ESR in CrCl3\mathsf{_3}-based graphite intercalation and bi-intercalation compounds

ESR experiments have been performed on stage-1, -2, -3, CrCl$_{3}$ graphite intercalation compounds as well as on CrCl$_{3}$-CdCl$_{2}$ and CrCl$_{3}$-MnCl$_{2}$ graphite bi-intercalation compounds. The measurements have been carried out at the X-band frequency and over the temperature range $4.2~{\rm K}\leqslant T\leqslant 294~{\rm K}$. The variation of the linewidth $(\Delta H)$ and the resonance field $(H_{\rm r})$ have been examined as a function of the temperature and the angle $\theta$ between the external field and the crystal $c$-axis. The results reflect the anisotropic 2D character of these systems. The room temperature angular dependence of $\Delta H$ follows a $(3 \cos^{2} \theta-1)^{2}$-like behavior and that of $H_{\rm r}$ has a $(3 \cos^{2} \theta-1)$-like form. For the singly intercalated systems, $\Delta H$ decreases with $T$ according to $(1-\Theta_{\rm cw}/T)$ in the high temperature region, then shows a local minimum at around 35 K followed by a critical-like divergence at lower temperatures. In the bi-intercalated compounds, $\Delta H$ vs. $T$ exhibits three types of behavior : for $T\geqslant 220$ K, $\Delta H$ behaves like $T^{2}$; for 120 K $\leqslant T\leqslant 220~{\rm K}$, $\Delta H$ seems to be proportional to $(1-\Theta_{\rm cw}/T)$; for $T\leqslant 120$ K, $\Delta H$ shows a gradual increase which becomes steeper and steeper with falling $T$. The $T^{2}$-like behavior may be explained in connection to a spin-lattice relaxation phenomenon which becomes important at high temperatures. The temperature dependence of $H_{\rm r}$ is characterized by an increase of $H_{{\rm r}\parallel}$ and a decrease of $H_{{\rm r}\perp}$ with decreasing temperature. This is consistent with the theoretical predictions developed for anisotropic low-dimensional systems, reflecting the increase in the anisotropy of low temperature susceptibility

Formation of active species and aging mechanisms in tin-based anodic materials [Formation des espèces actives et mécanismes de vieillissement dans des matériaux anodiques à base d'étain]

cited By 0International audienceVarious spectroscopic methods have been used in lithium ion batteries to study structural and electronic modifications of electrode materials in relation to their electrochemical properties. The experimental results have been analyzed by various models including semi-empirical and ab-initio calculations. Systems based on tin oxides or tin sulfides have been studied. It is shown that the choice of aging mechanisms will depend on the aimed battery applications. In thermodynamically stable systems, there are no losses of lithium during the first cycle but the capacities are limited and the reversibility depends on the preservation of the crystalline lattice. In metastable systems, there are significant losses of lithium during the first cycle but the capacities are high and the reversibility does not depend on the preservation of the crystalline lattice. Those observations open the way to searching new materials with particular attention paid to their texture and particle size

Formation des espèces actives et mécanismes de vieillissement dans des matériaux anodiques à base d'étain

Les nouveaux matériaux anodiques pour batteries “Lithium-ion” présentent de fortes capacités liées à la formation d'espèces actives composites métastables en interaction avec un support et leur vieillissement à la formation d'espèces parasites stables. Leurs identifications et l'analyse fine de leurs comportements au cours des cycles décharge/charge nous a permis d'établir les mécanismes réactionnels mis en jeu. La démarche de recherche développée permet d'analyser les modifications structurales et électroniques des matériaux d'électrodes en relation avec leurs propriétés électrochimiques. Elle est basée sur la complémentarité de méthodes expérimentales (Diffractions X ou Neutrons, Spectrométrie Mössbauer, Spectroscopies d'Absorption XAFS et de Photoélectrons XPS) dont les résultats sont analysés par différents modèles incluant des calculs semi-empiriques (Liaisons Fortes) et ab-initio (LAPW). Les exemples présentés ici sont choisis dans des systèmes à base d'oxydes ou sulfures d'étain

On the behavior of the LixNiO2 system: an electrochemical and structural overview

Lithium nickel oxide exhibits a departure from stoichiometry (Li1-zNi1+zO2) consisting in the presence of extra-nickel ions within the lithium sites. Using optimized experimental synthesis conditions, compositions very close to the ideal stoichiometry (z = 0.02) can be obtained. By using the sensitivity of the lithium site isotropic temperature factor to the stoichiometry, the amount of extra-nickel ions can be determined in a very precise way. The loss of reversibility at the first cycle is mainly related to the change in the oxidation state of the extra-nickel ions, which induces a local collapse of the structure and makes difficult the lithium re-intercalation, A systematic structural study of LixNiO2 phases has been performed by extended X-ray absorption fine structure (EXAFS) as well as X-ray and electron diffraction. In the case of the starting Li0.98Ni1.02O2 phase, a local distortion of the NiO6 octahedra, resulting from a dynamic Jahn-Teller effect of low spin trivalent nickel ions has been evidenced from the EXAFS study. For the partially de-intercalated materials (0.50 < x < 0.75) which crystallize in the monoclinic system, the EXAFS study shows that the NiO6 octahedra are only slightly distorted due to the occurrence of a hopping phenomenon between Ni-IV and Ni-III. Electron diffraction experiments show the existence of a superstructure due to a peculiar lithium-ion ordering. Systematic electrochemical studies have shown that this ordering is strongly sensititve to the presence of extra-nickel ions