Hu, Xueying; (2025) Study on ionic polyelectrolytes for highly stable Zinc ion batteries. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Aqueous zinc-ion batteries (AZIBs) are one of the most promising alternatives to lithium-ion batteries due to their high volumetric capacity, low cost, and non-flammability. However, the hydrogen evolution reaction (HER) and uncontrollable dendrite growth on Zn anode make for poor reversibility and low Coulombic efficiency, resulting in shortened cycle life of AZIBs hence hindering practical applications. This PhD thesis investigates economical and environmentally friendly ionic polyelectrolyte coatings through simple thin film preparation technology to effectively inhibit side reactions and dendrites growth during the cycling, thereby promoting the commercial application of AZIBs. Specifically, bio-inspired polyanionic electrolyte alginate acid (SA) was investigated and utilized to initiate in situ solid electrolyte interface (SEI) layers on the Zn anode in Chapter 3. Attributed to the anionic groups of -COO of alginate, the formed SEI can regulate the desolvation structure of Zn2+ and facilitates the formation of compact Zn (002) crystal planes. Correspondingly, it exhibits outstanding stability with an average Coulombic efficiency of up to 99.8 % over 1,400 cycles. In Chapter 4, a fluorine booster PDDA-OTf was prepared via the flocculation between poly(diallyl dimethyl ammonium) polycations (PDDA) and the CF3SO3 - anions (OTf− ) as a protective layer to suppress the corrosion and passivation of the Zn anode. The PDDA-OTf layer can decompose through the Hofmann elimination to form an in situ ZnF2-enriched SEI layer during the electrochemical process, which embeds at the surface of the Zn anode, providing excellent mechanical strength. As a result, the PDDA-OTf layer exhibits an ultra long lifespan of the Zn anode exceeding 2,528 h at 1 mA cm-2 and 1 mAh cm-2 with Coulombic efficiency of 99.2% over 800 cycles at 0.5 mA cm-2 and 0.25 mAh cm-2 . Based on the above two chapters, a universal strategy for stabilizing the Zn anode through layer-by-layer (LbL) self-assembly of polyelectrolytes was proposed in Chapter 5, an ion sieving accelerating channel was constructed to unify the Zn deposition by introducing an eco friendly layer-by-layer self-assembly of a flocculant poly(allylamine hydrochloride) (PAH) and its tautomer poly(acrylic acid) (PAA). The dual-ion channels, created by strong electrostatic interactions between carboxylate anions (COO⁻) and ammonia cations (NH₃⁺), can promote the uniform Zn deposition along the (002) plane, exhibiting a CE of 99.8% after 1600 cycles. An Ah-level pouch cell (17.36 Ah) with a high mass loading (> 8 mg cm⁻²) also demonstrates exceptional performance, retaining a capacity of 93.6% for at least 250 cycles at 1.7 C-rate (35.3 min).

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