Recent progress in nonflammable electrolytes and cell design for safe Li-ion batteries

Nonaqueous lithium-ion batteries (LIBs) are critical energy storage technologies for portable electronics and electric vehicles (EVs) owing to their high operating voltages (>3.5 V) and energy densities (& SIM;250 W h kg(-1)). However, they are suffering from the safety concerns induced by the uncontrolled thermal runaway owing to a series of exothermic reactions and the employment of highly flammable electrolytes. Here, we start with the fundamentals of the thermal failure mechanism of LIBs associated with the assessment matrix and testing protocols from the material-level to the battery-level. Next, we provide a critical review of nonflammable electrolytes' design principles, including the classical mechanisms of flame retardancy, as well as the recent progress in nonflammable electrolytes' design strategies. In addition, a comprehensive summary of the design strategies of flame-retardant cell components beyond electrolytes is also included considering that nonflammable electrolytes alone cannot guarantee battery-level safety for high-energy-density LIBs. Finally, our perspectives on the design strategies of safe high-energy-density LIBs are discussed to promote their practical applications in the future.