Page 572 - Handbook of Battery Materials

546 17 Liquid Nonaqueous Electrolytes

and 60 mPa·s [158]. On graphite electrodes, reversible lithium intercalation oc-
curs due to formation of a protecting SEI generated from the FSI anion [154,
159]. In an electrolyte consisting of 0.3 M LiTFSI in [PYR 13 ][FSI], the graphite
electrode shows a charge–discharge capacity of 130 mAh·g −1 that does not im-
prove on addition of VC. The Coulombic efficiency increases above 90% within
50 cycles [155]. Otherwise, in the case of 0.3 M LiPF 6 in [PYR 13 ][FSI], adding
VC slightly deteriorates the irreversibility of lithium deposition and dissolution,
while the specific capacity of about 300 mAh·g −1 is twice that in LiTFSI-based
electrolytes [155]. However, electrolytes based on [PYR 13 ][FSI] containing LiPF 6
have lower cycling stabilities than that containing LiTFSI [155]. The mixtures of
[PYR 13 ][FSI] and [PYR 14 ][TFSI] with LiTFSI or LiPF 6 show electrochemical win-
dows above 5 V for all compositions. In both cases the ionic conductivity increases
with increasing amount of [PYR 13 ][FSI] [154]. Compatibility of 0.7 M LiFSI in
[PYR 13 ][FSI] was tested with lithium–graphite anodes and LiFePO 4 cathodes [160].
Impedance measurements indicate higher diffusion resistance in IL electrolytes
due to higher viscosities and comparable interface resistance similar to that of
common organic electrolytes, indicating that a stable SEI is formed on the graphite
electrode. Charge–discharge cycles between 0 and 2.5 V vs Li/Li + show a re-
versible capacity of 367 mAh·g −1 for the lithium–graphite anode after the first
cycle and a slight increase during the next four cycles. At this point, Coulombic
efficiency reached more than 80% [160]. The LiFePO 4 cathode was pre-treated with
[PYR 13 ][FSI] in vacuum at elevated temperature, thus giving an initial capacity
of 140 mAh·g −1 at 1 C rate and Coulombic efficiencies of 100%, a considerable
improvement over a nonpretreated one [160]. Unfortunately, the capacity drops
significantly for rates higher than C/2, which is worse than for cells with common
electrolytes.
For 1-alkyl-3-methylimidazolium TFSI ILs it was found that the length of the
alkyl chain influences cell performance with an LiCoO 2 cathode [161]. All solutions
−1
containing 0.32 mol·kg −1 LiTFSI show conductivities above 1 mS·cm , decreas-
ing with alkyl chain length, due to higher viscosity. Discharge capacities of about
135 mAh·g −1 are close to the theoretical value of Li x CoO 2 (0.5 < x < 1). Particu-
larly, alkyl chains longer than hexyl enhance cycling performance and Coulombic
efficiencies. After 100 cycles capacities retain values of more than 110 mAh·g −1
[161]. Imidazolium-based ILs suffer from the low cathodic stability of the cation at
+
1.1 V vs Li/Li . Therefore lithium and graphite are unsuitable as anode materials
for these electrolytes. For an Li 1+x [Li 1/3 Ti 5/3 ]O 4 anode, reversible lithium interca-
lation and de-intercalation is possible, and a good cell performance is obtained
up to 50 cycles [162]. Compatibility of 1 M LiPF 6 in 1-ethyl-3-methylimidazolium
bis(trifluoromethylsulfonyl)imide ([EMIm][TFSI]) with a graphite anode was re-
ported by addition of 5% VC with only a slight capacity loss over 150 cycles
[163, 164].
Lithium cell performance of electrolytes of 0.4 and 0.8 M LiTFSI in 1,2-diethyl-
3,4-dimethylimidazolium bis(trifluoromethylsulfonyl)imide ([DEDMI][TFSI]) with
an LiCoO 2 cathode was investigated because of the enhanced cathodic stability of
alkylated imidazolium salts [165]. The conductivities of 1.4 and 0.8mS·cm −1 of 0.4