Aqueous Electrolyte Regulation Towards Long-Lasting Zinc Metal Anode

To mitigate the negative impacts on human life caused by the greenhouse effect, people around the world are making great efforts to use clean energy instead of traditional fossil fuels. In this process, the large-scale energy storage systems (EESs) play a crucial role because most of clean energy sources are intermittent and volatile. In recent ten years, aqueous zinc-ion batteries (AZIBs) have attracted great attention as a promising candidate for large-scale EESs due to their high safety, low cost, and environmental friendliness. Nevertheless, issues related to the zinc metal anode, such as hydrogen evolution reactions (HER), corrosion reactions, and dendrite growth, significantly hider the industrial application of AZIBs. Electrolyte regulation is one of the most efficient methods to address these problems. In this doctoral thesis, the issues related to the zinc anode and the recent developments in electrolyte regulation methods, including solvation structure regulation, electric double layer (EDL) regulation, and solid electrolyte interphase (SEI) regulation, are thoroughly summarized. Based on these theories and experiences, one cosolvent and two electrolyte additives have been developed, significantly improving the electrochemical properties of the zinc metal anode.

In the first work, logP, where P is the octanol-water partition coefficient, a general parameter to describe the hydrophilicity and lipophilicity of chemicals, is proposed as a standard for selecting co-solvents for Zn(CF₃SO₃)₂ electrolyte, which demonstrated by testing seven different types of solvents. The solvent with a similar logP value to the salt anion CF₃SO₃- can interact with CF₃SO₃-, Zn²⁺ and H₂O, leading to a reconstruction of the electrolyte solvation structure. To prove the concept, methyl acetate (MA) is demonstrated as an example due to its similar logP value to CF₃SO₃-. Both the experimental and theoretical results illustrate that MA molecules not only enter into the solvation shell of CF₃SO₃-, but also coordinate with Zn²⁺ or H₂O, forming a MA and CF₃SO₃- involved core-shell solvation structure. The special solvation structure reduces H₂O activity and contributes to forming an anion-induced ZnCO₃-ZnF₂-rich SEI. As a result, the Zn||Zn cell and Zn||NaV₃O₈·1.5H₂O (NVO) cell with MA-involved electrolyte exhibit superior performances to that with MA-free electrolyte.

In the second work, a functional group assembly strategy is proposed to design electrolyte additives for modulating the EDL, thereby realizing a long-lasting zinc metal anode. Specifically, by screening ten common functional groups, N, N-dimethyl-1H-imidazole-1-sulfonamide (IS) is designed by assembling an imidazole group, characterized by its high adsorption capability on the zinc anode, and a sulfone group, which exhibits strong binding with Zn²⁺ ions. Benefiting from the adsorption functionalization of the imidazole group, the IS molecules occupy the position of H₂O in the inner Helmholtz layer of the EDL, forming a molecular protective layer to inhibit H₂O-induced side reactions. Meanwhile, the sulfone group in IS, acting as a binding site to Zn²⁺, promotes the de-solvation of Zn²⁺ ions, facilitating compact zinc deposition. Consequently, the utilization of IS significantly extending the cycling stability of Zn||Zn and Zn||NVO full cell.

In the third work, ethylene glycol bis(propionitrile) ether (DENE) is developed as a dual-function electrolyte additive to prolong the cycling stability of the zinc metal anode. Due to the abundant polar groups in the structure of DENE, it interacts with H₂O through hydrogen bond (H-bond), thereby breaking the original H-bond network of H₂O. Furthermore, DENE exhibits a lower lowest unoccupied molecular orbital compared to H₂O, causing it to decompose before H₂O during the battery charging process to form a DENE-derived SEI. These synergistic effects inhibit the HER, corrosion reactions, and zinc dendrite growth, leading to a dramatic increase in the lifespan of both the Zn||Zn and Zn||NVO cells.

In summary, this doctoral thesis develops an electrolyte cosolvent selection method, an EDL regulating additive design method, and a dual-function electrolyte regulation additive for promoting the cycling stability of the zinc metal anode. Based on findings described above, I believe this thesis can offer valuable insights and references for further researchers, facilitating the expedited commercialization of AZIBs.