Towards sustainable smart displays: A review of lithium-free electrochromic devices

Electrochromic (EC) materials are capable of reversible and controllable transformation of optical properties through electrochemical reactions. Lithium (Li)-based electrolytes are widely employed in EC devices (ECDs) due to their high ionic conductivity and rapid intercalation kinetics, leading to fast switching speeds and enhanced coloration efficiency. However, challenges such as volatility, reactivity, safety, and cyclic stability remain critical bottlenecks in Li-based EC systems. ECDs offer significant advantages such as low power consumption, high contrast ratio, and dynamic optical control, making them suitable for a spectrum of applications. Central to these is the development of Li-free ECDs and to this end, electrolytes employing ions such as sodium, potassium, magnesium, ammonium, and hydrogen (protons) have seen rapid advancement. Furthermore, ensuring compatibility with flexible substrates to meet the mechanical and durability requirements of next-generation electronics remains a critical challenge. Achieving these multifaceted attributes simultaneously is often challenging due to the conflicting requirements. Consequently, a diverse range of EC materials, in combination with Li-free electrolytes, has been extensively investigated.This review discusses recent progress in Li-free ECDs and their integration with flexible substrates and electrodes. Emphasis is placed on advances in material synthesis, device architecture, electrochemical performance, flexibility, and environmentally benign fabrication methods. Key challenges related to long-term device reliability, optical performance, and mechanical resilience are examined alongside innovative solutions involving nanoscale material engineering and sustainable processing. Finally, the review outlines promising future directions aimed at high-performance, sustainable, flexible smart displays with greener, more adaptable materials and device platforms.