Electrolytes for rechargeable lithium batteries

Rechargeable lithium-ion batteries (RLIB) are excellent candidates for the next-generation power sources because of their high gravimetric and volumetric energy with respect to other cell chemistries. Actually, RLIBs supply most of the portable electronic devices (e.g., mobile phones, digital cameras, and laptops) and power tools. Recently, lithium battery applications have been expanding for several technologies, such as aerospace technology, electric vehicles, hybrid electric vehicles, and storage from renewable power sources. The lithium-ion battery technology is based on the use of electrode materials able to reversibly intercalate lithium cations, which are transferred between two host structures (positive and negative electrodes) during the charge and discharge processes. Commercial lithium-ion batteries commonly use liquid electrolytes based on suitable lithium salts (generally LiPF6) and organic solvents (generally alkylcarbonates such as EC, DEC, DMC), volatile and flammable, which represent a major problem for device safety. This issue has strongly pushed the scientific community toward the investigation of alternative, highly safe solvents. In recent years, gel polymer electrolytes have progressively replaced the above-mentioned liquid electrolytes. Additionally, a wide variety of alternative solvents as well as additives have been proposed, mainly aiming to improve the safety (flame retardants) and the compatibility (redox shuttles) of the electrolytes with high-voltage cathodes. In addition, completely dry, solvent-free electrolytes, both polymeric and inorganic, are under investigation worldwide because of the possibility to realize all solid-state lithium batteries, which are undoubtedly appealing from safety and engineering points of view and open new perspectives on applications. Finally, a new class of nonvolatile and nonflammable fluids, called ionic liquids (e.g., molten salts at room temperature), were successfully proposed in place of hazardous and toxic organic solvents for lithium battery electrolytes. Here, a survey of the most appealing types of electrolytes, proposed for modern rechargeable lithium batteries and taking a look at future trends is reported.