Functional Electrolyte Additives: A Pinch of Salt/Solvent to an Electrolyte for High Energy Density Lithium‐Ion and Lithium–Metal Batteries

Active electrode material in-conjunction with an electrolyte allows to expand the horizons of electrochemical properties and safety of Li-ion/Li-metal batteries. The role of electrolyte is pivotal in determining the performance metrics of rechargeable Li-based batteries. However, the unavoidable side-reactions imposed on the electrode active material and electrolyte due to the thermodynamic and kinetic conditions in a battery limits its performances. Electrolytes being ubiquitous for both the electrodes, a rational formulation containing a functional additive holds a great promise to tune the interfacial properties of electroactive material and to control the parasitic side reactions. Functional electrolyte additives are target specific, which serve the purpose of mitigating critical concerns related to material's properties, electrochemical performance, and safety of batteries scattered under different identities/chemistries. This review exclusively focuses on the significant impact of functional electrolyte additives on advancing the electrochemical performance, cycle-life, and safety of Li-ion/Li-metal batteries. The classification of additives and their mechanistic role is detailed on the basis of target-oriented actions involving film formation and its abnormal growth, capacity retention, overcharge/over-discharge, high-voltage, wide-temperature operation, acid and water contaminant, gas evolution, flammability, and aluminum corrosion. A perspective on the electrolyte and additive engineering considerate with the battery performances is detailed.