Sulfur and Selenium Composites as Cathodes for High Performance Rechargeable Li-S and Li-Se Batteries

Mukkabla, R and M, Deepa (2018) Sulfur and Selenium Composites as Cathodes for High Performance Rechargeable Li-S and Li-Se Batteries. PhD thesis, Indian institute of technology Hyderabad.

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Abstract

Lithium (Li)-ion batteries (LIBs) have now reached a level of maturity in terms of research, understanding and commercialization. However the usable capacities and energy densities of commercial LIBs are not sufficient to meet the ultrahigh specific energy requirements of next-generation electric, hybrid, and plug-in hybrid electric vehicles. Among the various high energy density alternatives to LIBs, Li-sulfur (LiS) and Li-selenium (Li-Se) battery technologies are attractive due to the several advantages they have over LIBs. The theoretical gravimetric and volumetric capacities of S (1672 mAh g -1 and 3277 mAh cm-3 ) and Se (678 mAh g-1 and 3240 mAh cm-3 ) are exceptionally high and so are their energy densities (S and Se: 2600 and 1500 Wh kg-1 ). While S is cheap, abundant and environmentally benign, Se is characterized by a high electrical conductivity, thus rendering them suitable as cathodes for Li-S and Li-Se batteries. The major challenge, however, in both Li-S and Li-Se batteries is the capacity fade: (1) caused by the dissolution and shuttle of polysulfides and polyselenides formed during discharge, and (2) due to the volume expansion that S and Se experience upon Li2S and Li2Se formation, that results in detethering of the active materials from the current collectors. Besides these issues, S is insulating which makes electron movement across the cathode cross-section difficult, thus reducing rate capability. In this thesis, the above-described challenges have been addressed to some extent by (a) the use of novel approaches and (b) composites of S and Se, which had not been reported till date. Sulfur composites were prepared with (a) graphite nanoplatelets (GNPs) by using a combination of in-situ and ex-situ methods wherein the GNPs served as a scaffold for tethering the nano-sulfur (nano-S) particles and (b) with functionalized multiwalled carbon nanotubes (MWCNTols) and coated with a poly(3,4-ethylenedioxypyrrole) (PEDOP). The Li-S/MWCNTols/PEDOP composite with S loading of 70% retained a capacity of 624 mAh gsulfur-1 after 200 chargedischarge cycles, with a Coulombic efficiency of 98.7%. Similarly, Li-Se cells were studied, where hybrids/composites of Se with (a) graphite platelets nanofibers (GPNFs), (b) cetyl trimethylammonium bromide (CTAB) decorated MWCNTs, (c) alkali activated carbon (AAC, derived from rice husk), and (d) conical carbon vii nanofibers (CCNFs), were used as cathodes. In the Li-Se/AAC cell, a tungsten oxide (WO3) interlayer was also implemented to enhance the cycling stability and the Liion storage capacity of the cell. While the WO3 layer conducts Li-ions, it simultaneously blocks the diffusion of dissolved polyselenides (Li2Sen), thereby reducing the capacity fade upon long-term cycling. Similarly, in the Li-Se/CCNFs cell, a poly(carbazole) (PCZ) layer was applied at the cathode to restrict the active material loss via the Li2Sen (n  3) dissolution and crossover. In all of the above the Li-S and Li-Se cells, while carbon nanomaterials by the virtue of their high electrical conductivity, effective surface area and open morphologies improve active material utilization, enable higher S and Se loadings, and confine the Li2Sn or Li2Sen species at the cathode to a good extent, further modification on the cell architecture (use of conducting polymer or WO3 layer), increases the cycle life, rate capability and capacity of the cells. Through elaborate structural and electrochemical studies, this thesis mainly focuses on the synthesis of Se and S based materials and mechanisms to address to some extent, the present limitations of the Li-S and Li-Se batteries.

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