Page 604 - Handbook of Battery Materials

578 17 Liquid Nonaqueous Electrolytes

solvent decomposition that mostly leads to Li 2 CO 3 and lithium alkyl carbonates in
the SEI and only to a small extent to anion reduction.
Solutions of LiBF 4 or LiPF 6 in PC mainly form LiF [342, 355]. Aurbach et al.
[340] report unavoidable contaminations of HF by traces of water due to hydrolysis
in solutions of LiPF 6 and LiBF 4 .Li 2 CO 3 and lithium alkyl carbonates (ROCO 2 Li)
finally react with HF to LiF, the main component in the SEI. Further products
like Li x PO y F z and Li x BO y F z arise from hydrolysis of PF 5 and BF 3 .Noalkyl
carbonates but nonspecified adsorbates are assumed to be formed in LiPF 6 /PC,
which are believed to be responsible for the reduced dendritic lithium growth
during deposition [356].
Electrolytes with sulfonium salts like LiOTf, LiTFSI, and LiMe show solvent
decomposition, resulting in Li 2 CO 3 and lithium alkyl carbonates [357]. Anion
reduction leads to Li 2 SO 3 and C 2 F 6 , which reacts afterwards to LiCF 2 CF 3 and LiF
[339, 340]. Furthermore, Laik et al. [358] propose another reaction, involving C–F
cleavage, that results in ·CF 2 SO 3 radicals rather than cleavage of the C–S bond. On
the other hand, LiBOB forms a completely different SEI compared to that formed by
the former salts. The BOB anion shows a multistep solvent decomposition reaction
that leads to several semicarbonate-like species, other salts such as Li 2 C 2 O 4 and
LiBO 2 , and polymeric components [341].
The complexity of SEI formation is topped off with reactions of the electrolyte
with contaminants and additives. Because of different reaction rates of all reactive
components with lithium, which yield surface films of different quality, additives
can be used to modify the surface films to highly conductive lithium films,
preventing the components of the electrolyte from further decomposition. There
are many successful examples of this approach in the open and patent literature,
both for lithium anodes and also for lithiated carbon electrodes.
Typical additives include, for example, 2-methylfuran and KOH in DIOX or THF
based solutions [359, 360], methyl formate (MF) in 2-Me-THF (THF, low content)
[361], CO 2 in PC [362, 363], DMC, or EC at lithium electrodes [72, 336], and CO 2 at
lithiated carbon electrodes [62, 63]. It is also interesting that even some gases (O 2 ,
SO 2 ) at low pressures form thin primary films in fast reactions [364–368].
Aluminum oxide, which is used as a drying agent, increases the cycling efficiency.
In 1992, Aurbach et al. [336] showed that after storing the electrolyte over Al 2 O 3 ,
cycling efficiency increased significantly. However, on bubbling argon through
the electrolyte after drying with Al 2 O 3 , the positive effect could not be observed.
The assumption that additives produced over Al 2 O 3 are gaseous was confirmed by
treatment with CO 2 , which reacts with lithium and alkoxides obtained from ethers
[369] to form a stable passive layer consisting of Li 2 CO 3 .
−
·CO 2 + e + Li →·CO 2 Li (17.41)
+
·CO 2 Li + CO 2 →·CO-O-CO 2 Li (17.42)
+ (17.43)
·CO-O-CO 2 Li + Li → CO + Li 2 CO 3
Even water in concentrations up to 200 ppm can increase cycling efficiency.
Table 17.14 shows reactions arising due to water addition. Reactions of water with