Dalhousie team reports ternary blends of electrolyte additives greatly enhance performance of NMC Li-ion cells
Batteries for electrified vehicles require much longer calendar and cycle lifetimes, as well as improved high temperature tolerance, than their portable consumer electronics counterparts. Electrolyte additives can be used to extend cell lifetime by suppressing parasitic reactions between charged electrodes and electrolyte by modifying the solid electrolyte interphase (SEI) at the negative electrode or the passivation layer formed on the positive electrode.
Researchers at Dalhousie University (Canada) led by Dr. Jeff Dahn now report that Li[Ni1/3Mn1/3Co1/3]O2 (NMC111)/graphite and Li[Ni0.42Mn0.42Co0.16]O2 (NMC442)/graphite pouch cells demonstrate greatly enhanced performance when ternary blends of electrolyte additives are added to the cells. Their work is published in a paper in an open access article in the Journal of the Electrochemical Society.
In the study, the Dalhousie team investigated the performance of NMC pouch cells with electrolytes with various sulfur- or phosphorous-containing electrolyte additives at a level of 2 wt% as well as electrolytes with the ternary electrolyte additive blends VC (vinylene carbonate) + MMDS (methylene methanedisulfonate) + TTSPi (tris(trimethylsilyl) phosphite) and PES (prop-1-ene-1,3-sultone) + MMDS + TTSPi. (I.e., the ternary blends contain one of VC or PES plus a sulfur containing molecule and TTSPi).
They found that, under long-term cycling tests at elevated temperature, the ternary electrolyte additive blends “VC-211” and “PES-211” showed excellent charge-discharge capacity retention by comparison to cells with only VC or PES additives alone. “PES-211” showed itself especially useful for NMC442/graphite pouch cells operated at 4.4 V.
MMDS and TTSPi significantly reduced the reactivity between lithiated (charged) graphite and the electrolyte. The ternary electrolyte blends of VC + MMDS + TTSPi and PES + MMDS + TTSPi show no reactivity between charged graphite and electrolyte to above 200°C. By contrast, electrolytes with no additives or with PES alone show significant reactivity with lithiated graphite below 200 °C. NMC/graphite cells with VC + MMDS + TTSPi or PES + MMDS + TTPSi additives will thus demonstrate long life and excellent safety, the researchers concluded.
The additive blend, “PES-211”, can yield significant cycle life improvements for NMC/graphite cells operating at both 4.2 V and 4.4 V and lower thermal reactivity for the charged graphite electrode. We encourage Li-ion battery manufacturers and researchers studying NMC-based Li-ion cells to try “PES211” in their experiments.—Ma et al.
Lin Ma, Jian Xia, and J. R. Dahn (2015) “Ternary Electrolyte Additive Mixtures for Li-Ion Cells that Promote Long Lifetime and Less Reactivity with Charged Electrodes at Elevated Temperatures” J. Electrochem. Soc. 162(7): A1170-A1174; doi: 10.1149/2.0181507jes