PhD Thesis Presentation
Transportations account for a great portion of CO2 emissions which result in the global warming. Consequently, a critical strategy to approach carbon neutrality is the electrification of land vehicles. Li-O2 batteries (LOBs) possess the highest theoretical specific density among all types of lithium batteries, making them an ideal candidate to replace the current Li-ion batteries (LIBs) for next-generation electric vehicles. However, designing highly active oxygen electrodes to kinetically accelerate the sluggish oxygen reduction/evolution reactions (ORR/OER) have not been achieved. Ternary metal-oxides and -sulfides have received much attention as potential electrocatalyst for high performance rechargeable batteries. This thesis is dedicated to preparing efficient oxygen electrodes with enhanced LOB electrochemical performance and revealing the reaction mechanisms behind the electrochemical performance.
Thanks to the thermally induced oxygen vacancies present across the intra/inter-crystalline sites and the large surface area of ultrafine particles, the oxygen deficient CoMn2O4 (CMO) electrodes ameliorate electrochemical performance of LOBs by offering (i) a higher initial capacity, (ii) a lower overpotential and (iii) better cyclic stability than the as-prepared CMO electrode. While the CMO electrode offers an excellent catalytic behavior in ORR, the oxygen vacancies mitigate the migration of Li+ ions and electrons and act as active sites for O2 in the OER. The ex situ characterization proves a lower kinetic charge transfer resistance and higher catalytic activities of the oxygen deficient CMO electrodes in the decomposition of discharge products.
MnCo2S4 nanosheets (NSs) are grown on carbon paper (MCS/CP) via facile electrodeposition followed by vulcanization. The electrochemical performance of the binder-free MCS/CP oxygen electrode is compared with that prepared from MnCo2O4 NSs on CP (MCO/CP). The MCS/CP electrode delivers an extremely high initial specific capacity of 10760 mAh g-1, twice that of MCO/CP. The former electrode sustains 96 cycles at an upper limit capacity of 500 mAh g-1 at 200 mA g-1, whereas the latter counterpart survives only a few cycles. The superior performance of MCS/CP is proven by four times higher electrical conductivity and 250% higher Li diffusion coefficient than MCO/CP. Three-dimensional (3D) interconnected web of two-dimensional (2D) MCS NSs offers a few micrometer open voids to accommodate discharge products and a large surface area with internal mesopores providing abundant active sites. The density functional theory (DFT) calculations reveal a lower adsorption energy for LiO2 on the surface of MCS than on MCO, which is responsible for the lower OER overpotential and the higher catalytic ability of MCS/CP.
2D trigonal phase MoS2 (1T-MoS2) nanosheets are prepared as the highly active electrocatalyst for LOBs for the first time. Metallic 1T-MoS2 synthesized by in situ liquid-redox intercalation and exfoliation is hybridized with functionalized carbon nanotubes (CNTs) to form freestanding, binder-free oxygen electrodes. The 1T-MoS2/CNT electrode exhibits excellent electrochemical performance: a high reversible capacity of 500 mAh g-1 at a current density of 200 mA g-1 for more than 100 cycles owing to the catalytically active surfaces of 1T-MoS2 accessible by Li+ ions and O2. The DFT calculations identify the catalytically active basal planes in 1T-MoS2 during ORR as well as the initial ORR path during LOB cycles. The results based on rotational ring disk electrode (RRDE) also support the findings from DFT calculations where the 1T-MoS2 basal planes are active for both ORR and OER, not the semiconducting hexagonal MoS2 (2H-MoS2) whose edges are only electrocatalytically active. This study sheds light on using metallic 1T-MoS2 as a multifunctional oxygen electrocatalyst for LOB applications with enhanced ORR and OER activities.
(Supervisor: Prof. Jang-Kyo Kim)