Page 601 - Handbook of Battery Materials

17.4 Bulk Properties 575

The composition of the passive layer is complex and the underlying chemistry is
rather complicated [331]. Apart from electrolytes, solvents, possible contaminants,
and additives the history of the electrode (e.g., cycling, storage, temperature, etc.)
play an important role. As consequence, it is crucial to figure out the reaction
mechanisms of the heterogeneous and homogeneous reactions, which depend on
the morphology of films and the solubility of reaction products. Tables 17.12–17.14
sum up different reactions on lithium and graphite electrodes.
In electrolytes based on solvent mixtures, both solvent constituent compounds
may react to form films of scarcely soluble materials. PC/THF mixtures yield
alkoxides and alkyl carbonates [345, 346], EC/ether blends mainly yield alkyl
carbonates, which are thought to be the reason for smaller lithium loss during
cycling [346]. For example, PC is reduced on graphite electrodes first at 0.8 V vs
Li/Li + and lower potentials and forms a passivating film consisting of Li 2 CO 3
and several lithium alkyl carbonates on the graphite surface [338]. Nevertheless,
co-intercalation of solvated lithium ions occurs in PC-based electrolytes [347], and
the decomposition to propylene gas leads to severe deterioration of the graphite
electrode [348, 349]. Because of this drawback of PC, other solvents are of interest.
EC, with the difference of only one methyl group compared to PC, reveals a very
effective passivation and diminishes the risk of solvent co-intercalation drastically
due to earlier film formation at higher potentials. In the first reduction step, a
one-electron transfer, cyclic and open-chain species are formed by a C–O cleavage.
With EC, only one open-chain radical occurs, whereas with PC there are three

Table 17.12 Reductive decomposition reactions of common solvents with lithium.

Reaction References

− +
DEC + 2e + 2Li → Li 2 CO 3 ↓+C 4 H 10 ↑ [332]
− + [333, 334] a
DMC + e + Li →·CH 3 + LiO 2 COCH 3 ↓
− + a
DMC + e + Li →·OCOCH 3 + LiOCH 3 ↓ [333]
− + [332]
DMC + 2e + 2Li → Li 2 CO 3 + C 2 H 6 ↑
− +
2DMC + 2e + 2Li → 2LiO 2 COCH 3 ↓+C 2 H 6 ↑ [332]
− + [332, 334]
EC + 2e + 2Li → Li 2 CO 3 ↓+C 2 H 4 ↑
− +
EC + 2e + 2Li → LiCH 2 CH 2 OCO 2 Li ↓ [333]
− + [333]
2EC + 2e + 2Li → (LiO 2 COCH 2 ) 2 ↓+C 2 H 4 ↑
− + [335] a,b
EMC + e + Li →·OCOCH 3 + LiOCH 2 CH 3
− + [335] a,b
EMC + e + Li →·OCOCH 2 CH 3 + LiOCH 3
− + [335] a,b
EMC + e + Li →·CH 2 CH 3 + LiO 2 COCH 3
− +
PC + 2e + 2Li → Li 2 CO 3 ↓+C 3 H 6 ↑ [334, 335]
− + [334, 335]
2PC + 2e + 2Li → CH 3 CH(OCO 2 Li)CH 2 OCO 2 Li ↓+C 3 H 6 ↑
− + [334, 335] a
LiO 2 COR + e + Li → R·+ Li 2 CO 3
a Alkyl (R·) and acyl (ROCO·) radicals react according to R·+ e + Li → RLi and
+
−
+
ROCO·+ e + Li → LiOR + CO, where LiOR may react with EMC, yielding ethers or other
−
alkoxides and carboxylate anions.
b
Asymmetric alkyl methyl carbonates like MPC or BMC react in a similar way to EMC.