Elsevier

Journal of Energy Chemistry

Volume 48, September 2020, Pages 364-374
Journal of Energy Chemistry

Attapulgite nanorods assisted surface engineering for separator to achieve high-performance lithium–sulfur batteries

https://doi.org/10.1016/j.jechem.2020.02.030Get rights and content

Abstract

Lithium–sulfur (Li–S) batteries have been recognized as one of the most promising candidates for next-generation portable electronic devices, owing to their extremely high energy density and low cost. However, the dissolution of lithium polysulfides (LiPSs) and consequent “shuttle effect” seriously hinder the practical deployment of Li–S batteries. Herein, multi-metal oxide nanorods named attapulgite are proposed as multifunctional ionic sieve to immobilize LiPSs and further promote the regulation of LiPSs. Attapulgite, consisting of Al, Mg, Fe, Si and O ions, possesses more polar sites to immobilize LiPSs in comparison with single metal oxides. In addition, the catalytic nature (Fe ions) of attapulgite avails the LiPSs conversion reaction, which is further confirmed by the linear sweep voltammetry and electrochemical impedance spectroscopy. Benefited from the synergistic effect of multi-metal oxide and conductive carbon, the Li–S battery with the modified separator delivers remarkable discharge capacities of 1059.4 mAh g−1 and 792.5 mAh g−1 for the first and 200th cycle at 0.5 C, respectively. The work presents an effective way to improve the electrochemical performance of Li–S batteries by employing attapulgite nanorods assisted separator surface engineering.

Graphical abstract

Attapulgite nanorods have excellent chemical adsorption and physical barrier to LiPSs. Besides, the high catalytic nature of attapulgite further promotes the LiPSs conversion. Therefore, the battery with attapulgite-modified separator shows excellent electrochemical performance.

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Introduction

With the wide application of power vehicles and large-scale energy storage systems, a considerable amount of requirement has been placed on the performance of the chemical power supply, especially on the energy requirements of the battery. Based on S8 + 16Li = 8Li2S, rechargeable Li–S batteries have received extensive attention due to their high theoretical capacity (1675 mAh g−1), which is much higher than those of current commercial secondary batteries [1], [2], [3]. In addition, elemental sulfur has the advantages of non-toxicity, abundant reserves and low cost, offering Li–S batteries have great commercial application prospects. Despite these remarkable metrics, the development of Li–S batteries is still hindered by following problems: (i) Due to the insulation nature of elemental sulfur and discharge products Li2S2/Li2S, a large amount of conductive material needs to be added, resulting in a decrease in the actual energy density of the Li–S batteries [4,5]. (ii) The intermediate phase LiPSs generated on the cathode side during discharge are highly soluble in the electrolyte, causing fast capacity degradation, low coulombic efficiency and corrosion of the metal lithium [6,7]. (iii) The different densities of sulfur and Li2S2/Li2S may cause severe expansion and contraction of sulfur cathode during lithiation and delithiation processes, resulting in cracking of the electrode structure [8].

In order to address these issues, a diverse set of strategies have been explored on the design of cathode materials with different structures [9], [10], [11], anode protection [12], [13], [14], [15], [16], [17] and new electrolyte systems [18,19], which effectively improved the electrochemical performance of Li–S batteries. In addition, constructing a functional interlayer between cathode and separator is also an effective method for inhibiting the migration of soluble LiPSs [20], [21], [22]. Among that, the commonly used functional interlayer are various porous carbonaceous materials [23], [24], [25], [26], [27], which can physically adsorb LiPSs and improve the utilization rate of active materials. However, the interaction between non-polar carbon materials and polar LiPSs by Van der Waals force is weak, causing limited capture and reuse of LiPSs during long cycling. Recently, the employment of polar metal oxides for the chemisorption of LiPSs to suppress the shuttle effect has attracted much attention for Li–S batteries [28], [29], [30], [31], [32]. Nevertheless, the low adsorption capacity and poor conductivity of metal oxides weakens the conversion kinetics of sulfur species [33], and it is difficult for mono-metal oxide to provide enough active sites to anchor LiPSs [34]. Therefore, it is promising to utilize the multi-metal oxide with chemisorption on LiPSs for the restriction of the shuttle process [35], [36], [37], [38].

Attapulgite, natural multi-metal oxide nanorods, can be used as adsorbents for water purification due to its high surface area and porosity feature. For the energy storage, attapulgite has been employed as the anode of lithium-ion batteries [39,40] and the sulfur host of lithium sulfur battery [41] due to its excellent chemical stability and strong adsorption capability. In light of this contribution, attapulgite nanorods could be a multifunctional ionic sieve interlayer to suppress the “shuttle effect” and further guide LiPSs management in working Li–S batteries. Moreover, the conductive acetylene black has been combined to promote electron transfer, as shown in Scheme 1. The composite of attapulgite nanorods and acetylene black (AB) possesses several metrics: (i) Compared with single metal oxides, attapulgite are able to provide more abundant polar sites to immobilize LiPSs. (ii) Variable and high-valence Fe ions introduced from attapulgite can further avail the LiPSs conversion [42]. (iii) The conductive network of AB can allow fast electrons transition between LiPSs and AB, which improves the utilization rate of active materials. Meanwhile, we found the relative amount of polysulfide adsorbent and the conductive agent plays an important role in the performance of Li–S batteries, which agrees with our previous work [43]. Profiting from the synergistic effect between multi-metal oxide and conductive carbon, the battery with attapulgite-modified separator holds remarkable discharge capacities of 1059.4 mAh g−1 and 792.5 mAh g−1 for the first and 200th cycle at 0.5 C, respectively.

Section snippets

Synthesis of attapulgite/acetylene black composites

The 3% polyvinylidene fluoride (PVDF) solution was prepared by adding an appropriate amount of PVDF into N-methylpyrrolidone (NMP) solvent, followed by magnetic stirring for 30 min. A specific weight ratio of attapulgite (haiyangfenti. Corp., China) and acetylene black (AB, Jinpu. Corp., China) were dispersed in PVDF solution, and then grinded with a ball mill for 5 h to form a homogeneous slurry.

Preparation of the modified separator

The synthesized slurries were coated evenly on one side of PP separator (Celgard 2325) using a

Results and discussion

Fig. 1(a) shows the microstructure of attapulgite checked by SEM, exhibiting interlaced nanorod-shaped structure. From the magnified SEM images (Fig. S1), the diameter of attapulgite is from 20 nm to 100 nm. Fig. 1(b) presents the TEM image of attapulgite nanorods. The HRTEM images in Fig. 1(c) further confirm the nanorod possesses the minimum diameter of around 20 nm. Such a nanoroidal structure has the large surface area and then exposes lots of active sites for trapping LiPSs. The STEM image

Conclusions

In summary, attapulgite nanorods are proposed to assist separator modification, which can effectively suppress the “shuttle effect” and further guide LiPSs management in Li–S batteries. Rich polar sites on the attapulgite surface and the catalytic nature of attapulgite can promotes the LiPSs conversion. In addition, benefiting from the admirable conductivity, the carbon network has been also designed to decreases the interfacial resistance and improves the utilization rate of active materials.

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

This work was supported by the National Natural Science Foundation of China (No. 51861165101, 51822706, 51777200), Beijing Natural Science Foundation (No. JQ19012) and DNL Cooperation Fund, CAS (DNL201912).

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