Abstract
Electrolytes lie at the heart of every battery, serving as the medium that allows ions to move between electrodes and enabling energy to be stored and released efficiently. Their properties, such as ionic conductivity, electrochemical stability, and thermal resilience, directly shape the performance, safety, and lifespan of energy storage systems. As demand for reliable batteries grows in electric vehicles, renewable energy integration, and portable devices, the design of better electrolytes has become a critical research priority. This review brings together insights from a wide range of studies to examine the principles, requirements, and limitations of five major electrolyte systems: aqueous, organic, ionic liquid, solid-state, and redox-active types. Each category demonstrates clear strengths but also important trade-offs. Aqueous electrolytes remain affordable and eco-friendly yet struggle with narrow voltage windows. Organic systems deliver high energy density but introduce flammability concerns. Ionic liquids promise exceptional stability but remain expensive and viscous. Solid-state electrolytes enhance safety and energy density, though they face manufacturing and conductivity challenges. Redox-active systems stand out for durability and scalability, particularly in grid-level applications, but lack compactness. Taken together, the findings emphasize that no single solution is universal. Instead, electrolyte design must be tailored to the context, balancing performance, safety, cost, and sustainability.