Rechargeable Lithium-Sulfur Batteries: Novel Electrode and Separator Design for High-Energy Density and Long-Cycle Life
Creators: name: Liu, Borui
Publisher: name: The Australian National University
Year: 2022

description: Rechargeable Li-S batteries are a promising next-generation energy storage technology. However, the loading of sulfur needs to be high enough to match the energy density of traditional LIB cathodes for practical concerns. Moreover, converted polysulfides during charge/discharge process can easily dissolve in the electrolytes and penetrate the separator ("shuttle effect"), compromising the cycling stability of Li-S batteries. Besides, the volume of sulfur expands by 80% during lithiation, and the low conductivity of sulfur hinders the polysulfide-conversion reactions. Therefore, it is urgent to find effective means to solve these issues. This thesis focuses on the development of novel electrode and separator designs for high-energy density and long-life Li-S batteries. Investigation methods include engineering conductive and porous electrodes as polysulfides/sulfur hosts and ion-sieving separators as polysulfides barrier as well as for Li anode protection. In the first study, a CPH-derived monolithic electrode architecture was investigated for Li-S batteries. The electrode architecture consists of a 3D free-standing and highly conductive N, P co-doped CPH-derived carbon network, enhanced by homogeneously dispersed MOF nanodomains, offering high S-loading capability. The carbon monolithic matrix provided superior host effect for sulfur and electrical connection among the MOF nanodomains, resulting in low charge transfer resistance, strongly enhanced sulfur loading and capacity retention. In the second study, a self-supporting ZIF-8 MOF film was investigated as a Li-S battery separator. This was the first time that a self-supporting MOF film was used as a battery separator. The cells with the ZIF-8 film separator exhibited slow capacity fading of 0.12% per cycle. It demonstrated that the subnanopores of the dense ZIF-8 separator can effectively hinder the shuttle of polysulfides and improve the cycle life of Li-S batteries. In the third study, a facile approach for the engineering of Janus conductive/insulating microporous ion-sieving membranes was developed as Li-S separators. The insulating side consisted of a standard PP separator, while the conductive side was made of closely packed multilayers of HAR-ZIF-67/graphene nanosheets having a thickness of few nanometers and a large specific surface area. Models and experiments revealed that this electrically conductive microporous nanosheet architecture promoted the use of polysulfide trapped by the membrane and decreased the polysulfide flux by a factor of 250 times over recent microporous membranes made of granular MOFs and standard battery separators. Li-S batteries using the Janus membranes achieved outstanding rate capability and long-term stability with 75.3% capacity retention for 1700 cycles. In the last study, a versatile MOF-based Janus membrane possessing a Li-ion microporous disperser and a polysulfide barrier, with subnanometer Li-ion channels and asymmetric electrical conductivity achieved via a facile temperature-dependent annealing process was engineered as the separator to enhance dual-electrode stability of high-areal-capacity Li-S batteries. Experiments and models revealed that the Li-ion disperser had a low bulk conductivity, ensuring MOF stability at low potentials, effectively regulating the uniformity of Li-ion mass transfer and achieving stable voltage hysteresis in Li|Li symmetric cells. Moreover, the polysulfide barrier improved the polysulfide utilization owing to its high conductivity. Both synergistically suppressed polysulfide shuttle and improved the overall cycling stability of Li-S batteries, resulting in 73% capacity retention for 1000 cycles. Additionally, pouch cells fabricated with the Janus membrane showed 82.4% capacity retention for 200 cycles.