Transportations account for a great portion of CO
2 emissions which result in the global
warming. Consequently, a critical strategy to approach carbon neutrality is the electrification
of land vehicles. Li-O
2 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 CoMn
2O
4 (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 O
2 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.
MnCo
2S
4 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 MnCo
2O
4 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 LiO
2 on the
surface of MCS than on MCO, which is responsible for the lower OER overpotential and the
higher catalytic ability of MCS/CP.
1
2D trigonal phase MoS
2 (1T-MoS
2) nanosheets are prepared as the highly active electrocatalyst for LOBs for the first time. Metallic 1T-MoS
2 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-MoS
2/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-MoS
2 accessible by Li
+ ions and O
2. The DFT calculations identify the catalytically active basal planes in 1T-MoS
2 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
2 where the 1T-MoS
2 basal planes are active for both ORR and OER, not the semiconducting hexagonal MoS
2 (2H-MoS
2) whose edges are only electrocatalytically active. This study sheds light on using metallic 1T-MoS
2 as a multifunctional oxygen electrocatalyst for LOB applications with enhanced ORR and OER activities.
1,2 The DFT calculations are conducted by Jiapeng Liu. The discussion related to the calculations are written by Zoya Sadighi.
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