PhD Oral Exam - Jalal Rahmatinejad, Chemical and Materials Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Expanding electrical energy storage beyond lithium-ion batteries (LIBs) is crucial due to their limitations in material availability, cost, and performance. LIBs rely on lithium, which has a limited supply, uneven global distribution, and increasing demand, leading to concerns about long-term sustainability and price volatility. Additionally, LIBs face intrinsic challenges such as safety risks from thermal runaway, capacity fading over extended cycles, and performance limitations at extreme temperatures. These constraints drive the search for alternative battery chemistries, such as sodium-ion, potassium-ion, magnesium-ion, zinc-ion, and aluminum-ion batteries. However, these alternatives introduce new challenges due to differences in ionic size, charge density, and electrochemical behavior, necessitating the development of advanced electrode materials. Molybdenum disulfide (MoS2), a 2D nanomaterial with larger interlayer spacing than graphene, shows promise for storing such ions but faces challenges from its semiconducting nature and side reactions.

This thesis begins with a comprehensive review of recent literature on strategies for modifying the structure of MoS2 and its nanocomposites to enhance ion storage capabilities for Na+, K+, Mg2+, Zn2+, and Al3+. Next, four novel composites were designed and synthesized, with their potential as active materials for post-LIBs comprehensively investigated. A range of material characterization techniques, including Brunauer-Emmett-Teller (BET) surface area analysis, thermogravimetric analysis (TGA), X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS), were employed to study their structural and chemical properties. To evaluate their electrochemical performance as active battery materials, various methods such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) were utilized.

Two anode composites for sodium-ion batteries (SIBs), MoS2@HPC (crystalline MoS2 in hierarchically porous carbon) and a-MoSx@HPC (amorphous MoSx in HPC), were designed and evaluated. The hybridization with HPC was found to enhance Na+ storage by improving capacity and cycling stability. At 0.5 A g−1, capacities of 550 mAh g−1 and 301 mAh g−1 were achieved by a-MoSx@HPC and MoS2@HPC, respectively, both outperforming pure MoS2, which delivered 253 mAh g−1. After 100 cycles, capacity retentions of 75% and 89% were maintained by a-MoSx@HPC and MoS2@HPC, respectively, in contrast to the 53% retention observed for pure MoS2.

Following this, a 1T/2H mixed-phase MoS2 (MP-MoS2) modified with a polyethylene ionomer (I@MP-MoS2) was investigated for Mg2+ storage in magnesium-ion batteries (MIBs) and Mg2+/Li+ storage in dual-salt magnesium-lithium-ion batteries (MLIBs). With 53% of metallic 1T phase, increased interlayer spacing (1.11 nm vs. 0.62 nm in MoS2), and enhanced electrolyte interaction, I@MP-MoS2 achieved 144 mAh g−1 at 20 mA g−1 in MIBs and 270 mAh g−1 in MLIBs, with 87% of capacity retention after 200 cycles.

Finally, a novel cathode design was investigated for MLIBs using a 2D/2D nanocomposite of 1T/2H-MoS2 and delaminated Ti3C2Tx MXene (1T/2H-MoS2@MXene). This structure improves Mg2+ kinetics, structural integrity, and reversible Mg2+/Li+ co-intercalation, achieving 253 mAh g−1 at 50 mA g−1 and retaining 36% of capacity at 1,000 mA g−1.

Overall, this thesis presents innovative MoS2-based materials for post-LIBs, addressing ion storage, conductivity, and stability challenges, contributing to next-generation energy storage solutions.