research that has not been published previously and
Acknowledgements
The authors acknowledge the financial support of the National Natural Science Foundation of China (No. 21805152, 51673106), the doctor foundation of Shandong province (No.ZR2018BB033), the Postdoctor Science Foundation of China (No. 2018M640616), Talent Fund of Shandong Collaborative Innovation Center of Eco-Chemical Engineering (XTCXQN16) and the Postdoctor Science Foundation of Shandong province (No.201902038).
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2023, Composites Part B: EngineeringCitation Excerpt :Moreover, the DNA-CNT/MXene/PP-based battery exhibited a reversible capacity of approximately 800 mAh g−1 when switching the current rate back to 0.2 C, which suggested that the battery had excellent reversible capacity over varying current densities (Fig. 4c). Specifically, the capacity decay was observed in the first 10 cycles at 0.2 C, which may be the incomplete activation of batteries in the initial stage, and followed by complete activation, the capacity decay is limited at higher C-rates [53]. Furthermore, as shown in Fig. 4d, the corresponding charge-discharge curves of the battery with the DNA-CNT/MXene/PP separator maintained two flat plateaus even at a large current density of 2 C, which confirmed the excellent polysulfide inhibiting ability and rapid redox kinetics of the DNA-CNT/MXene/PP separator [54].
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These authors contributed equally to this work.