Research Article
MOFs-derived integrated flower shaped porous carbon anchored with core-shell Ni-NiO nanoparticles as efficient multifunctional electrode for Li–S batteries

https://doi.org/10.1016/j.jallcom.2022.166764Get rights and content

Highlights

  • Carbon paper as a highly conductive substrate for 3D integrated electrode.

  • Designed a MOFs-derived Ni-NiO/NC multifunctional sulfur host.

  • Explored the catalytic activity of Ni-NiO nanodots on different sulfur species.

  • The influence of catalyst on system overpotential is estimated.

Abstract

Designing electrodes with structures suitable for storing and fixing sulfur to overcome the inherent shortcomings of lithium-sulfur (Li–S) batteries is an effective approach to achieving commercial application. Indeed, multifunctional electrodes constructed with sulfur immobilization substrate and efficient catalyst has dramatically improved the utilization of sulfur. However, structurally designing the substrate and achieving uniform dispersion of catalytic remain huge challenges. In this paper, a metal-organic frameworks derived flower shaped nitrogen doped carbon (NC) materials anchored with core-shell Ni-NiO nanoparticles loaded on carbon paper (CP) was fabricated, which equipped with 3D conductive network and hierarchical porous structure physically restrict polysulfide shuttling. Furthermore, well-proportioned Ni-NiO nanoparticles were induced into host to bond with polysulfides further catalyzed the electrochemical reaction. Notably, the S@Ni-NiO@NC/CP exhibited high specific capacity with 1332.9 mAh/g initial specific capacity at 0.5 C and maintained at 896.3 mAh/g after 200 cycles. And S@Ni-NiO@NC/CP can achieves an initial capacity of 1095.7 mA h g−1 even with high sulfur loading of 4.6 mg/cm2. Electrochemical test results indicated that the introduction of Ni-NiO catalyst can improve the kinetics and reduce the reaction overpotential (ηE), and further affects the value of concentration overpotential (ηC) ultimately leads to the increase of battery capacity.

Introduction

As emerging energy storage systems, Li–S batteries have great potential to be used in mobile power sources and small power stations, since the ultra-high theoretical specific capacity (1675 mAh/g) and energy density (2600 Wh/kg) which is four times as high as commercial lithium-ion batteries [1], [2], [3]. Meanwhile, besides inexpensiveness, Li–S battery is non-toxicity and ease to storage, which contributes to reduce the battery cost. Nevertheless, the limited lifespan, poor rate performance, and low practical energy density adversely impact their commercial application. Apart from the insulating nature [4], [5], [6], the volume change of the active materials during charge-discharge [7], [8], and the formation of lithium dendrites [9], [10], the performance of Li–S batteries were influenced mainly by the irksome “shuttle effect” [11], [12], [13] and slow kinetics of sulfur conversion [14], [15]. The production, dissolution and accumulation of polysulfides (LiPSs) in electrolyte and cannot be effectively converted into short-chain sulfides resulting in loss of active material and low-rate capacity.

In recent years, most of the research work focuses on how to restrain the so-called “shuttle effect”. Currently, the popular strategy involves the following aspects: (1) adsorption methods including chemical adsorption and physical adsorption, (2) limiting the diffusion of polysulfide though pore structure, (3) modified separator to limit the movement of polysulfides toward the lithium cathode, and (4) electrocatalytic strategy, catalysts are embedded in the sulfur electrode to accelerate the conversion of polysulfide to sulfide. Based on the above strategies, many research efforts have been dedicated to the development of organic combinations of the porous carbon and catalytic materials as a comprehensive solution to the problems described above. The composites of transition metal compounds (TMCs) and carbon material has achieved a great improvement in battery performance, such as TiN-VN heterostructures doped carbon nanofibers [16], CoS2 embedded multichannel carbon nanofiber [17], 1 T-MoS2 nanotubes wrapped by N-doped grapheme [18] and so on. However, they are inevitably faced with the large particle size and uneven distribution of catalyst in the matrix through the secondary load. At the same time, the unstable binding between the TMCs and the conductive substrate leads to the easy detachment of the catalyst from the surface of the conductive substrate, resulting in a decrease of the catalytic effect.

According to above strategies, cathode materials have the following characteristics to demonstrate excellent performance: (1) The highly conductive sulfur host has a proper pore structure including a large pore volume to store sulfur, a higher specific surface area to provide abundant reaction places and a suitable pore size to physically alleviate the “shuttle effect”; (2) Sufficient electrocatalysts are uniformly and firmly anchored into the conductive substrate. Therefore, the construction of sulfur anode requires precise structural design and careful material selection to meet the multifunctional requirements.

In recent years, research have confirmed that metal-organic frameworks (MOFs) with suitable pore structure [19], [20], [21], [22] can derive to heteroatom-doped porous carbon. Moreover, the exposed heteroatoms and metallic sites may be ideal candidate for multifunctional sulfur host materials though high-temperature calcination [23], [24], [25], [26]. Chu et al. synthesized an In-MOFs at room temperature, after carbonization and activation at high temperature, obtained three-dimensional cotton-padded layered porous carbon (CPHPC) stacked by hexagonal highly graphitized carbon nanosheets [27]. The carbon conductive network of CPHPC has superior electrical conductivity and effective limitation of polysulfide produced by sufficient contact between carbon matrix and sulfur. Finally, the material only has 0.014 % capacity loss rate after 3800 cycles even at high current density of 10 C.

The transition metal oxide (MxOy, M = Ni, Cu, Co, etc.) with rock structure possess the empty d orbital can accept the lone electron pair of the polysulfide. So that a coordination bond can be formed between polysulfide and metal which act as catalytic center to fix and smooth the conversion of sulfur-containing substance [14], [28], [29]. NiO materials, as a kind of metal oxides, are widely used in the cathode of Li–S batteries on account of the variable valence state of Ni center, can provide high electrocatalytic activity. Xu et al. synthesized an efficient catalysis sulfur host material consists of ZnO (core) and Co-doped NiO (shell) [26]. The result shows high discharge capacity of 738.56 mAh g−1 after 500 cycles at 0.5 C with slight capacity fade of 0.048 % per cycle. Even at 1 C, 501.05 mAh g−1 could be retained after 500 cycles, and the capacity retention rate was 62.13 %. However serious electrically inert and small active surface area prevents it becoming a highly efficient electrode material. Therefore, Ni-NiO core-shell structure was constructed as high efficiency electrode material to supply rapid electron conduction and catalytic active sites [30], [31], [32], [33]. At the same time, the close contact and electron interaction between metal and metal oxide can reinforce the intrinsic catalytic activity of catalyst and improve the utilization rate of catalyst [34]. Therefore, Ni-NiO with core-shell structure can smooth the transfer of sulfur substances and improve the performance of lithium sulfur batteries.

In this paper, nickel-based MOFs particles were grown on carbon paper (CP) implementing the hydrothermal method to build an integrated material precursor (Ni-MOFs/CP). Through further pyrogenic decomposition, Ni-MOFs/CP was converted to porous carbon doped with Ni(core)-NiO(shell) (Ni-NiO@NC/CP), as schematically described in Fig. 1. For comparison, Ni and NiO doped porous carbon integrated materials (Ni@NC/CP and NiO@NC/CP) were also used as the electrodes. Experiments have shown that the synthesized Ni-MOFs precursor has a flower-like morphology, which remains after thermal treatment. The 3D conductive network connected all sulfur storage units with low connections resistance to resist the fragmentation of the conductive network. The hierarchical pores created by the pyrolysis of MOFs can be used to store and confine sulfur, as well as to mitigate the electrode damage caused by volume expansion. Moreover, the pores generated by MOFs blocks were filled with electrolytes to provide an efficient transmission channel for lithium ions. The core-shell Ni-NiO nanoparticles with high catalytic activity are uniformly distribute in the carbon matrix, forming a superior host for the rapid transformation of polysulfide. Notably, emergence of oxygen-containing groups limits diffusion of polysulfide to the lithium anode by electrostatic repulsion and assists the migration of Li+ to the surface of the electrode increasing the rate of electrochemical reactions [35]. Hence, compared with the S@Ni@NC/CP, and the S@NiO@NC materials, S@Ni-NiO@NC/CP can achieve the highest discharge specific capacity of 1322.9 mAh/g and lowest capacity loss rate (0.16 % per cycle) after 200 cycles. At the same time, the specific capacity of the S@Ni-NiO@NC/CP can still show great specific capacity at high rate of 2 C.

Section snippets

Results and discussion

The morphology and component of as-prepared materials were examined. The SEM figure shows that the raw CP base was weaved by carbon fiber with a uniform diameter of about 6.7 µm (Fig. S1a). After solvothermal reaction, the flower-shaped Ni-MOFs particle with a diameter about 29 µm uniformly anchors on the CP almost filling the gaps between the carbon fiber as shown in Fig. S1b. Compare with the Ni-MOFs/CP precursor, Ni-NiO@NC/CP (Fig. 2a) and Ni@NC/CP (Fig. S1c) basically maintain their

Conclusion

In brief, Ni-NiO nanodots embellished porous carbon integrated material Ni-NiO@NC/CP with N-atom and oxygen-containing groups co-doping was manufactured through simple solvothermal and pyrolysis craft. The integrated electrode structure with CP as current collector links MOFs-derived sulfur storage host (Ni-NiO@NC) can provide low contact resistance. Ni-NiO@NC with reasonable pore structure can limit the polysulfide shuttle through physical action and the introduction of oxygen defect sites is

CRediT authorship contribution statement

Qian Guo: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Data curation, Writing – original draft, Project administration. Xiaoxiao Liu: Resources, Data curation, Formal analysis. Xiaotao Ma: Investigation, Resources. Yu Li: Resources, Funding acquisition. Donghong Duan: Investigation, Funding acquisition. Xianxian Zhou: Investigation, Writing – review & editing, Funding acquisition. Fuxiang Li: Investigation, Resources, Funding acquisition. Shibin Liu:

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.

Acknowledgements

This work was supported by the Debrider and Ecological Engineering Technology Co., Ltd, the Applied Basic Research Program of Shanxi Province (201901D211064 and 0302123118), the Natural Science Foundation of Shanxi Province (201903D321056), and National Natural Science Foundation of China (22178244 and 2217080894).

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