From medium- to high-entropy hydroxides for hybrid supercapacitors: a review

The quest for devices with significantly higher power and energy density has made hybrid supercapacitors a promising alternative for energy storage. These devices have gained traction by combining the exceptional power density of supercapacitive materials with the high energy density of battery-type materials in a single system. Among promising battery-type materials, medium-entropy and high-entropy hydroxides (ME-hydroxides/HE-hydroxides) have attracted growing research interest due to their unique structural characteristics and their potential for tailoring functional properties. This new class of materials represents a significant departure from the traditional concept of low-entropy materials, paving the way for innovative advancements in energy storage technologies. Considering the significant advancements in the past five years, this review focuses on the recently developed ME- and HE-hydroxides for hybrid supercapacitors. It covers their synthesis methods, effective strategies, promising trends, and performance as positive electrode materials. Additionally, the review addresses the inappropriate use of the term “high-entropy”. Finally, the challenges and prospects in designing ME- and HE-hydroxides for hybrid supercapacitors are discussed, offering guidance for the development of new materials to advance future energy storage technologies.