Elsevier

Electrochimica Acta

Volume 305, 10 May 2019, Pages 247-255
Electrochimica Acta

Deposition of thin δ-MnO2 functional layers on carbon foam/sulfur composites for synergistically inhibiting polysulfides shuttling and increasing sulfur utilization

https://doi.org/10.1016/j.electacta.2019.03.061Get rights and content

Abstract

Practical application of Lisingle bondS battery is greatly impeded by the critical challenges of large volume expansion, intrinsic insulation problem of active materials, and serious polysulfides shuttling. Thus, it is still quite essential for us to search an efficient strategy to synergistically overcome these problems. Herein, we design and in-situ deposit a functional δ-MnO2 layer onto the outside surface of porous carbon foam/sulfur composites to synergetic inhibit the polysulfides shuttling and increase sulfur utilization, finally improve the electrochemical performance of Lisingle bondS batteries. As verified by UV–vis and XPS results, the strong chemical interaction between the MnO2 layer and polysulfide intermediates is characterized to be an internal disproportionation reaction by transferring the absorbed long-chain polysulfides into insoluble polythionate complex. Also, the unique structure possesses high conductivity and good volume expansion tolerance, further guaranteeing the electrode a good cycling stability, rate performance and low self-discharge behavior. The MnO2 coated carbon foam/S electrode displays a remarkable capacity of 1547 mA g−1 at 0.1 C, and achieves a reversible capacity of 629 mA g−1 after 300 cycles at 1 C. Even with sulfur loading of 2.6 and 4.3 mg cm−2, the designed electrodes still deliver an initial discharge capacity of 1106 and 963 mA g−1 at 0.2 C.

Introduction

Lithium-sulfur (Lisingle bondS) batteries are widely regarded as a promising candidate for next-generation energy storage systems owing to its high energy density, low cost and environment benignity. It is well known that Lisingle bondS batteries can deliver an ultra-high theoretical specific capacity of 1675 mA g−1 and energy density of 2600 Wh kg−1 through the redox reaction of S+2LiLi2S(1). However, there are still many challenges impeding its practical application. Firstly, there will be as large as 80% volumetric exchanges during the electrochemical reaction, which will induce a disastrous collapse in the electrode structure. Secondly, sulfur and the discharge product of Li2S are electronic and ionic insulators, further leading to a low specific capacity and poor rate performance. Moreover, the soluble lithium polysulfides will shuttle between the positive and negative terminals, which will lead to the loss of active materials in cathode, vast capacity fading, short life spans and poor stabilities [[1], [2], [3]]. Thus, searching for an efficient strategy to accommodate the volume changes and enhance the conductivity as well as prohibit the polysulfides shuttling are significantly required, which are still the center topics of current Lisingle bondS technologies.

Nowadays, conductive porous carbon materials coupling with stable composition, high specific surface area, good electronic conductivity and adjustable architectures are pervasively employed as sulfur host materials to improve the energy storage performances [[4], [5], [6]]. However, only limited physisorption effects existed between pure carbon and polysulfides owing to the non-polar and non-affinity features, which could not fulfill the demands of high performance Lisingle bondS batteries. Recently, researchers have reported that by employing heteroatoms such as nitrogen, phosphorus, oxygen and boron doped/co-doped carbon materials and/or other surface functionalized carbonaceous materials as sulfur hosts could introduce strong chemisorption interaction with polysulfides, thus contributing to high specific capacities, long-term stability as well as good rate capabilities [[7], [8], [9], [10]]. The doping atoms with different electronic structures could polarize carbon materials by affecting the electronic cloud distribution of carbon atoms, further improving the chemical affinity of the host material for polysulfides. Generally, these heteroatoms serve as polysulfides-trapping centers, dominated through the formation of Li-X bonds (X refers to the doping element, such as N, P, O, etc.) rather than directly interacting with S atoms in the lithium polysulfides complex to inhibit polysulfides intermediates shuttle [[11], [12], [13], [14], [15]]. This chemical inhibition is closely related to the doping amount, somehow effective but inadequate. The reason is that heteroatoms doping will usually reduce the specific surface area of carbon matrix, leading to a low S mass loading (less than 60%) and lowering the energy density of the system. Additionally, the doping strategies are always intricate with high power and time consumption, which is not suitable for practical application [[16], [17], [18]].

Very recently, the polar materials were also employed to retain the active materials within the cathode due to the strong chemical bonding between these polar hosts and polysulfides complex with polarity characteristics [[19], [20], [21], [22], [23], [24]]. It was already verified that polar transition metal oxides such as VO2 and δ-MnO2 with a redox potential in the range of 2.4 V < E° ≤ 3.05 V (vs. Li+/Li) could trigger an internal disproportionation reaction to transfer the absorbed long-chain polysulfides into polythionate complex on their surfaces of metal oxides. During the subsequent reactions, the polysulfides will be continually anchored onto the long chain of insoluble polythionate complex until the final formation of Li2S2 or Li2S. This anchoring effect is more direct with high-efficiency which is known as the “Wackenroader reaction” (equation (1)) [25,26]. However, the deficiency is that these polar materials are always electronic insulating, for example, the electronic conductivity of MnO2 is as low as 10−6 S cm−1, which is not suitable for individually applied as sulfur host with high active mass loading [[27], [28], [29]].

Intriguing by the above concerns, it is interesting and hopeful to composite carbon materials and the polar transition metal oxides to construct a synergetic sulfur host materials with high conductivity, strong physical adsorption and chemical bonding effects to polysulfides [30]. Thus, herein, we construct an efficient sulfur host by in-situ depositing δ-MnO2 onto the surface of carbon foam/sulfur composites with optimal conductivity and chemically absorption capability. The deposition process, achieved by reacting of carbon foam/sulfur composite in potassium permanganate solution in a time as short as 10 min,is highly simple, facile and efficient, holding great promise for scalable synthesis. The UV–vis and XPS results identify the strong chemical interaction between δ-MnO2 layers and sulfur atoms in polysulfides, which could inhibit the shuttling effects directly and powerfully. As a result, the δ-MnO2@CF/S materials could achieve a specific capacity of 1547 mA g−1 at 0.1 C and a promising cyclic and rate performances with 627 mA g−1 after 300 cycles at 1 C, referring to 83% of the capacity retention.

Section snippets

Synthesis of carbon foam (CF)

The precursors of CF were prepared by a cost-effective Pechini approach. [31] Typically, 3.84 g citric acid and 1.48 g Mg(NO3)2 were added into 10 ml deionized water under sonication for 30 min. The obtained homogeneous mixture was further transferred into an oven for gelation at 120 °C for 24 h. The obtained orange polymer gel was annealed at 800 °C in argon flow for 60 min. After thoroughly removing the decomposition products by 1 M H2SO4 and repeated washing with DI water, the grey CF

Results and discussion

The synthesis procedure of δ-MnO2@CF/S cathode material is schematically illustrated along the Taiji circle in Fig. 1. The modified Pechini method was used to prepare the polymer gel precursor (Fig. S1) [31]. After annealing and etching, the black and loose CF could be employed as sulfur host materials directly. Sulfur was impregnated into the pores of CF via a melt-diffusion strategy with ∼65 wt% loading in the S/CF composite. At last, the CF/S composite was immersed in a 0.05 M KMnO4 aqueous

Conclusion

Aiming at enhancing the electrochemical performance of the cathode for a potential practical application, we construct an efficient sulfur host by in-situ depositing δ-MnO2 onto the surface of carbon foam/sulfur composites with optimal conductivity and chemically absorption capability. The deposition process, achieved by reacting of carbon foam in potassium permanganate solution in a time as short as 10 min,is highly simple, facile and efficient, holding great promise for scalable synthesis.

Supporting information

Additional figures, such as photo graphics, elemental mapping, EDS spectra, XPS results, electrochemical results, TG curve, could be found in Supporting Information Section.

Notes

The authors declare no competing financial interest.

Acknowledgment

This work is financially supported by Fundamental Research Funds for the Central University (No. 2018CDYJSY0055, 106112016CDJZR325520), China Postdoctoral Science Foundation (2018M633316), the National Natural Science Foundation of China (No. 21503025, 21603019), Key Program for International Science and Technology Cooperation Projects of Ministry of Science and Technology of China (No. 2016YFE0125900), Venture & Innovation Support Program for Chongqing Overseas Returnees (cx2017060, cx2017115

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