Hybrid polyacrylamide/carbon coating on sulfur cathode for advanced lithium sulfur battery
Introduction
Lithium sulfur battery is a promising energy storage device because of theoretical capacity of up to 1673 mAhg−1 based on the complete conversion from S to Li2S [1]. The merits of abundant sulfur source, low cost and environmental friendliness are also attractive considering current situation of resource depletion and environmental pollution around the world. Nevertheless, the utilization of active material encounters great challenge due to the insulating nature of elemental sulfur and its discharge products Li2S. Additionally, the volumetric change between lithiated and non-lithiated sulfur species can cause pulverization and structural damage of the electrode [2]. What’s more, the high solubility of intermediate polysulfides in liquid electrolytes will lead to undesired shuttle effect and corrosion reaction to Li anode [3]. These shortcomings are the main reason of rapid capacity decay and low coulombic efficiency of the battery.
To address these problems, a variety of strategies on modifying sulfur cathode have been attempted [4]. Combining sulfur with conductive carbons and polymers has been intensely investigated [5], [6], [7] because these matrices can trap the soluble polysulfide and accommodate the volume change to some extent, as well as act as conductivity enhancers. However, polysulfide dissolution and structural deterioration are always inevitable during long-term cycle for these composite designs. Another alternative approach is introducing a surface layer on the cathode, which has been proved to be effective to confine the polysulfides within the cathode zone and improve the capacity and cyclability of the cell [8]. However, the defect of either bad flexibility or low (even non-) conductivity [9], [10], [11] weakens the contribution of the coating to cell performance. Meanwhile, some studies are involved with complicated fabrication technology [8], [11], unsuitable for large-scale production. Besides, few reports had paid attention to further explore the mechanism of restricting polysulfides brought about by the extra coating.
Herein, a commercialized conductive slurry consisting of polyacrylamide (PAM) and two kinds of carbon black were first introduced as the surface coating of sulfur cathode. Generally, PAM [12], [13], [14], [15] is a water-soluble polymer with good adhesivity and hydrophilicity, favoring a flexible and stable framework owing to the intertwined polymer chain and strong hydrogen bond interaction from amino group (-NH2). Hence it should be appropriate that coating PAM-contained slurry on the cathode surface towards the liquid electrolyte. The comparative study evaluates the effect of cathode construction and coating type on the electrochemical performance of Li-S cells. It is found that the novel PAM/C hybrid coating is capable of restricting polysulfides by a combination of chemical and physical interaction (that is, dual polysulfide-restricting strategy), and promotes the utilization of active material by providing a highly-conductive active surface.
Section snippets
Synthesis of S/C Composite
A calculated S/C composite containing 75 wt% sulfur (Alfa, 99.5%) and 25 wt% carbon nanotubes (CNTs) were thoroughly milled and mixed in an agate mortar. The homogeneous powder was transferred into a sealed stainless steel autoclave and placed in a tube-type furnace at 155 °C for 12 h, following at 250 °C for 2 h, then cooled down to room temperature to obtain S/C composite.
Cathode fabrication and cell assembly
The conductive slurry was provided by Shenzhen Perfect Power Technology Co., Ltd, consisting of polyacrylamide (PAM) as polymer
Results and discussion
The dried PAM/C slurry was characterized by FTIR spectra and the result is shown in Fig. 1. The absorption peak at around 3340 cm−1 and 3198 cm−1 is ascribed as the free and associated amino group (-NH2), respectively. The band centered at 2941 cm−1 and 2861 cm−1 is characterized to asymmetric and symmetric stretching vibration of methylene (CH2), respectively. The peak appearing at 1650 cm−1 and 1612 cm−1 represents for amideI(CO stretching vibration) and amideII(NH bending vibration), respectively.
Conclusions
A novel sandwich-structured sulfur cathode with hybrid PAM/C coating is fabricated for Li-S batteries. A high initial capacity of 1351 mAh g−1 and reversible capacity of up to 900 mAh g−1 after 300 cycles are demonstrated in this coated cathode. The synergistic effect of physical confinement by surface coating and chemical bonding between heteroatoms in the coated polymer and sulfur species effectively suppresses the migration and shuttle of polysulfides, which is considered to be an important role
Acknowledgments
The authors thank the financial support of National Natural Science Foundation of China (51474243 and 51574288) and Natural Science Basic Research Plan in Shaanxi Province of China (No. 2016JQ5040).
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These authors contributed equally to this work.