Nanostructured Sulfur and Composites for Lithium-Sulfur Batteries

INTRODUCTION

Over the past decade emphasis has been laid on environmentally clean technology in the automotive industry due to which the market for secondary batteries has considerably evolved. Lithium ion battery has dominated the field of secondary batteries and the technology has evolved at a bludgeoning rate over the past few years to meet the ever growing energy needs of consumers. However, with the pressing demand for increasing the specific energy storage capacity in automotive industry, the current focus of research has shifted towards the development of lithium sulfur (Li–S) batteries owing to the high theoretical specific capacity exhibited by sulfur as compared to the cathode materials currently present in the market. Li–S battery shows a theoretical capacity of 1675 mAh/g corresponding to the formation of Li₂S during lithiation which makes sulfur a promising electrode to replace the layered transition metal oxides (~150 mAh/g) and LiFePO₄ (~170 mAh/g) used in the present systems. The theoretical specific energy and volumetric energy density of Li – S system is ~2.6 kWh/kg and ~2.8 kWh/L corresponding to the complete formation of Li₂S(1). However, Li–S systems suffer from poor performance issues, high capacity fade rate and failure due to the dissolution of lithium polysulfide and their high mobility in the electrolyte; insulating nature of sulfur and Li₂S; volumetric and morphology change of the sulfur electrode; and shuttling of polysulfide between the negative and positive electrodes leading to the passivation of the surface of the electrode.

In recent years researchers have developed porous carbon – sulfur nanocomposites(2), graphene oxide – sulfur(3), constrained sulfur in hollow carbon nanofiber(4) and polymer encapsulated sulfur particles(5) to improve the performance of Li – S system.

In the current study, sulfur nanoparticles (S_NP) have been synthesized by wet chemical methods in various media and coated with conductive polymer to improve the conductivity and prevent the dissolution of polysulfides in the electrolyte. Subsequently, the active material was mixed with Super P – binder and slurry coated on aluminum substrate to be used as cathode. Electrochemical characterization of these electrodes was carried out for Li/Li+ ion in a voltage range of 1.5V – 2.4V in a 2025 coin cell using 1 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) in1,3-dioxolaneand 1,2-dimethoxyethane (volume ratio 1:1) as electrolyte.

X-Ray diffraction studies show the presence of sulfur and carbon in the active composite material prepared by high energy mechanical milling of S_NP–graphite and precipitation of S_NPon graphite as shown in Fig 1. Furthermore, scanning electron microscopy and transmission electron microscopy were carried on initial active material and electrochemically cycled electrodes to determine the composition and stability of the electrodes. Charge – discharge characteristics, cyclability, rate capability and fade rate of the active material have been studied as a potential cathode material for Li – S battery. Results of these studies will be presented and discussed.

References

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