Jeff Dahn’s battery technology research team at Dalhousie University in Canada has developed a new means of improving high-voltage, lithium-ion battery cell performance through the use of cyclic carbonates as the enablers for ethylmethyl carbonate (EMC)-based electrolytes, rather than conventional options, according to recent reports.
The research showed that the cyclic carbonates in question — VC (vinylene carbonate), FEC (fluoroethylene carbonate), and DiFEC ((4R,5S)-4,5-Difluoro-1,3-dioxolan-2-one) — reportedly worked well as the enablers for the aforementioned EMC-based electrolytes when used in NMC442/graphite battery cells tested at high voltages (up to 4.4 V or 4.5 V).
The work clarified that the conventionally used ethylene carbonate (EC) electrolyte additive is detrimental to lithium-ion battery cell health (cycle and calendar life) at high voltages. EC use (for passivation of graphite electrodes during initial cycles) is now known to be associated with oxidation, gas generation, and impedance growth — meaning that removal has been shown to increase the working life of high-voltage lithium-ion cells.
Here’s an explanation of the work directly from the paper: “In this paper, four ‘enablers’ including EC, VC, FEC and DiFEC were compared head to head in NMC442/graphite pouch type Li-ion cells. Other enablers such as SA, MEC, PES will not be included in this paper but will be discussed in latter publications. Experiments were made using ultra high precision coulometry (UHPC), a precision storage system, electrochemical impedance spectroscopy (EIS) and a gas measurement. Gas evolution during formation and cycling, coulombic efficiency, charge endpoint capacity slippage during cycling, and EIS spectra before and after cycling, were examined and were compared to EC-based electrolyte with some promising additive blends.”
As an example, the combo of EMC with specific amounts of some of the enablers mentioned above resulted in battery cells with improved performance as compared to battery cells with EC-containing electrolytes with additives, when tested up to 4.5 V.
The paper provides more: “The work in this paper suggests that EC itself is the root cause of many issues associated with the operation of NMC/graphite cells to high potential. Electrolyte oxidation reactions at high voltages cause gas evolution and impedance growth, leading to cell failure. These parasitic reactions become very problematic at 4.5 V even with state of the art electrolyte additives PES211 in EC:EMC electrolyte. … This work demonstrates that cyclic carbonates such as VC, FEC, and DiFEC can act as the enablers for EMC-based electrolytes which function well in NMC442/graphite cells tested up to 4.4 or 4.5 V.”
The paper notes that more work needs to be done in order to “optimize the amount of these and other enablers and to find other co-additives that can be used together with these enablers to improve cell performance. It is very likely that other enablers can also function well. It is also very likely other linear carbonates besides EMC can function well in electrolytes without EC. Further work may also include the exploration of cycling performance at high temperature, low temperature, high rate as well as the performance in different cell chemistries (ie. LiCoO2 (LCO)/graphite and LiNi0.80Co0.15Al0.05O2 (NCA)/graphite Li-ion cells). It is essential that other researchers get involved in such searches.”
A couple of final things that should be noted with regard to this work: A patent has been filed for the work by 3M; financial support was provided by NSERC and 3M Canada through the Industrial Research Chairs program; and, while Dahn is now in an exclusive 5-year research partnership with Tesla that began in June, most or all of this work predates that agreement.
The new work is detailed in a paper published in the Journal of Power Sources, and another paper has been submitted by the researchers to the Journal of the Electrochemical Society.
Image via Dalhousie University
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