Structural regulation chemistry of lithium-ion solvation in nonflammable phosphate-based electrolytes for high interfacial compatibility with graphite anode

With the booming development of lithium-ion batteries, safety has become one of the most primary focuses of current researches. Although there are various approaches to enhance the safety of lithium-ion batteries, phosphate-based electrolyte holds the greatest potential for practical application due to their non-flammability. Nonetheless, its compatibility issue with the graphite anode remains a significant obstacle to its widespread use. Herein, an effective method is proposed to improve the compatibility of electrolyte with graphite (Gr) anode by rationally adjusting the proportion of lithium salt and solvent components to optimize the Li+ solvation structure. By slightly increasing the Li+/triethyl phosphate (TEP) ratio, TEP alone cannot fully occupy the inner solvation sheath and therefore less polar ethylene carbonate (EC) has to be recruited, and the solvation structure gradually changes from Li+–[TEP]4 to Li+–[TEP]3[EC] with the coexistence of EC and TEP. Simultaneously, EC molecules in the Li+–[TEP]3[EC] could be preferentially reduced on graphite compared to the TEP molecules, resulting in the formation of a uniform and durable solid-electrolyte interphase (SEI) layer. Benefiting from the optimized phosphate-based electrolyte, the Gr|Li battery exhibits a capacity retention rate of 96.8% after stable cycling at 0.5 C for 470 cycles which shows a longer cycle life than the battery with carbonate electrolyte (cycling at 0.5 C for 450 cycles). Therefore, this work provides the guidance for designing a non-flammable phosphate-based electrolyte for high-safety and long cycling-life lithium-ion batteries.