Improving Zn electrodes and electrolytes towards rechargeable zinc ion batteries

Abstract

Zinc ion batteries (ZIBs) have been considered promising energy storage devices due to their high safety and low toxicity. However, Zn metal anodes face the challenges of dendrite growth and severe side reactions, which deteriorate the cycle life of ZIBs. Therefore, improving the electrochemical performance of Zn anodes is crucial. Various approaches have been explored to enhance the performance of Zn anodes, such as designing 3D structures, fabricating surface protection layers, and modifying electrolyte composition. Nevertheless, their performance is far from satisfactory for practical applications. In this thesis, by combining experimental studies and theoretical calculations, I have demonstrated three new strategies to enhance the electrochemical performance of Zn anodes. In Chapter 2, 3D Zn composite electrodes with carbon nanofiber (CNF) backbones (Zn@CNF) were developed as model electrodes to reveal how depth of discharge (DOD) and current density affect their performance. In Chapter 3, an ultra-thin (~45 ± 5 nm) protective layer was created on Zn surfaces using a facile and rapid chemical treatment by polyphosphoric acid, which dramatically improved the stability of Zn/electrolyte interfaces. This work offers a readily employable Zn electrode protection method for scalable applications. In Chapter 4, a branched sugar, dextran, was developed as a multifunctional and universal electrolyte additive to enable high-performance ZIBs. Experimental and theoretical results revealed that dextran played four functions: forming a surface protective layer to minimize side reactions, facilitating stepwise [Zn(H2O)6]2+ desolvation, preferably adsorbing on Zn(0002) planes to supply desolvated Zn2+ and homogenizing electric field. This thesis provides some in-depth understanding of Zn anode’s electrochemical working mechanisms, and the findings also guide the future development and commercialization of ZIBs.