Researchers at the Graduate School of Engineering and Graduate School of Science at the University of Tokyo have designed and synthesized a fluorinated cyclic phosphate solvent, 2-(2,2,2-trifluoroethoxy)-1,3,2-dioxaphospholane 2-oxide (TFEP), for use in lithium-ion batteries. The solvent molecule has a fused chemical structure of cyclic carbonates that can form a stable solid electrolyte interphase and organic phosphates that can trap hydrogen radicals and prevent combustion.
In a paper in Nature Energy, they report that an electrolyte formula composed of 0.95 M LiN(SO2F)2 in TFEP/2,2,2-trifluoroethyl methyl carbonate shows excellent non-flammability with zero self-extinguishing time and enables the highly stable operation of graphite anodes (~0.1 V versus lithium) and high-voltage LiNi0.5Mn1.5O4 cathodes (~4.7 V versus lithium), and thereby outperforms traditional electrolytes. They conclude that the work opens up new frontiers in electrolyte developments towards safe lithium-ion batteries with higher energy densities.
Different parts of the TFEP molecule account for different improvements. © 2020 Yamada et al.
The conventional electrolyte for lithium-ion batteries is a combination of 1 M LiPF6 with a cyclic carbonate-based solvent (for example, ethylene carbonate).
TFEP combines the cyclic carbonate-like structure (required for the formation of a stable solid-electrolyte interface (SEI) via a ring-opening polymerization) with the possession of a phosphorous atom (to trap the hydrogen radicals required for combustion to take place) to give an EC-free, LiPF6-free non-flammable electrolyte.
A battery’s voltage is limited by its electrolyte material. The electrolyte solvent in lithium-ion batteries is the same now as it was when the batteries were commercialized in the early 1990s. We thought there was room for improvement, and we found it. Our new fluorinated cyclic phosphate solvent (TFEP) electrolyte greatly improves upon existing ethylene carbonate (EC), which is widely used in batteries today.—Professor Atsuo Yamada
EC is notoriously flammable and is unstable above 4.3 volts; TFEP, on the other hand, is nonflammable and can tolerate greater voltages of up to 4.9 volts. This extra voltage in an otherwise identically sized package can mean the batteries can last longer before they need another charge.
Zheng, Q., Yamada, Y., Shang, R. et al. (2020) “A cyclic phosphate-based battery electrolyte for high voltage and safe operation.” Nat Energy doi: 10.1038/s41560-020-0567-z