Nanostructured oxygen electrodes for lithium-oxygen batteries : performance and mechanistic insights

Abstract

Towards a carbon-neutral and more sustainable society, the emission-free, renewable but intermittent energies such as solar and wind necessitate energy storage devices for practical applications. Furthermore, the large-scale commercialization of EVs has been hindered by the driving range and cost due mainly to the currently dominant Li-ion battery (LIB) systems. In addition, the rapid growth of electronics industries governed by Moore’s law continuously minimizes the size and weight of portable electronics. On contrary, the energy storage capability of LIBs intrinsically limited by the chemistry fails to achieve comparable breakthroughs, and becomes the technological bottleneck. Lithium-oxygen batteries (LOBs) have therefore emerged as a promising choice, who deliver an ultrahigh theoretical gravimetrical energy density of 3505 Wh kg^-1. However, many of challenges should be resolved, if LOBs are to be commercialized. Aside from the electrochemical performance, a detailed understanding of the reaction mechanisms is compulsory to master the underlying electrochemical processes. Oxygen electrode is the central component of LOBs, and largely affects the electrochemical performance and reactions taking place. This thesis is consequently dedicated to synthesizing nanostructured oxygen electrodes so as to improve the electrochemical performance and unravel the reaction mechanisms. Co-Ni encapsulated carbon nanofiber electrodes are synthesized by electrospinning. The inherently interconnected, graphitic network makes it unnecessary to include binders and carbon additives, therefore eliminating the possible side reactions from binder and facilitating the ion and electrical transport in the electrodes. The in-situ encapsulation of Co-Ni nanoparticles facilitates uniform dispersion of catalysts and prevents the catalyst aggregation after cycles. The influence of Li₂O₂ morphology and its nucleation mechanism are probed by experiments along with the first-principle calculations. It is revealed that the LOBs with Li₂O₂ films deliver unexpectedly improved capacities, longer cycles and significantly reduced overpotentials assisted by NiFeO_x nanofiber catalysts. The energetically favored Li 2a vacancies under LiO₂-rich conditions, small crystallites and large contact areas with the electrode/electrolyte explain the anomalous performance enhancement. Li₂O₂ films are formed by a heterogeneous nucleation mechanism and the voltage applied, electrolyte, electrode surface and use of catalysts are identified as the parameters controlling the mechanisms. 2D porous RuO₂ nanosheets are synthesized using graphene oxide as the template. The holey RuO₂ nanosheets are rationally assembled with carbon nanotube interlayers to form a hybrid with highly enhanced mass/electron transport through the conductive, porous structure. The 2D/1D hybrid electrodes deliver exceptional performance in LOBs. The first-principle calculations combined with microscopy reveal that Li₂O₂ interacts strongly with RuO₂ and thermodynamically follows a Stranski-Krastanov growth mode to form a dumbbell-shaped heterostructure. The highly LiO₂-soluble dimethyl sulfoxide-based electrolyte is employed for the first time to prepare Li ion oxygen batteries (LIOBs) through refining the solid-electrolyte interphase and concentrated electrolyte. This approach effectively mitigates the energy mismatch present between the lowest unoccupied molecular orbitals of electrolyte and the work functions of Si/C anode. The LIOBs deliver cyclic stability and ultrahigh gravimetric energy and power densities up to 1897 Wh kg^-1 and 1396 W kg^-1, respectively.

Date of Award	2017
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology