The first author Dong Dejian, a PhD student in CUHK’s Department of Mechanical and Automation Engineering and member of Professor Lu's team, shows the prototype of the zinc ion batteries using highly concentrated zinc acetate electrolyte. Image: Lu et al, Natural Communications

Hydrotropic agents for aqueous zinc batteries

1 August 2023

A research team led by Professor Lu Yi-chun of the Chinese University of Hong Kong has taken an important step in creating a high-performance, eco-friendly electrolyte for aqueous zinc batteries. 

The team used hydrotropic agents to significantly increase the solubility of zinc acetate in battery electrolytes, overcoming key technical barriers.

Lithium ion (Li-ion) batteries provide a stable energy output and have satisfactory lifespan but use toxic, flammable organic electrolytes, which come with serious safety concerns. In contrast, aqueous zinc systems are non-flammable and use a cheap and environmentally salt.

However there are technical barriers including a hydrogen evolution reaction and dendrite growth at the anode side which can cause short circuits. Existing approaches to mitigating the needle-like zinc often involve the use of a large amount of halogen-containing salts, which raises issues of environmental sustainability.

Lu and her colleagues experimented with an approach which is often used in the pharmaceutical industry: using hydrotropic solubilisation agents to improve the solubility of the acetate. Using this approach, the team was able to improve the solubility of the zinc salt up to 14 times. One of the effective hydrotropic solubilisation agents demonstrated in the study was urea, which is non-toxic, eco-friendly and often used in topical dermatological products to promote hydration of the skin. Potassium acetate and acetamide were also tested as hydrotropic agents with similar results.

The test system demonstrated a satisfactory life span of more than 4,000 cycles with fast (six-minute) charging. Common side reactions such as hydrogen gas evolution were are virtually eliminated.

Professor Lu said, “Our work provides a rational, universal approach to breaking the solubility limit of cost-effective, eco-friendly salts for sustainable, high-performance battery applications. It will unlock the potential of many eco-friendly materials to be used for clean energy applications.” The findings were published in Nature Sustainability.