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388 13 Rechargeable Lithium Anodes
Later, Saito et al. [59] studied anodes with a layered structure consisting of
Li/protective film/additive/protective film/Li/ protective film/additive/–. They
made the anode by dropping the additive on a lithium sheet, folding the lithium
sheet, and then compressing the folded lithium with an oil press. They repeated
this process more than 10 times. The FOM in LiAsF 6 –EC/2MeTHF electrolyte
was 7.41, 13.5, and 37.0 for a lithium anode without additives, a lithium anode
with toluene in the electrolyte, and a layered-structure lithium anode containing
toluene, respectively.
Other interesting examples of these less reactive additives to improve lithium
cycling efficiency are siloxanes [60, 61]. The influence of four poly-ether modi-
fied siloxanes as electrolyte additives on charge–discharge cycling properties of
lithium was examined. As siloxanes, diethylene glycol methyl-(3-dimethyl(trimethyl
siloxy)silyl propyl)ether (sample A), diethylene glycol methyl-(3-dinethyl(trimethyl
siloxy)silyl propyl)-2-methylpropyl ether (sample B), diethylene glycol methyl-
(3-bis(trimethylsiloxy)silyl propyl)ether (sample C), and diethylene glycol-3-
methyl-bis(trimethylsiloxy)silyl-2-methyl propyl)ether (sample D) were investi-
gated. The chemical structures of samples B and D are shown in Figure 13.4. As
a base electrolyte solution, 1 M LiPF 6 - EC/ethylmethyl carbonate (EMC) (mixing
volume ratio = 3 : 7) was used (EM). Lithium cycling efficiencies of lithium-metal
anodes improved, and an impedance of anode/electrolyte interface decreased
on adding poly-ether modified siloxanes. Among these siloxanes, sample B and
sample D exhibited much better performance. Figure 13.5 shows the Cole-Cole
plot of Li/Li cells after first charge (plating Li on SUS). In this figure, R 1
corresponds to the impedance of electrolyte solution. R 2 and R 3 correspond to
the impedance of the interface between Li and the electrolyte solution, that is,
between SEI and electrolyte solution. Two components of impedance (R 2 and R 3 )
are also observed in EM alone. Two components of impedance may arise from
the double layer structure of SEI, for example, layers of organic compounds such
as lithium alkyl carbonate and inorganic compounds such as LiF [62]. After first
charge, the impedance of the electrolyte/Li interface in EM + sample D (10 vol%)
was smaller than that in EM alone. This result suggests that the impedance of the
surface film or layer of lithium in EM + siloxanes is smaller than that in EM alone.
Figure 13.6 shows the proposed models for the mechanism of the enhancement
of lithium cycling efficiency (Eff) by adding siloxanes. Just after charging (lithium
deposition), freshly deposited lithium is chemically active. On the lithium surface,
EM is chemically reduced by lithium and produces the surface film. The reduction
B D
CH 3 CH 3
CH CHCH O-(C H O) CH 3 CH -CHCH O-(C H O) CH 3
2
2 4
2
2
2 4
2
2
2
(H C) SiO-Si(CH ) (H 3 C) 3 SiO-SiO-Si(CH 3 ) 3
3
3
3 2
CH 3
Figure 13.4 Chemical structure of polyether-modified siloxanes.