Aqueous rechargeable sodium-ion energy storage

Abstract

Aqueous rechargeable sodium-ion energy storage (ARSIES) is proposed as a promising alternative to the state-of-art lithium-ion batteries due to its abundance of natural reserves (water and sodium salts), excellent fire safety and environmental benignity. Towards enabling ARSIES for large-scale energy storage applications, it is imperative to develop electrode material with high charge storage capacity with superior rate capability and low-cost electrolytes that can enable long cycle lifespan. Due to the nature of the aqueous system, the main challenges lie in the various parasitic side reactions associated with water that limits the electrochemical stability window and undermines the cycle stability of the cell. In this thesis, a tunnel-type sodium manganese oxides (NMO), is used as a model cathode, owing to its high theoretical storage capacity and the ease of fabrication. The charge storage and its failure mechanism of the NMO in the aqueous system, are thoroughly investigated and characterized. A series of strategies based on electrode modification and electrolyte engineering are designed and optimized to realize the ideal full cell for the ARSIES. Also, as a proof-of-concept, an array of green, ultralow-cost, and low concentrated electrolytes has been successfully developed based on the novel concept of hydrogen-bonding interaction between co-solvents. The unique hydrogen bonding interactions in the hybridized electrolytes can effectively suppress the water activities as well as the parasitic side reactions associated with water, thereby enabling the superior electrochemical performances with long cycle stability of NMO. The novel findings from this study and the proposed future works could provide guidelines and inspire more research and developments for the practical application of next-generation ARSIES.