Fundamental study of engineered cathode and Li-ion conducting electrolyte architectures for high energy density Lithium Sulfur batteries

Rapid development in electric vehicle and portable electronic device technologies has made it imperative to identify new higher energy density (∼500 Wh/kg) battery systems than currently available Li-ion batteries (∼250 Wh/kg). Lithium-sulfur with a theoretical capacity of ∼1675 mAh/g has emerged as the leading next generation Li-ion battery system. However, sulfur cathodes are hamstrung by inherent poor electronic conductivity (∼1×10-15 S/m), volumetric expansion (∼80%) and more importantly, soluble polysulfide formation causing inferior cyclability, areal capacities and rate capabilities. This PhD dissertation overcomes these limitations by engineering electron conducting novel cathode architectures using metal organic frameworks (MOFs) and complex framework materials (CFMs) as primary sulfur hosts confining the volume expansion along with composite polymer electrolytes (CPEs) and directly deposited sulfur architectures (DDSA) combining polysulfide trapping agents (PTAs). The sulfur infiltrated MOF (S-MOF) and CFM hosts (S-CFMs) exhibit stable capacities of ~1000-1050 mAh/g with minimal fade rate (~0.12-0.19% loss/cycle) due to Lewis acid-base interactions of the MOF with polysulfides and concomitant sulfur bonding with the CFM carbon architecture yielding complete lithiated polysulfide species retention within the MOF and CFM structures. The CPEs and DDSA-PTA composite systems also exhibit excellent stability (~0.14% loss/cycle) upon extended cycling to (~200 cycles), highlighting the potential of these systems in preventing polysulfide dissolution in Li-S batteries. Fundamental aspects of the different strategies employed to resolve the core sulfur cathode problems of polysulfide dissolution and inferior electronic conductivity of sulfur are outlined. Results from the extensive materials, chemical and electrochemical characterization of these individual systems are also discussed