An integrated development of high-capacity Lithium-sulfur (Li-S) batteries : cathodes, separators, and lithium-metal anode

Chang, Chi-Hao, Ph. D.
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Lithium-sulfur (Li-S) batteries have been receiving great recognition for the past few years. This is largely due to the fact that sulfur is an environmentally harmless, and cost-effective element in high abundance. Most importantly, it offers the highest capacity among all solid-state cathode materials. However, the poor electrochemical utilization resulting from the low active material conductivity, and fast capacity fade caused by the freely migrating polysulfides (Li₂S [subscript n], 4 ≤ n ≤ 8) limit the practical implementation of Li-S battery technology to replace the current lithium-ion technology. This dissertation focuses on improving the electrochemical performance by developing new battery materials and advanced cell components for the Li-S-cell. First, a simple method is presented to design a thin coating layer on the cathode-side of the polymeric separator, which significantly limits the polysulfide migration. The functional coating layer offers either physical trapping capabilities or chemical immobilization toward migrating polysulfide species within the cathode region during cycling, resulting in a great improvement on discharge capacity and cycling performance. Second, a sophisticated cathode design is proposed to increase the sulfur loading and enhance the areal capacity. A facile procedure was used to integrate polysulfide trapping layers and polysulfide blocking layers into the sulfur cathode of the Li-S cell. The designed cathode, also called "the tandem (layer-by-layer) cathode," not only efficiently utilizes the active material but also effectively suppresses the polysulfide migration. Third, a new electrolyte additive was successfully synthesized to mitigate the polysulfide migration. The electrolyte additive and polysulfides form bulkier polysulfide complexes that are then size-selectively sieved by the separator. Moreover, a protected layer/coating abundant with functional groups of high lithium affinity effectively stabilize the lithium-metal anode surface and improve the reversibility of the lithium-metal cell. Additionally, a new polymer/graphene composite was synthesized and used as a new cathode material for the Li-S chemistry, which also shows better electrochemical performance compared to that from the elemental sulfur cathode. Finally, several techniques are utilized and integrated to assemble a practically viable Li-S battery cell prototype (pouch-cell). The electrochemical performance and the morphological changes are also investigated. This work successfully achieves high-capacity Li-S cells, which are able to compete with the conventional lithium-ion batteries.