Building high-energy density lithium-sulfur batteries
The increasing consumption of limited fossil fuel resources is exerting much pressure on the modern society. Renewable energies, such as solar and wind, are attracting much attention; however, efficient use of these sustainable energies requires economical and efficient electrical energy storage (EES) systems. Among the various possibilities of EES systems, lithium-sulfur (Li-S) batteries are attracting much attention due to their high energy density and low cost. However, the practicality of Li-S technology is hindered by technical obstacles, such as low sulfur utilization and fast capacity decay, caused by the insulating sulfur and the shuttling of corrosive polysulfides. Much progress has been made in recent years towards enhancing the performance of Li-S batteries, but further understanding on the Li-S battery chemistry is needed to realize its practical use. In this dissertation, three critical aspects of Li-S batteries are discussed. First, the strategy of surface modification is used to control the deposition of sulfur and its reduction products. An interlayer configuration using surface-treated carbon paper is analyzed in the beginning, followed by the introduction of a novel surface-hydroxylated graphene-sulfur nanocomposite cathode with superior high-rate performance. Moreover, the hydroylated graphene-sulfur composite cathode is coupled with a fluorinated ether electrolyte that suppresses polysulfide shuttling. The mechanisms of suppressed polysulfide shuttling and lithium-anode surface chemistry are investigated. Second, Li/polysulfide batteries and protection of lithium-metal anode are presented. Various cathode conductive matrices in Li/polysulfide batteries are compared regarding their polysulfide confinement capability and electrochemical performances. It is further identified that the lithium-anode corrosion is the main obstacle to increase the sulfur content in the cathode. Based on the characterization data, a novel anode-protection mechanism is proposed, which may solve the problem of electrode degradation in the case of high sulfur contents at high rates. Third, the development of an alternative, prelithiated sulfur cathode Li2S is presented. Li2S cathode coupled with a lithium-free anode can avoid the use of unstable lithium-metal anode. However, the insulating nature of Li2S and the lack of flexibility to form nanoparticles prevent its practical applications. To solve these problems, a low-cost activation of Li2S bulk particles is presented.