The Design and Synthesis of Cathode Materials for High-Performance Li-S Batteries
Creators: name: H J, Bharathkumar
Publisher: name: RMIT University
Year: 2025
Subjects: classificationCode: 340305
subject: Physical properties of materials
classificationCode: 340303
subject: Nanochemistry
classificationCode: 340301
subject: Inorganic materials (incl. nanomaterials)

description: Among different rechargeable batteries, Li-S batteries (LSBs) are considered as the potential alternative to Li-ion batteries (LIBs) due to their high theoretical capacity (1675 mA h g-1) and energy density (2600 W h kg-1). Furthermore, sulfur is inexpensive and available in abundance compared to the commercial mixed metal oxide and olivine type (LiFePO4) cathodes which are being used in commercial LIBs whose energy density is also limited to 200 W h kg-1. However, the commercialization of LSBs is hindered by some of the fundamental challenges such as polysulfide dissolution and shuttle, sluggish conversion kinetics of polysulfides and volume expansion during the charge-discharge process in LSBs. These result in poor cycle life and Coulombic Efficiency (C.E.) of the batteries. This thesis investigates different sulfur host materials to address these issues with the aim to improve the cycle life of LSBs. Towards this objective, firstly, a nickel (II) phthalocyanine tetra sulfonate crosslinked polypyrrole was used as a sulfur host where the Ni2+ displayed strong electrostatic polyvalent interactions with LiPS intermediates and improved the performance of LSBs with a stable performance over 500 charge-discharge cycles. Furthermore, density functional theory (DFT) studies revealed strong interactions with different polysulfides which further supported the improved electro-chemical performance. Further, two organic redox-active small molecules based on diketopyrrolopyrrole (DPP) were synthesized with phenyl and pyridine substituents. The pyridine substituted DPP (PyDPP) possessed higher adsorption energy for lithium polysulfides due to the strong pyridinic N-Li interactions which also amplified the adsorption energy of carbonyl oxygen of DPP compared to that of phenyl DPP (PhDPP). Thus, LSBs with PyDPP outperformed PhDPP comprising batteries in terms of specific capacity, rate performance and long-term cycling stability. Also, the band edges of the radical anions of PyDPP and PhDPP are commensurate for efficient charge transfer with Li2S thus improving the sulfur utilization. Subsequently, a series of porous organic polymers (POPs) was synthesized with different functional moieties (acetylene, carbonyl and pyridine) to impart chemisorption for polysulfides and improve the electrocatalysis. Interestingly, the polymers possess similar pore size distributions which match the size of polysulfides, thus imparting physicochemical confinement for polysulfides and improving the cycling life of the LSBs with POPs. Among the four polymers, the polymer with both carbonyl and pyridinic N displayed the best performance with capacity close to 1000 mA h g-1 at 1C with stable performance over 600 cycles which is attributed to multiple polysulfide interacting groups which also improved their electrocatalysis. DFT studies also revealed the strong interactions between the polysulfides, and the functional moieties present in the POPs. Finally, the thesis summarizes the research findings and future directions for further advancement in the field of LSBs and their potential applications.