Page 569 - Handbook of Battery Materials

17.2 Components of the Liquid Electrolyte 543

cathode gave a charge–discharge capacity of 152 mAh·g −1 in the voltage range
+
of 4.2−3.4 V vs Li/Li , close to the theoretical value of Li x CoO 2 (0.5 < x < 1)
and retained 119 mAh·g −1 after 50 cycles, which is still 85% of the initial value
[141]. A slight capacity decrease with cycle number is a typical observation for SEI
formation. Lower lithium salt concentrations improve the capacity of the LiCoO 2
cathode because of enhanced transport properties, yielding a slightly higher value
of 155 mAh·g −1 at the first cycle in the 0.8 M LiTFSI/[P 222(2O1) ][TFSI] electrolyte
[141].
N,N,N,N-Cyanomethyl-trimethyl-ammonium bis(trifluoromethylsulfonyl)imide
◦
([CTMA][TFSI]), with a melting point of about 35 C, is still liquid at room
temperature because of supercooling, and it thus can be used as an electrolyte
solvent. [CTMA][TFSI] has sufficiently high electrochemical stability but
suffers from a very low conductivity of 10 −4 mS·cm −1 [142]. However, on
adding LiTFSI the viscosity decreases and conductivity increases, in contrast
to noncyano-containing ILs, where viscosities increase on adding a lithium
salt. This untypical behavior may contribute to an interaction of the cyano
group with the lithium cation instead of the original interaction with the TFSI
anion, leading to a lower viscosity [142]. LiTFSI/[CTMA][TFSI] electrolytes show
reversible lithium deposition and dissolution, which indicates that a stable
protective film is generated on the lithium metal surface, preventing further
decomposition of the electrolyte. The cyano group is probably less stable than
alkyl groups toward reduction, so that the protective SEI is formed at higher
potentials. For example, in LiTFSI/[EMIm][TFSI] electrolytes with more than
20 wt% of N,N,N,N-cyanomethyl-ethyl-dimethyl-ammonium TFSI, reversible
lithium deposition and dissolution are observed [143], while neat [EMIm][TFSI]
decomposes before lithium deposition takes place and therefore produces no
protective film. Mixtures of cyano-substituted quaternary ammonium TFSI
salts with [EMIm][TFSI] show reversible charge–discharge behavior. Blends of
N,N,N,N-cyanoethyl-trimethyl-ammonium ([CEMA]), [EMIm], and LiTFSI reach
a capacity of about 110 mAh·g −1 with the LiCoO 2 cathode, while without of
[EMIm][TFSI] no useful result is achieved [144].
N-Methyl-N-propylpiperidinium bis(trifluoromethylsulfonyl)imide ([MPPip]
[TFSI]) has a wide electrochemical window of about 5.5 V. On adding
LiTFSI, the conductivity decreases from 1.51 mS·cm −1 to 0.51 mS·cm −1 at
◦
25 C [145]. In the voltage range of 3.2–4.2 V vs Li/Li + the capacity of a
Li/LiTFSI/[MPPp][TFSI]/LiCoO 2 cell decreases gradually until it reaches a
relatively constant value after 20 cycles with a Coulombic efficiency quite
close to 100% [145]. The capacity of a LiCoO 2 cathode in a 0.4 mol·kg −1
LiTFSI/[MPPpi][TFSI] electrolyte cycled in the range of 2.5–4.5 V vs Li/Li +
stabilized around 150 mAh·g −1 after a few cycles with a charge efficiency
exceeding 99.5% [146]. Lithium deposition and dissolution show only poor
−1
cycling performance on a nickel surface in a 0.4 mol kg LiTFSI/[MPPip][TFSI]
electrolyte without additives. Addition of EC leads to constant cycling efficiencies
of about 92% [146]. Charge–discharge curves with a lithium-graphite anode show
irreversible behavior without any additives. Addition of 10 wt% VC enhances the