Cathode engineering for high energy density aqueous hybrid-ion batteries

Abstract

As the demand for cleaner energy evolves and scales up, with the growth of sustainable energy and electric vehicles, a more stable energy source is crucial to support this demand. However, to implement renewable energy with current electric grids, we must first tackle the persisting issue of unpredictable fluctuations in both supply and demand. Therefore, better energy storage solutions must be developed to alleviate this issue. One example of these solutions is using electrochemical energy storage devices, such as lithium-ion batteries. These batteries have become widely used due to their high volumetric energy density and specific energy, allowing them to be small enough to be used in mobile applications. Despite these benefits, they suffer many drawbacks such as high toxicity, fire hazards, and limited distribution of raw materials. In response, a wave of new battery technologies has appeared in literature, with aqueous zinc-ion batteries among the most promising. These batteries, while having lower energy density compared to lithium, can operate much more safely, posing a much lower fire and explosion hazard. However, due to the risks of side reactions in both the anode and cathode, zinc-ion batteries still suffer from low cycle life and low capacity.

With the aim of unlocking and understanding the potential of aqueous zinc batteries, this study will investigate how solvent engineering, along with the dry cathode process can be a simple solution to improve battery performance, while maintaining simplicity and cost for faster integration into mass production. Considering these practical requirements, a high mass loading (>10 mg/cm²) was employed in this study. To maximize capacity and cycling stability, the optimal separator was determined to be the glass fiber membrane. Furthermore, it was discovered that despite Zn(CF3SO3)2 (or Zn(OTf)2) being the most common electrolyte for aqueous zinc-ion batteries, in certain testing conditions, the mixtures of 0.5M ZnSO4 with 0.5M Zn(CF3SO3)2, and also 2M ZnSO4 with 0.04M KAl(SO4)2 showed superior performance, delivering higher capacity over more cycles. In particular, 0.5M ZnSO4 with 0.5M Zn(CF3SO3)2 mixture was able to reach under 250 mAh/g at 0.2 A/g, and could cycle for over 5000 cycles at 5A/g. The 2M ZnSO4 with 0.04M KAl(SO4)2 also performed well, reaching close to 203 mAh/g at 0.2 A/g, and could retain 65% of its peak capacity at 5000 cycles at 5A/g. Although these results demonstrate the large potential of solvent engineering as a promising method to improve this type of battery, they also highlight the need for a deeper understanding of all interactions within a battery. Therefore, this study aims to spark interest in new research for electrolytes for aqueous zinc-ion batteries.