Influence of the Fluorination Degree of Organophosphates on Flammability and Electrochemical Performance in Lithium Ion Batteries

Abstract

Four symmetric compounds deriving from the alkyl chain containing tripropyl phosphate were designed and synthesized by varying substitution of fluorine in the side chains and added as co-solvents to yield 10%, 20% and 30% concentrated electrolyte formulations. The formulated electrolytes were physicochemically and electrochemically characterized and compared to a state-of-the-art organic carbonate-based electrolyte in regard to the flammability as well as any occurring trade-off in cycling performance. The addition of phosphates resulted in superior flammability behavior of the electrolyte as the flammability could be severely reduced with increased concentration of the phosphates. As the addition of such phosphate compounds to the electrolyte usually comes with a trade-off in cycling performance, electrochemical behavior was thoroughly investigated regarding ionic conductivity, anodic stability limit and cycling stability in lithium metal and lithium ion cells. The influence of the varying fluorine content as well as position of the substituted fluorine was determined and discussed. The tripropyl phosphate derivatives showed very promising cycling results hand in hand with a significant improvement achieved regarding the flammability of the electrolyte.

Fluorine and Lithium: Ideal Partners for High‐Performance Rechargeable Battery Electrolytes

“… Cycling behavior in NMC/graphite cells of the 30 % phosphate mixtures (tripropyl phosphate (TPrP), tris(3,3,3‐trifluoropropyl) phosphate (3F‐TPrP), tris(2,2,3,3‐tetrafluoropropyl) phosphate (4F‐TPrP), and tris(2,2,3,3,3‐pentafluoropropyl) phosphate (5F‐TPrP)) compared with the reference electrolyte at 20 °C with Li as the RE. The charging rate was set to 0.1 C for the charge and discharge process over 5 formation cycles …”

“…Furthermore, the SET results reveal that the terminal fluorination is more important than the fluorine content. This leads to the conclusion that the fully fluorinated propyl group is important for the development of a safe electrolyte formulation as well as for electrochemical performance …”

Tris(2,2,2-trifluoroethyl) Phosphate as a Cosolvent for a Nonflammable Electrolyte in Lithium-Ion Batteries

“…10 The same results have been reported in previous reports that the conductivity of carbonate-based electrolytes is reduced after adding alkyl phosphates or fluorinated alkyl phosphates as FRs. 12,24 In a word, the conductivity of the LiPF 6 -GBL/TFP (70:30) electrolyte is higher than that of the LB 301 + 30 wt % TFP electrolyte and it also reaches over 7 mS cm −1 . Combined with the good wettability and nonflammable characteristic of the LiPF 6 -GBL/TFP (70:30) electrolyte, its conductivity is enough to meet the requirement of LIBs.…”

Design Strategies of Safe Electrolytes for Preventing Thermal Runaway in Lithium Ion Batteries

“…What’s more, the F element contributes to the formation of a stable SEI film at the electrode interface and improves the interfacial capability between the electrolyte and the electrodes. In addition, the F element can also make the molecules and ions move more quickly by reducing the viscous force among molecules, thus reducing the viscosity of the electrolyte and resulting in improved ionic conductivity. , In early studies, Xu et al synthesized a series of fluorinated alkyl phosphates including tris­(2,2,2-trifluoroethyl)­phosphate (TTFP) and bis­(2,2,2-trifluoroethyl) methylphosphonate (BMP) . Both the fluorinated alkyl phosphate compounds significantly enhanced the flame retardancy in comparison to nonfluorinated alkyl phosphate, while exhibiting no negative effect on the electrochemical stability of the electrolyte.…”