Low-Flammble Electrolytes for Stable Operation of High Energy-Density Lithium-Ion Batteries

Abstract

Due to its flame retarding effect, trimethyl phosphate (TMPa) was previously employed to suppress the flammability of the conventional LiPF6-organocarbonates electrolytes used in lithium ion batteries (LIBs). However, the presence of TMPa in conventional electrolytes interferes with the formation of effective solid electrolyte interphase (SEI) on graphite (Gr) electrodes. Consequently, the introduction of TMPa in electrolytes leads to significantly deteriorated electrochemical performance of LIBs. To resolve the incompatibility issue between TMPa and Gr electrode, the concept of localized high-concentration electrolyte (LHCE) was applied to TMPa. In the TMPa based LHCE (E-TMPa for abbreviation), the unique solvation structure of the ion sheaths facilitates the decomposition of anions of the conducting salt. The synergetic decompositions of TMPa and anions create an effective SEI on Gr that allows reversible lithiation/delithiation of the Gr electrodes. However, the SEI formed in E-TMPa only offers sufficient protection for <200 charge/discharge cycles. The Gr||NMC811 (LiNi0.8Mn0.1Co0.1O2) cells using E-TMPa suffer from a sudden capacity loss resulting from solvent co-intercalation and Gr exfoliation due to SEI failure. To further improve the SEI quality, electrolyte additives were introduced into E-TMPa. Among all the studied additives, fluoroethylene carbonate (FEC) stood out as the best electrolyte additive because of the formation of effective SEI in this FEC containing electrolyte (E-TMPa-F). Gr||NMC811 cells using E-TMPa-F achieved a capacity retention of 85.4% after 500 charge/discharge cycles within the voltage range of 2.5-4.4 V, which is significantly higher than that of cells using a conventional electrolyte (75.4%). In addition, the high TMPa content in LHCEs effectively suppressed the flammability of the electrolytes. Meanwhile, because of the reduced flammability and excellent compatibility with electrodes, E-TMPa-F is considered to be a promising electrolyte for constructing high energy-density LIBs with improved safety performance.