| Summary: | 博士 === 國立成功大學 === 材料科學及工程學系碩博士班 === 95 === Li+ conductors are key materials for technological applications as all- solid-state lithium batteries. Unfortunately, many crystalline phases having high Li+ conductivity are unstable in the presence of a metallic lithium anode. Although interfacial reactions between such conductors and the lithium anode are always inferred, very little effort has been devoted to studying this interfacial instability, which is usually examined by observing the coloration of sample with the naked eye. The scope of our study is, therefore, focused on the detailed reaction mechanism and its suppression.
Among the various ceramic Li+ conductors, we employed a perovskite-type La2/3–xLi3xTiO3 as a model material for our fundamental study into the interfacial instability. This selection stems from the availability of the crystal structure and the easily comprehensible ion-transport mechanism. In this study, we found that when this La0.56Li0.33TiO3 sample and lithium were placed in contact at room temperature for 24 h, the results of X-ray photoelectron spectrometry (XPS) indicate that 12% of the tetravalent Ti4+ ions were converted into trivalent Ti3+ ions and the valence conversion and degree of conversion were limited by the structural rigidity of the host crystal. The secondary ion mass spectrometry (SIMS) analyses suggests the existence of a local electric field near the contact surface and indicates that the 6Li+ isotope ions were inserted into the specimen through the effect of this field. The mechanism of the lithium-activated RT interfacial reaction is associated with the reduction of Ti4+ transition metal ions from tetravalent to trivalent states, which resulted in the increase of electronic conductivity, and the local-electric-field-induced Li+ insertion into La3+/Li+-site vacancies of La0.56Li0.33TiO3. Moreover, although this metallic-lithium- activated donor doping process semiconductorized the sample on its surface, the ionic conduction of bulk sample was altered to mixed ionic/electronic conduction, which includes a spontaneous electronic transition without directly depending on the interfacial instability. Through our identification, this transition is the lithium-ion- motion dependent electron hopping process. As a result, it was roughly found that the phenomena mentioned above were caused by the presence of Ti4+–transition metal ions and the highly vacant structure in La2/3–xLi3xTiO3 system. Thus, La0.50Li0.50TiO3 has higher reaction inhibition against metallic lithium, and we also found that the perovskite-type LaAlO3 can be incorporated into La0.50Li0.50TiO3 to form a xLaAlO3–(1–x)La0.50Li0.50TiO3 solid solution (0.0 ≤ x ≤ 1.0; i.e., La0.50+0.50xLi0.50–0.50x - Ti1–xAlxO3), indicating that the Al3+ ions can be completely substituted for the Ti4+ ions. However, the samples with 0–40 mol % LaAlO3 have the Li+ ion conduction properties to be used as solid electrolytes of electrochemical devices. After measuring its electrical transition by using metallic lithium electrodes and digital multimeter, the altered process indicate that the sample’s electron concentration decreased with the incorporated amount of LaAlO3, in accordance with the decrease in the resulting electronic conductivity form 1.23× 10–2 S cm–1 to 4.33×10–4 S cm–1. Consequently, Al3+ ions substituted for the Ti4+ ions can assist La0.50Li0.50TiO3 to suppress the interfacial reaction between solid electrolyte and metallic lithium.
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