Elsevier

Materials Today Nano

Volume 13, March 2021, 100106
Materials Today Nano

Unraveling the effects of anions in NixAy@CC (A=O, S, P) on Li-sulfur batteries

https://doi.org/10.1016/j.mtnano.2020.100106Get rights and content

Abstract

Nickel-based compounds have been regarded as potential sulfur carriers in Li-sulfur (Li-S) battery. However, the relationship between the anions in those compounds, which are crucial to the intrinsic properties, and their electrochemical performances remains unclear. Herein, nickel compounds with the same morphologies, i.e., Ni5P4, NiO, and NiS, grown on carbon cloths (noted as NixAy@CC (A = O, S, P)) have been rationally designed to act as sulfur hosts in Li-S batteries to investigate the compositional effects on their catalytic activities and chemical absorption abilities. Density functional theory (DFT) results demonstrate that such variations in electrochemical performance are attributed to the interaction between polysulfides (PSs) and nickel compounds, leading to different adsorption energies and interfacial states. Among them, Ni5P4@CC exhibits the best electrochemical performance, delivering capacities of 1349.5, 1020.4, 803.7, 656.7 mAh g−1 at rates of 0.1, 0.5, 2, and 3C, respectively. Moreover, super stable cycling performance has been achieved at 1C and 3C by Ni5P4@CC-based electrode, showing an ultralow decay ratio of 0.0357% and 0.0173% per cycle for over 1000 and 2000 cycles, respectively. In contrast, NiS@CC and NiO@CC show 0.0796% and 0.0787% per cycle at the cycling test.

Introduction

Li-S batteries are regarded as one of the most promising candidates for the next generation of energy storage devices due to their high theoretical capacity and energy density, ca. 1675 mAh g−1 and 2600 Wh kg−1, respectively [1]. Besides, sulfur is environment friendly with abundant resources in earth, making Li-S batteries even more appealing for commercialization [2,3]. However, it is still difficult to achieve reliable Li-S batteries due to the loss of polysulfides (PSs) resulting from the dissolution-precipitation redox process and sluggish reaction kinetics caused by the inherently poor conductivity of the active materials. Therefore, immobilizing PSs and improving kinetics are of importance to optimize Li-S batteries. Combining sulfur with other materials, such as various carbon materials [1] and metal-based materials, including iron [4,5], nickel [6,7], cobalt [[8], [9], [10]], and vanadium-[11,12] based compounds, is an effective strategy to achieve better performance. Carbon materials can significantly improve the conductivity of the cathode which have been extensively used in Li-S batteries. Proper design of nanoporous carbon structure and sulfur composites can remit the shuttle effect by confining sulfur in the nanopores [13,14]. Metal compounds, whose conductivity is not comparable to carbon materials, have been proved to be able to catalyze the redox reactions of polysulfides, hence, improving reaction kinetics during the electrochemical processes [8,10,15]. Moreover, due to their polar structure, metal compounds can adsorb soluble PSs effectively to inhibit the dissolution of active materials [[16], [17], [18]].

To achieve superior performance, enormous efforts have been made to understand the relationship between the specific characters of the metal compounds, including electronic and electrode structure, and their interaction with sulfur species in Li-S batteries [19]. However, the intrinsic reasons about how these metal compounds influence the electrochemical performance still remain blurry, which impedes the fundamental understanding of the mechanism of Li-S batteries. Among the metal compounds, Ni-based compounds have been studied in electrochemical devices, such as electrochemical catalysis [20] and alkaline-ion batteries [[21], [22], [23]] because they can provide enough active sites and promote redox during electrochemical processes. Researches also applied nickel oxide [6], nickel sulfides [24], and nickel phosphide [25,26] to Li-S batteries due to their ability to effectively storing PSs. As many factors such as the anion, morphology, microstructure, and synthesis method significantly affect the electrochemical performance in Li-S batteries, it is difficult to understand the intrinsic reasons causing various performances among these compounds and make a meaningful comparison for results from different laboratories. Therefore, it is of significance to ensure the coherent experiment conditions including same testing method and identical morphology to reveal the mechanism of these nickel-based compounds on Li-S batteries precisely.

Herein, a group of Ni-based compounds (NixAy (A = O, S, P)) with identical morphology have been synthesized to investigate how the different anions affect the electrochemical performance in Li-S batteries. In order to make the comparison more sheer and correct, pure Ni-based compounds grown on the surface of the carbon cloth (CC) have been synthesized (noted as NixAy@CC) rather than the mixture of the metal compound and nanostructured carbon skeleton. The results show that nickel phosphide (Ni5P4@CC) endows the most impressive performance with the highest specific capacity of 1349.5, 1020.4, 656.7 mAh g−1 at 0.1C, 0.5C, and 3C, respectively. More impressively, only 0.0173% capacity decay ratio per cycle is evidenced upon 2000 cycles at a current density of 3C, showing the excellent cycling stability. A detailed mechanism for this enhancement in performance has been studied. Using density functional theory (DFT) calculation, the adsorption between NixAy (A = O, P, S) and PSs has been analyzed, exhibiting that Ni5P4 possesses the suitable interaction to trap PSs and also favors fast conversion kinetics. NiO/NiS possesses too strong/weak adsorption to PSs, and hence, exhibits worse performances. In addition, electrochemical characterizations, including cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS), reveal that these compounds present significant differences in their electrochemical performances including specific capacity, cycling stability, and redox kinetics.

Section snippets

Results and discussion

The synthesis route of NixAy@CC composites is illustrated in Fig. 1a. Firstly, Ni-OH nanoflakes (Ni-OH@CC) are in situ grown on the surface of CC by a conventional hydrothermal method. After a subsequent thermal annealing under air atmosphere, Ni-OH nanoflakes turn into NiO nanoflakes (NiO@CC). NiS@CC and Ni5P4@CC are synthesized through the conversion of the as-prepared NiO@CC by hydrothermal method and gas-solid reactions, respectively. Fig. 1 b-d show the morphologies of NiO@CC, NiS@CC, and

Conclusion

In conclusion, we synthesized different nickel compounds grown on carbon cloth, including NiO@CC, NiS@CC, and N5P4@CC via simple hydrothermal method and subsequent post treatments. Results show that N5P4@CC presents the highest capacity and most stable cycling performance among these three samples. Through DFT calculation, it has been found that a suitable binding energy to PSs is formed by Ni5P4 rather than NiO (too strong) and NiS (too weak), which can effectively suppress the loss of the

Preparation of Ni-OH@CC

The growth of Ni-OH on carbon cloth is prepared according to the previous research [31]. Typically, 1.35 g of Ni(NO3)2·6H2O, 1.08 g of urea, and 0.75 g of NH4F were dispersed into 60 mL deionized water (DIW) assisted by sonication for 30 min to get a clean green solution. After that, the solution was transferred to a 100 mL Teflon liner. One piece of carbon cloth was set on one piece of glass slide with its upper end and bottom covered by heat-proof tape. Subsequently, the as-prepared carbon

Credit author statement

Kuikui Xiao: Investigation, Data curation, Writing – original draft preparation, Zhixiao Liu: Software, DFT calculation, Zhen Chen: Writing- Reviewing and Editing, Xun Cao: Software, Analysis, Characterization, Zheng Liu: Visualization, Software, Yizhong Huang: Validation, Software, Huiqiu Deng: Validation, DFT calculation, Xiaohua Chen: Supervision, Validation, Ze Xiang Shen: Supervision, Validation, Jilei Liu: Supervision, Validation, Methodology.

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.

Acknowledgments

X. H. Chen acknowledges the financial support from the National Natural Science Foundation of China (51572078, 51772086 and 51872087) and the Scientific Research Fund of Hunan Province (2015JJ2033). J. L. Liu thanks the financial support from the National Natural Science Foundation of China (No. 51802091, 51902102), Creative Research Funds from Hunan Province (Grant No. 2018RS3046), Outstanding Young Research Funds from Hunan Province (2020JJ2004) and the Major Science and Technology Program of

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