Improving Electrochemical Performance in Lithium-Sulfur Batteries using Carbonaceous Materials

Abstract

Recently, the increment of energy consumption of high technological advancement and the requirement of environmentally-friendly energy sources make a necessity of new conceptual energy source. Notably, among various approaches for promising energy sources, electrochemical energy storage system seems appealing for its high energy conversion efficiency and low product of pollutant. Lithium-sulufur secondary battery is one of the most promising electrochemical redox couple system for storage chemical energy directly to electrical energy. Although a plenty of research has been conducted in this field, several issues still remain, including its low electrical conductivity, irreversible loss of polysulfides and volume expansion during battery cycling. In this thesis, the various carbonaceous materials was controlled to solve this issues. In chapter 1, the electrochemical conversion system based on lithium anode and sulfur cathode is introduced. The selected terminologies are formally refered, and the history of lithium based secondary battery is briefly explained. In addition, the advantage and drawback of lithium sulfur battery system are mentioned and its theoretical reaction mechanism based on previous studies also simply noticed. Moreover, there are the concepts of various research papers and its solving approaches. Finally, the experimental conditions of conventional battery test and advanced mesaurements are reported in this thesis. In chapter 2, the electrochemically critical parameters in the Li-S battery, the overpotential (ΔV), the capacity from the dissolution region (Q1), and the capacity from the precipitation region (Q2), are identified to trace the electrochemical behavior of the electrode during the charge/discharge operation, which can aid in the deep understanding of the enhancement mechanism in the different model situation. The effect of cycling rate, conductive additive content, and oxygen functional group on the battery performance has been studied as the model systems. In this study, it is suggested that cycling conditions should be carefully considered and critical parameters are derived when exploring the performance of Li-S battery or designing batteries based on a new concept or novel architecture. In chapter 3, we have synthesized GO-S/CB composites that micron-sized sulfur particles are encapsulated by GO sheets. The structural properties and chemical properties of GO-S/CB composites were characterized by various microscopic and spectroscopic techniques. Various electrochemical analyses were conducted to elucidate the role of GO that has rich oxygen functional groups and its effect on the electrochemical properties. The charge-discharge profiles revealed the significantly enhanced cycling and rate performance of the GO-S/CB electrode, indicating that GO plays a key role in trapping dissolved polysulfide and in improving electronic conductivity. Therefore, in chapter 4, we have designed GQDs-S/CB composites as a high-performance cathode material for Li-S batteries. The nano-sized GQDs induce a tightly packed structure via charge interaction with S and CB, which results in enhanced conductivity by shortened electron conduction paths. Furthermore, C-S bonding is generated in-situ during the operation of the battery, which originates from the high functional-edge density of the GQDs. Thus, loss of active materials into the electrolyte is minimized. The adsorption of nano-sized sulphur particles onto the GQD interfaces by C-S bonding was confirmed by TEM, and further supported by XPS and Raman analysis and DFT calculations. The GQDs-S/CB composites significantly improve cycling and rate performances, with high reversible capacities at both high and low current density. This excellent cycling behavior was demonstrated through the analysis of discharge profiles. We believe that our results provide a new avenue for material scientists to tailor oxygen-rich functional groups of nano-sized carbon for the application in various batteries. In chapter 5, the synthesis of sulfur copolymers via inverse vulcanization for enhanced cathode materials in Li?S batteries is reported. We demonstrate that this inexpensive, bulk copolymerization can sufficiently modify the properties of sulfur to improve the battery performance without the need for nanoscopic synthesis or processing. This system also demostrates for the first time that high capacity polymeric electrodes can be fabricated while also suppressing capacity fading after extended battery performance to 500 cycles. In chapter 6, the SDrGO chemically synthesized the DIB and sulfur with reduced graphene oxide. To make uniformly incorporated structure, oleylamine (OLA) functionalities are applied on the synthesis method. The S-C boning from DIB support the wrapping of soluble polysulfides and reduced grapehene oxide support the increased electrical conductivity, which make improved cycling and rate performance. Various electrochemical techniques support the deeply understanding for analysing reaction phenomena on this system. In chapter 7, the structural integrity at the nanoscale of S-P3HT/CB accounts for the enhanced rate capability by shortened diffusion length of reactant. In summary, we introduced the copolymerization of allyl-terminated P3HT with sulfur enabled by a radical reaction between the allyl end-group and a radical sulfur species. This approach allows the covalent linkage of sulfur and P3HT yielding in S-P3HT copolymer homogeneously distributed in a sulfur matrix. The homogeneous incorporation of this semiconducting polymer lowers the electrical resistance, thus, an improved battery performance can be observed for S-P3HT copolymer containing electrodes. In chapter 8, PEO/PAA multilayers on sulfur electrode effectively improved the capacity retention of lithium-sulfur batteries, by successful protection of polysulfide from irreversible loss. This simple and inexpensive method is expected to be widely utilized in various types of electrochemical devices. Future work for further optimization of electrochemical performance is currently underway by nanostructural tailoring of surface layers.