Page 603 - Handbook of Battery Materials

17.4 Bulk Properties 577

Table 17.14 Possible reactions of lithium, lithium salts, and
decomposition products with water at graphite electrodes.

Reaction References

− + 1 [333]
H 2 O + e + Li → LiOH ↓+ H 2 ↑
2
− + 1
LiOH + e + Li → Li 2 O ↓+ H 2 ↑ [333]
2
LiOH + HF → LiF + H 2 O [342, 343]
Li 2 O + 2HF → 2LiF + H 2 O [343]
LiMF n + H 2 O → LiF + 2HF + MOF n−3 [342, 344]
LiMF n + H 2 CO 3 → [342]
LiF + 2HF + CO 2 + MOF n−3
Li 2 CO 3 + 2HF → 2LiF + H 2 CO 3 [343, 344]
(ROCO 2 Li) 2 + H 2 O → Li 2 CO 3 + CO 2 ↑ [336]
+(ROH) 2
ROCO 2 Li + HF → LiF + CO 2 ↑+ ROH [344]

lithium can be cycled close to its optimal capacity hundreds of times in, for example,
EC/DMC based solutions of LiAsF 6 , LiTFSI, or LiPF 6 [336].
With a high dielectric constant and flash point, EC perfectly meets the require-
◦
ments for practical use. Nevertheless, the relatively high melting point of 36.5 C
makes it impractical for low-temperature applications. For practical applications,
combinations with solvents of lower viscosity and melting point like carbonates
(e.g., DMC, EMC, DEC) or ethers (e.g., DME, THF), despite low flash points, are
necessary. Reductive decomposition of carbonates leads to lithium alkyl carbonates
that passivate lithium, for example, DEC, where lithium is dissolved to form lithium
ethyl carbonate and Li 2 CO 3 [332, 350]. Superior solvents may be asymmetric alkyl
methyl carbonates [351] like EMC [335] or MPC [352]. Methyl pentyl carbonate
(MPentC) and butyl methyl carbonate (BMC) show very good passivation abilities
when used as co-solvents [353]. In particular, BMC reaches very good performances
even in PC-based electrolytes. Due to earlier formation of the SEI compared to PC,
solvent co-intercalation can be prevented. Furthermore, BMC is a good alternative
co-solvent for EC-based electrolytes. Table 17.12 gives some reactions of common
solvents with lithium.
Decomposition reactions of solvents are important for a stable SEI, preventing
further electrolyte decomposition and exfoliation. However, the morphology of the
SEI is influenced by the lithium salt too. Table 17.13 and some examples give a
short overview of SEI formation ability of lithium salts:
When salts are more reactive with lithium than the solvent, such as LiOTf and
LiTFSI in PC [354] or LiTFSI and LiMe in DIOX (where LiTFSI is more reactive
then LiMe [337]), their reduction components dominate the film and its behavior.
For LiAsF 6 and LiClO 4 in PC [354] and LiAsF 6 in DIOX or other ethers [337],
the situation is reversed. In LiClO 4 /PC or LiAsF 6 /PC, the native film on lithium
consisting of LiOH, Li 2 CO 3 , and Li 2 O, is rather stable, and hardly reacts with
the electrolyte. However, LiAsF 6 ,LiClO 4 , and lithium halogenides show above all