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D. Le-diagramme, Nyquist de l'échantillon Mg100 (Figure 62) montre un seul demi-cercle duquel on en déduit une conductivité 1.3 10 -9 S.cm -1 à 21°C. La conductivité de ce Nasicon formé dans les phases ? et ?' déjà peu conductrices en Li [2] est donc extrêmement faible, Composés mixtes Li/Mg

, Figure 62) est complexe du fait de la présence de 2 porteurs de charges possibles (Li + et Mg 2+ ) et du grand nombre de phases en présence. Néanmoins, la conductivité ionique mesurée à température ambiante pour l'échantillon Li100 est supérieure de 5 ordres de grandeur à celle mesurée pour Mg100. On en déduit que la mobilité de Li + est très supérieure à celle de Mg 2+ et ce quelle que soit la phase considérée. Ainsi, les contributions respectives des phases contenant majoritairement des ions Li + et de celles contenant majoritairement des ions Mg 2+ sont probablement séparées dans le diagramme de, L'analyse des diagrammes de Nyquist des échantillons contenant du lithium et du magnésium, Mg25, Mg33, Mg50 et Mg75

, on observe systématiquement un demi-cercle aux hautes fréquences (noté A), une portion de demi-cercle aux moyennes fréquences (noté B) et dont la taille relative par rapport au demi-cercle A augmente avec la composition en Mg, et enfin une portion aux basses fréquences (notée C), Nous allons chercher à attribuer chacune de ces portions à une phase spécifique pour chaque échantillon

, nous avons mis en évidence la coexistence de phases Nasicon lithiées et magnésiées. Plus particulièrement pour les composés contenant majoritairement du lithium (Mg25 et Mg33), les résultats obtenus laissent clairement supposer que le lithium a formé deux phases ?'-Li et ?'-Li et que le magnésium a formé une phase ?'-Mg. Pour les composés contenant majoritairement du magnésium (Mg50 et Mg75), le lithium serait dans une phase ?'-Li et le magnésium dans des phases ?'-Mg et ?-Mg qui peuvent toutefois contenir une fraction minoritaire de lithium, La phase lithiée qui est présente dans les quatre échantillons mixtes est donc la phase ?'-Li

. Li and . Li, On peut donc suggérer que la contribution A correspond à la diffusion des Li + dans la phase ?'-Li. A noter toutefois que pour Mg25 et Mg33 il existe deux autre phases lithiées, les phases ?

, Ainsi il est possible que leurs contributions dans le diagramme de Nyquist de Mg25 et Mg33 soient des demi-cercles de plus petit diamètre (résistance plus faible) que la contribution de la phase

, Quant à la contribution B, il s'agit probablement de la contribution de la phase ?'-Mg et/ou ?-Mg qui contient aussi de façon très probable une petite proportion d'atomes de lithium comme précédemment discuté. La résistance de cette phase est impactée par la proportion d'atomes de Li ayant réussi à s'y insérer, et qui doit croitre avec la teneur globale en Li dans l'échantillon. En conséquence, comme la mobilité des ions Li + est plus élevée que celle des Mg 2+

, Enfin la contribution C peut être interprétée comme la contribution d'une phase de type ?'-Mg ou ?-Mg contenant exclusivement du magnésium et qui serait donc encore plus résistive que la contribution B; ou plus simplement comme l'impédance de l'interface pastille/or qui ne serait pas une capacité

H. Xu, S. Wang, H. Wilson, F. Zhao, and A. Manthiram, Y-Doped NASICON-type LiZr2(PO4)3Solid Electrolytes for Lithium-Metal Batteries, Chem. Mater, vol.29, issue.17, pp.7206-7212, 2017.

F. Sudreau, D. Petit, and J. P. Boilot, Dimorphism, phase transitions, and transport properties in LiZr2(PO4)3, Conductivité ionique, vol.83, pp.78-90, 1989.

, La conductivité ionique totale mesurée pour l'échantillon Li100 est de l'ordre de 10 -5 S.cm -1 , en accord avec les valeurs de la littérature, pp.10-19

, ce qui nous laisse penser que la mobilité du Li + est bien plus élevée que celle des Mg 2+ dans les structures de type Nasicon, et que donc c'est cet ion qui est le porteur de charges principal dans les composés mixtes Li/Mg. La charge plus élevée du cation Mg 2+ induit probablement de plus fortes interactions avec le réseau anionique, S.cm -1 )

. Dans-les-composés-mixtes-li-mg, on mesure une conductivité ionique intermédiaire entre celle de Li100 et celle de Mg100, qui décroit avec la teneur globale en magnésium. Cela s'explique par la formation de phases ? et ?' d'une part, moins conductrices que la phase ?, et également par la réduction du nombre de porteurs de charges plus mobiles

, Cette étude montre donc que la substitution de Li + par Mg 2+ dans Li1.15Y0.15Zr1.85(PO4)3 n'est à première vue pas bénéfique pour les propriétés de conduction de matériaux de type Nasicon puisque la stabilisation de la phase ? est perdue et que les ions Mg 2+ sont moins mobiles que les ions Li +