Showing papers by "Dany Carlier published in 2008"

On “Really” Stoichiometric LiCoO2

Abstract: The Li NMR T1 relaxation time is shown to be a very severe criterion for determining the stoichiometry of LiCoO2. When Co3O4 containing minimum impurity level of paramagnetic ions such as Fe or Ni is used as starting material (together with Li2CO3), a T1 value as long as 18 s is obtained but this requires a rather long thermal treatment for complete elimination of excess Li. Such highly stoichiometric Li1CoO2 exhibits a rather early insulator to metal transition upon Li deintercalation. Magnetic susceptibility and specific heat capacity investigations show the exclusive presence of diamagnetic LS Co. Introduction LiCoO2 has been the subject of quite a lot of investigations, as reviewed by Antolini (1). Being still the most common positive electrode material for Li-ion batteries, it is quite significant that some aspects of its structure or properties are still under debate. Close similarity of its structure with the Na based cobaltites that presently receive considerable attention for their thermoelectric behaviour (2 , 3) also makes LiCoO2 a material of particular interest. 1 Corresponding author, menetrier@icmcb-bordeaux.cnrs.fr 2 Electrochemical society active member 1 ha l-0 03 24 04 2, v er si on 1 23 S ep 2 00 8 Author manuscript, published in \"Electrochemical and Solid-State Letters 11, 11 (2008) A179-A182\" DOI : 10.1149/1.2968953

High-temperature phase transition in the three-layered sodium cobaltite P ′ 3 -Na x CoO 2 ( x ∼ 0.62 )

Abstract: The high temperature phase transition in the three-layered ${P}^{\ensuremath{'}}3{\text{-Na}}_{x}{\text{CoO}}_{2}\phantom{\rule{0.3em}{0ex}}(x\ensuremath{\sim}0.62)$ has been investigated by means of heat capacity measurement, x-ray diffraction, and $^{23}\text{N}\text{a}$ magic angle spinning (MAS)--NMR spectroscopy in the 300--550 K range. The phase transition occurs nearby ${T}_{S}=350\text{ }\text{K}$. Below ${T}_{S}$, the unit cell is monoclinic (space group $C2/m$). Above ${T}_{S}$, the monoclinic cell is reversibly converted into a rhombohedral cell (space group $R3m$). The crystallographic change mainly manifests into Na rearrangement in the interslab from a low symmetry position to a higher symmetry position. A global picture for both systems of the $(x,y,z)$ off-center position of Na could be understood as a balance between on-site ${\text{Na}}^{+}{\text{-Co}}^{3+/4+}$ electrostatic repulsions ($z$ shift) and in-plane ${\text{Na}}^{+}{\text{-Na}}^{+}$ electrostatic repulsions ($xy$ shift). We suggest that ${\text{Na}}^{+}$ interlayer redistribution is the driving force of the phase transition. $^{23}\text{N}\text{a}$ MAS-NMR spectroscopy has been used to investigate changes in the environment and in the distribution of the sodium cations occurring by raising the temperature. The gradual suppression of the second-order quadrupolar interactions and the resulting new resonance is consistent with the sodium site exchange mechanism. Changes in the resistivity at ${T}_{S}$ suggest a strong coupling between the ${\text{Na}}^{+}$ and ${\text{CoO}}_{2}$ layers.