Dalhousie team explores impact of different electrolyte solvents and electrolyte additives on high-voltage Li-ion cells
One pragmatic approach to delivering the high energy-density Li-ion batteries required for longer EV range is to boost the operating voltage of batteries above the current ~4 volts. However, the performance of some higher voltage electrode materials is poor in conventional carbonate-based electrolytes due to increased electrolyte oxidation at high positive potentials, leading to cell failure stemming from gas generation and impedance growth.
As a result, successfully operating higher voltage Li-ion cells may require a combination of new electrolyte solvents, electrolyte additives as well as surface coatings. A team at Dalhousie University (Canada) led by Professor Jeff Dahn has explored the impact of different electrolyte solvents and electrolyte additives in high-voltage coated and uncoated NMC442 (LiNi0.4Mn0.4Co0.2O2)/graphite cells and compared them head-to-head using an automatic storage system (up to 4.7 V) and automated EIS/cycling measurements (up to 4.5 V). A paper detailing their findings is published in the Journal of Power Sources.
The researchers measured voltage drop and impedance growth during cycling experiments as well as gas evolution during both cycling and storage. They also performed long-term cycling experiments (to 4.5 V) to compare the charge-discharge cycling stability of cells containing these electrolyte systems.
They used eight electrolyte blends containing different electrolyte solvents and additives in both LaPO4-coated and uncoated NMC442/graphite pouch cells.
Storage experiments showed that the voltage drop during storage at 4.3 or 4.4 V for both coated and uncoated cells was very similar for the same electrolyte choice.
At 4.5 V or above, the LaPO4-coated cells had a significantly smaller voltage drop than the uncoated cells except when fluorinated electrolytes were used.
Automated charge discharge cycling/impedance spectroscopy testing of cells held at 4.5 V for 24 h every cycle showed that all cells containing ethylene carbonate:ethyl methyl carbonate electrolyte or sulfolane:ethyl methyl carbonate electrolyte exhibited severe capacity fade.
By contrast, cells containing fluorinated electrolytes had the best capacity retention and smallest impedance growth during these aggressive cycling/hold tests.
Long-term cycling experiments to 4.5 V confirmed that cells containing fluorinated electrolyte had the best cycling performance in the uncoated LiNi0.4Mn0.4Co0.2O2/graphite cells while cells containing sulfolane:ethyl methyl carbonate electrolyte had the best cycling performance in coated LiNi0.4Mn0.4Co0.2O2/graphite cells.
It is clear that coatings, electrolyte solvents and electrolyte additives all play a role in determining the final cell performance. Successfully operating Li-ion cells to high voltage may require a combination of all these strategies.
This work was mainly focussed on NMC442/graphite cells charged to high potentials. FEC:TFEC-based electrolytes show promise for cycling at high potential by limiting the impedance growth (although initial impedance is high). However gas evolution at high potentials was still a problem even when gas reducing additives like PES were added. The gassing problem of FEC:TFEC-based electrolytes may be due to the use of large amounts of FEC which is apparently not stable at high temperature and/or high potentials. Alternative solvents are required. We encourage other researchers to assist in the search.—Xia et al.
Jian Xia, K.J. Nelson, Zhonghua Lu, J.R. Dahn (2016) “Impact of electrolyte solvent and additive choices on high voltage Li-ion pouch cells,” Journal of Power Sources, Volume 329, Pages 387-397 doi: 10.1016/j.jpowsour.2016.08.100