Low-Temperature Tolerant Gel Polymer Electrolytes for Rechargeable Zn-Air Batteries
Creators: Cui, Jiayao
Publisher: name: University of Alberta Library
Year: 2023

<p>Global warming and climate change are driving the use of cleaner energy sources to replace fossil fuels. Wind and solar energy are renewable and sustainable clean energies and can generate electricity. However, both wind and solar energy depend on the weather conditions. Storing the electricity in batteries is a way to create a more consistent supply. Li-ion batteries are the most widely used batteries currently, but their high price hinders their application in grid storage. In contrast, metal-air batteries, especially Zn-air batteries (ZABs), are gaining traction because of their safe operation and lower price compared with Li-ion batteries. Gel polymer electrolytes (GPEs) are emerging materials for ZABs as the GPE can act as an electrolyte and a separator in the ZAB and prevent short circuits caused by Zn dendrite formation. This thesis focuses on extending the working temperature range of the ZAB to low temperatures (as low as -41℃) as well as reducing the interfacial resistance between the GPE and the electrode.<br><br>
The first study involved fabricating two GPEs (GPE-KOH and GPE-KOH-KI) for ZABs through the polymerization reaction of poly(acrylic) acid and KOH with (GPE-KOH) and without (GPE-KOH-KI) ZnO, followed by an immersion in a solution containing KOH, KI and ZnO (GPE-KOH-KI) . ZABs using these two GPEs were tested at different temperatures and current densities. Both GPEs demonstrated excellent low temperature resistance and competitive performance in a ZAB compared with the literature. The ZAB using GPE-KOH was able to cycle at 10 mA cm-2 and -28℃ and at 5 mA cm-2 and -41℃ for 100 h (200 cycles). The initial and final efficiencies were 50% and 41% (-28℃, 10 mA cm-2) and 42% and 32% (-41℃, 5 mA cm-2), respectively. When tested at 21℃, the ZAB using GPE-KOH exhibited a peak power density of 127 mW cm-2 and successfully cycled for 260 h (520 cycles) at 10 mA cm-2 before experiencing accelerated performance degradation. The initial efficiency was 61% and the efficiency at 260 h was 42%. The addition of KI to the electrolyte changed the conventional charging reaction to a reaction with a lower thermodynamic barrier. The battery efficiency was improved significantly with a maximum increase of 36 % relative to ZABs without KI. It was proposed that only the reaction at the Zn electrode was fully reversible, while the reaction at the air electrode was not, which would result in accumulation of KIO3 as the battery cycles. When tested at 21℃, the ZAB using GPE-KOH-KI had a lower peak power density of 98 mW cm-2  compared with the ZAB using GPE-KOH, due to I- occupying active oxygen reduction reaction sites, and was able to cycle for 100 h (200 cycles) at 10 mA cm-2 with initial and final efficiencies of 71% and 52%, respectively. When tested at -41℃ and 5 mA cm-2, the battery could cycle for 100 h (200 cycles) with initial and final efficiencies of 52% and 43%, respectively.<br>
    The second study explored a new fabrication methodology to fabricate the GPE (without KI) by an in-situ method in the jig for ZAB fabrication. Our initial hypothesis was that the in-situ fabrication method could reduce the interfacial resistance between the electrolyte and the electrode and improve battery cyclability, compared with ZABs using ex-situ fabricated GPE, because the precursor solution has better fluidity and wettability and can establish a better contact with the air electrode. A battery cell was designed and fabricated via 3D printing to realize in-situ fabrication of GPE for ZABs. The interfacial resistance was not reduced, nor was the battery cyclability improved by in-situ fabrication of GPE. The battery cyclability was similar at 2 mA cm-2 for both synthesis methods, but the charging performance was worse for the ZAB using in-situ fabricated GPE when cycled at 5 mA cm-2 and 10 mA cm-2. Scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectroscopy revealed that GPE completely penetrated through the air electrode when in-situ fabrication of GPE was used. This caused electrolyte flooding, reduced catalytic activity, and blocked GDL pores, leading to poorer battery efficiency and worse charging performance during battery cycling.  Further studies are needed to test our initial hypothesis.</p>