Dahn team develops ethylene-carbonate-free electrolytes for better-performing high-voltage Li-ion cells
Conventional electrolytes for Li-ion batteries contain ethylene carbonate (EC) and other additives. However, the cycling performance of Li-ion cells using these carbonate-based electrolytes has been poor at higher voltages (≥4.4 V) due to increased electrolyte oxidation at the surface of the cathode as the potential increases. This increased oxidation results in salt consumption, gas evolution and impedance growth, all reducing the energy density and the lifetime of Li-ion cells.
A team led by Professor Jeff Dahn at Dalhousie University (Canada) has now demonstrated that EC is actually detrimental for Li-ion cells at high voltages and that cyclic carbonates such as VC (vinylene carbonate), FEC (fluoroethylene carbonate) and DiFEC ((4R,5S)-4,5-Difluoro-1,3-dioxolan-2-one) can act as the enablers for EMC (ethylmethyl carbonate)-based electrolytes which function well in NMC442/graphite cells tested up to 4.4 or 4.5 V. A paper on their work is published the Journal of Power Sources. The team has also submitted a related paper to the Journal of the Electrochemical Society.
EC has been considered essential for the passivation of the graphite electrodes surface during the first cycle. However, left-over EC in the electrolyte may be continuously oxidized at the positive electrode in cells operated to high voltage. This may lead to gas generation and impedance growth. Dahn and his team recently showed (as reported in the paper submitted to JES) that the removal of EC from carbonate-based electrolytes yielded high voltage Li-ion cells with longer life-time.
The Dahn team has also shown that EC-free-linear alkyl carbonate-based electrolytes with a small amount of “enabler”—vinylene carbonate (VC)—allowed NMC(442)/graphite cells to be cycled up to 4.4 V with longer cycle and calendar life, compared to cells with a state-of-the-art electrolyte incorporating additives. As an example, NMC(442)/graphite cells filled with 1 M LiPF6 EMC:VC (98:2) + 2% PPF (pyridine phosphorus pentafluoride) had longer cycle life at both room temperature and 55 ˚ C when cycled up to 4.4 V than cells with EC-based electrolytes incorporating state of the art additive blends.
The EMC:VC (98:2) electrolyte system also has acceptable conductivity, wets separators quickly, shows good tolerance to high voltage and provides cells with low polarization growth when cycled up to 4.4 V.
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.——Jia et al. 2016a
The combination of EMC with appropriate amounts of these enablers yielded cells with better performance than cells with EC-containing electrolytes incorporating additives tested to 4.5 V.
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.
… Further work should be done 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.—Xia et al. (2016a)
3M has filed a patent on this work.
The work was supported financially by NSERC and 3M Canada under the auspices of the Industrial Research Chairs program. (3M and NSERC had funded Dahn’s Industrial Research Chair in Materials for Advanced Batteries since 1996. This supported ended in June 2016, and the Tesla relationship took over.)
Jian Xia, Remi Petibon, Deijun Xiong, Lin Ma, J.R. Dahn (2016a) “Enabling linear alkyl carbonate electrolytes for high voltage Li-ion cells” Journal of Power Sources 328 124-135 doi: 10.1016/j.jpowsour.2016.08.015
Jian Xia, Remi Petibon, A. Xiao, W. M. Lamanna, and J. R. Dahn (2016b) “Some Fluorinated Carbonates as Electrolyte Additives for Li(Ni0.4Mn0.4Co0.2)O2/Graphite Pouch Cells” J. Electrochem. Soc. 163(8): A1637-A1645 doi: 10.1149/2.0831608jes (open access)