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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.
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