The reduction stability of Li3AlF6-based Li+ solid electrolytes under the oxygen atmosphere

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At present, the passivation mechanisms in all-solid-state lithium-ion batteries with Li3AlF6-based electrolytes, including those related to the microstructure and chemical composition of the anode interphase, are not understood. Therefore, the cause of the reduction stability of Li3AlF6-based solid electrolytes has been investigated in this study. The Li3AlF6-Li2SO4 composite is a Li+-ion conductor with high tolerability under high voltages (over 6 V vs. Li+/Li). However, the theoretical onset voltage of Li3AlF6 reduction is higher than the charging voltage of graphite, indicating that reduction of Li3AlF6 (Li3AlF6 + 3Li+ + 3e− → 6LiF + Al) proceeds prior to the Li+ intercalation of graphite. Nevertheless, all-solid-state lithium-ion batteries with graphite anodes operate when Li3AlF6-Li2SO4 is used as the solid electrolyte. Hence, it is speculated that a passivation layer is formed at the anode interphase. In this study, Li/Mo cells with Li3AlF6-Li2SO4 (or Li3AlF6) were fabricated and polarized at −0.5–1.0 V vs. Li+/Li. Subsequently, the interphase microstructure was investigated. At an O2 concentration of ∼0.5%, the Mo/Li3AlF6-Li2SO4 interphase was covered by a Li2O layer after polarization at 0.5 V. This Li2O layer was also observed on the surface of Li3AlF6, where oxygen was not included as a constituent element. The thickness and coverage of the Li2O layer decreased significantly following polarization under an O2-poor atmosphere (1–2 ppm), indicating that the O2 source was the measurement atmosphere. The results also revealed Li3AlF6-Li2SO4 to be intrinsically stable at 0.5 V, as the reduction products were not clearly observed, irrespective of the presence of the Li2O layer. Although Li3AlF6-Li2SO4 was reduced at −0.5 V, this voltage is considerably lower than the theoretical onset voltage (1.06 V). Additionally, the reduction of the solid electrolyte was terminated because of the homogeneous coverage of the LiF (the decomposition product of Li3AlF6-Li2SO4) layer at the interphase. Hence, it can be concluded that the reduction stability of the Li3AlF6-based electrolyte is attributable to the sluggish kinetics of the Li3AlF6 reduction. Even if Li3AlF6-Li2SO4 undergoes reductive decomposition under overcharging conditions, a continuous reduction reaction is unlikely because of the formation of a LiF passivation layer at the interphase. Because of this kinetic stability, the anode materials whose operating voltage is lower than the reduction limit of Li3AlF6 can be used. On the other hand, Li2O formation should be controlled from the perspective of charge-discharge performances of the battery.