Gel Polymer Electrolytes and Battery Designs for Rechargeable Zinc-Air Batteries

• Author / Creator

  Tran, Thuy NT

• To accelerate the world's transition to renewable sources, rechargeable Zn-air batteries (ZABs) have regained interest in recent years for distributed energy storage. ZAB technology has several advantages including operational safety, high theoretical energy density and low cost. This is mainly due to Zn being environmentally benign and abundant in the Earth’s crust, while the other electroactive species (oxygen) can be withdrawn directly from the air. Although primary ZABs have been commercially available for almost a century, there are still numerous issues to be addressed for their rechargeable counterparts. For example, problems arise during recharging of ZABs due to instability of the air electrode. Additionally, batteries using aqueous electrolytes suffer from carbonation, leakage and evaporation of the electrolyte, and dendrite formation on the Zn electrode, all of which can be mitigated by using gel polymer electrolytes (GPEs). The thesis focuses on (1) preparation techniques, characterization and compositional optimization of GPEs, and (2) configurations for solid-state ZABs to improve energy efficiency and cycle life.
  The first study involved the synthesis and characterization of different hydrogel networks, i.e., poly(vinyl alcohol) (PVA), poly(acrylic acid) (PAA) and poly(4-vinylbenzenesulfonate-co-[3-(methacryloylamino)propyl]trimethylammonium chloride) (PAM). Their chemical stability, electrochemical windows and mechanical properties are evaluated. The relationship between ionic conductivity, water uptake and temperature are also discussed. This study demonstrated that PAA-based GPEs show considerable promise for use as electrolytes in ZABs.
  The second study investigated cyclability of solid-state ZAB using tri-electrode designs such as sandwich and planar cells. Various additives for the PAA-based GPEs, such as crosslinker and zinc oxide (ZnO), were also evaluated. Crosslinking density affects the physical state of GPEs, which in turn affects the oxygen evolution reaction, since oxygen bubbles may be trapped in the GPE-air electrode interface. Meanwhile, the effects of ZnO on discharge and charge were studied separately. The addition of ZnO to the oxygen reduction reaction side of the sandwich cell enhanced the cycling performance of ZABs; i.e., the battery can withstand at least 100 cycles at 5 mA cm–2 with an efficiency of 62% during the first cycle.
  The third study compared cyclability of ZABs using aqueous electrolytes and GPEs with a bi-electrode design. In general, ZABs with GPEs exhibit better initial performance than those with aqueous 6 M KOH. Although the efficiency of the ZAB using the aqueous electrolyte gradually improved during cycling, there is visible evidence of flooding and degradation of the air electrode. Gels with lower crosslinking concentrations are weaker, but they have higher conductivity and better water retention, whereas gels with higher crosslinking concentrations can reduce dendrite growth, but they can facilitate passivation of the Zn electrode, resulting in early failure of the battery.
  The fourth study optimized PAA as a promising host to support alkaline electrolytes and other additives, including ZnO and zinc acetate in ZABs. Most importantly, stiffness and adhesion of the PAA matrix to the air electrode influence battery performance. Alternative ideas for battery designs besides bi- and tri- electrode configurations are also discussed. With a vertical double air electrode configuration, ZABs using PAA-based GPEs show unprecedented performance including high specific energy (1036 Wh kgZn−1), excellent cycling stability (190 cycles at 2×10 mA cm−2) and high power density output (2×135 mW cm−2). The study represents a viable option to replace aqueous electrolytes for high performing ZABs.

• Subjects / Keywords

  • hydrogel electrolytes
  • Zn-air
  • battery designs

• Graduation date

  Fall 2020

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-sp6b-ya43

• License

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