Materials from the Mn(0.5−x)CaxTi2(PO4)3 (0≤x≤0.50) solid solution were obtained by solid-state reaction in air at 1000 °C. Selected compositions were investigated by powder X-ray diffraction ...analysis, 31P nuclear magnetic resonance (NMR) spectroscopy and electrochemical lithium intercalation. The structure of all samples determined by Rietveld analysis is of the Nasicon type with the R3̄ space group. Mn2+/Ca2+ ions occupy only the M1 sites in the Ti2(PO4)3 framework. The divalent cations are ordered in one of two M1 sites, except for the Mn0.50Ti2(PO4)3 phase, where a small departure from the ideal order is observed by XRD and 31P MAS NMR. The electrochemical behaviour of Mn0.50Ti2(PO4)3 and Mn(0.5−x)CaxTi2(PO4)3 phases was characterised in Li cells. Two Li ions can be inserted without altering the Ti2(PO4)3 framework. In the 0≤y≤2 range, the OCV curves of Li//LiyMn0.50Ti2(PO4)3 cells show two main potential plateaus at 2.90 and 2.50-2.30 V. Comparison between the OCV curves of Li//Li(1+y)Ti2(PO4)3 and Li//LiyMn0.50Ti2(PO4)3 shows that the intercalation occurs first in the unoccupied M1 site of Mn0.50Ti2(PO4)3 at 2.90 V and then, for compositions y>0.50, at the M2 site (2.50-2.30 V voltage range). The effect of calcium substitution in Mn0.50Ti2(PO4)3 on the lithium intercalation is also discussed from a structural and kinetic viewpoint. In all systems, the lithium intercalation is associated with a redistribution of the divalent cation over all M1 sites. In the case of Mn0.50Ti2(PO4)3, the stability of Mn2+ either in an octahedral or tetrahedral environment facilitates cationic migration.
A detailed characterization of the structural modifications and redox processes occurring upon lithium deintercalation from the Lix0Co1−yMgyO2 materials (x0=1.0 and 1.10; y=0.0, 0.03, 0.05 and 0.06) ...was performed in order to determine the effect of Mg doping on the cycling properties. Using electrochemical tests, X-ray diffraction (XRD), 7Li MAS NMR and electrical properties measurements, we show that the LixCo1−yMgyO2 system exhibits a solid solution existing in the whole deintercalation range studied (0.30≤×≤1.0). These phases exhibit reversible capacities equivalent to that of LiCoO2 upon cycling with a good structural stability. Moreover, the 7Li MAS NMR study shows that the structural defects (O vacancies and intermediate spin Co3+ ions) which are present in the starting Mg-doped phases govern the electronic properties upon lithium deintercalation. Indeed, regardless of the presence of Mg ions in the structure, a behavior similar to that of the LixCoO2 (1
Lithium nickel oxide derivatives are promising positive electrode materials for the next generation of lithium-ion batteries. Partial substitution of certain cations for nickel in this family of ...oxides significantly modifies their properties and is therefore an attractive route to develop an optimised oxide electrode which satisfies the demanding requirements for rechargeable battery applications. In this paper the interest is focused on the effect of cobalt, iron, aluminium and magnesium for a general discussion of the effect of cationic substitution on the properties based on a review of results mostly obtained in our laboratories. Although iron substitution does not seem interesting for the practical aspect, iron Mössbauer spectroscopy allows very precise characterisations, interesting to understand the general behaviour of this family of materials. We deal with the optimisation of the synthesis conditions in order to obtain the most electrochemically active materials. The relations between the nature of the substituting cation, the presence of foreign cations in the lithium site, the electrochemical behaviour and the redox processes upon electrochemical cycling are discussed in detail. A new view of the relation between this latter point and the cationic distribution formed during the material synthesis is proposed.
HT-Li x 0 Co1 - y Mg y O2 (where x 0 is the Li/(Co + Mg) ratio; x 0 = 0.98, 1.0, 1.10; y = 0.0, 0.03, 0.06, and 0.10) materials were synthesized via a solid-state reaction. These samples were ...characterized by X-ray diffraction, 7Li MAS NMR spectroscopy, and electrical properties measurements. The XRD study showed that pure phases are obtained for 0.0 ≤ y < 0.10 and x 0 ≥ 1.0. 7Li MAS NMR spectra of the Mg-doped phases exhibit two types of new signals at 55 ppm and 325, 7, −9, and − 27 ppm in addition to the signal at 0 ppm resulting from the presence of diamagnetic CoIII ions. On the basis of our general knowledge of Li NMR in layered oxides with electron spins, we suggest that Mg doping in LiCoO2 always leads to the simultaneous presence of CoIV ions (sharing an itinerant electron hole with neighboring CoIII ions) and, to a smaller extent, of intermediate spin Co3+(IS) ions trapped in a square-based pyramidal environment because of an oxygen vacancy. This feature is further enhanced by lithium overstoichiometry.
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