Recent Progress of Urea-Based Deep Eutectic Solvents as Electrolytes in Battery Technology: A Critical Review

Urea, a basic chemical compound, holds diverse applications across numerous domains, ranging from agriculture to energy storage. Of particular interest is its role as a hydrogen bond donor (HBD). This specific characteristic has propelled its utilization as an essential component in crafting deep eutectic solvents (DESs) for battery electrolytes. Incorporating urea into DESs presents a promising avenue to address environmental concerns associated with traditional electrolytes, thereby advancing battery technology. Conventional electrolytes, often composed of hazardous and combustible solvents, pose significant environmental risks upon improper disposal potentially contaminating soil and water and threatening both human health and ecosystems. Consequently, there is a pressing need for eco-friendly alternatives capable of upholding high performance and safety standards. DESs, categorized as organic salts resulting from the blending of two or more compounds, have emerged as promising contenders for the next generation of electrolytes. Urea stands out among DES electrolytes by enhancing ion transport, widening the electrochemical window stability (ESW), and prolonging battery cycle life. Further, its non-toxic nature, limited flammability, and elevated thermal stability play pivotal roles in mitigating environmental concerns and safety issues associated with traditional electrolytes. Laboratory testing of urea-based DES electrolytes across various battery systems, including Al-ion, Na-ion, and Zn-ion batteries, has already been demonstrated. This review examines the evolution of urea-based DES electrolytes by elucidating their structure, molecular interaction mechanisms, performance attributes, and preparation methodologies.