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Following our previous work on the tavorite-like LiFePO(4)·OH and FePO(4)·H(2)O phases, we report here the magnetic and NMR characterizations of analogous LiMnPO(4)·OH, MnPO(4)·H(2)O and VPO(4)·H(2)O phases together with the DFT calculations of the NMR shifts. The first two compounds exhibit Curie-Weiss type magnetic behavior with Curie constants close to the theoretical ones for HS Mn(3+), while the vanadium compound is very close to a pure Curie-type behavior. (7)Li, (31)P and (1)H MAS NMR spectra are reported for the three compounds, and show strong Fermi-contact shifts for the first two nuclei, while the sign and magnitude of the (1)H shifts are very different for the three phases. DFT calculations (FLAPW in GGA+U approximation) using the WIEN2k code and the experimental susceptibilities are shown to reproduce closely the experimental data. This situation is compared to the case of the homologous and isostructural Fe compounds, which exhibit much more complex magnetic behaviors.

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RESUMEN

Lithium iron phosphate is one of the most promising positive-electrode materials for the next generation of lithium-ion batteries that will be used in electric and plug-in hybrid vehicles. Lithium deintercalation (intercalation) proceeds through a two-phase reaction between compositions very close to LiFePO(4) and FePO(4). As both endmember phases are very poor ionic and electronic conductors, it is difficult to understand the intercalation mechanism at the microscopic scale. Here, we report a characterization of electrochemically deintercalated nanomaterials by X-ray diffraction and electron microscopy that shows the coexistence of fully intercalated and fully deintercalated individual particles. This result indicates that the growth reaction is considerably faster than its nucleation. The reaction mechanism is described by a 'domino-cascade model' and is explained by the existence of structural constraints occurring just at the reaction interface: the minimization of the elastic energy enhances the deintercalation (intercalation) process that occurs as a wave moving through the entire crystal. This model opens new perspectives in the search for new electrode materials even with poor ionic and electronic conductivities.

RESUMEN

Deintercalated "Li(x)NiO2" materials (x = 0.25, 0.33, 0.50, 0.58, and 0.65) were obtained using the electrochemical route from the Li0.985Ni1.015O2 and Li0.993Ni1.007O2 compounds. Refinements of X-ray diffraction data using the Rietveld method show a good agreement with the phase diagram of the Li(x)NiO2 system studied earlier in this laboratory. Electronic conductivity measurements show a thermally activated electron-hopping process for the deintercalated Li0.5NiO2 phase. In the Li(x)NiO2 materials investigated (x = 0.25, 0.33, 0.50, and 0.58), 7Li NMR shows mobility effects leading to an exchanged signal at room temperature. A clear tendency for Li to be surrounded mainly by Ni3+ ions with the 180 degree configuration is observed, particularly, for strongly deintercalated materials with smaller Li+ and Ni3+ contents, even upon heating, when this mobility becomes very fast in the NMR time scale. This suggests that Li/vacancy hopping does occur on the NMR time scale but that Ni3+/Ni4+ hopping does not occur independently. The position of Li seems to govern the oxidation state of the Ni around it at any time; the electrons follow the Li ions to satisfy local electroneutrality and minimal energy configuration. The observed NMR shifts are compatible with the Li/vacancy and Ni3+/Ni4+ ordering patterns calculated by Arroyo y de Dompablo et al. for x = 0.25 and x = 0.50, but not for x = 0.33 and x = 0.58.

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The metastable O2-LiCoO(2) phase undergoes several reversible phase transitions upon lithium deintercalation. The first transition leads to an unusual oxygen stacking in such layered compounds. This stacking is found to be stable for 0.52 < x < or = 0.72 in Li(x)()CoO(2) and is called T(#)2. We studied this phase from a structural viewpoint using X-ray and neutron diffraction (ab initio method). The new stacking derives from the O2 one by gliding every second CoO(2) slab by (1/3, 1/6, 0). The lithium ions are found to occupy very distorted tetrahedral sites in this structure. We also discuss the possibility of this T(#)2 phase to exhibit stacking faults, whose amount depends on the method used to prepare this deintercalated phase.

RESUMEN

We have examined the oxidation states and local atomic structures of Ni, Fe, and Co in Li(x)Ni0.7Fe0.15Co0.15O2 as a function of Li content during the first charge in a Li/Li(x)Ni0.7Fe0.15Co0.15O2 nonaqueous cell. We show that the composition of the material in the pristine state is more accurately described by Li0.95Ni(II)0.09Ni(III)0.66Fe(III)0.15Co(III)0.25O2 Half of the Ni(II) resides in Li-vacant sites. Both Fe and Co substitute for Ni within the NiO2 slabs with no significant amounts of Fe or Co that can be attributed to Li-vacant sites. The local structure parameters are consistent with oxidation states observed on the basis of the XANES data. The Ni K-edge energy continuously shifts to a higher energy with decrease in Li content due to oxidation of Ni(II) to Ni(II) and Ni(III) to Ni(IV). After the complete oxidation of Ni(III) to Ni(IV), the Fe K-edge energy begins to increase with further decrease in Li content indicating the oxidation of Fe(III) to Fe(IV). The Co K-edge energy at half-height, on the other hand, is unchanged during the whole range of Li deintercalation indicating that no significant change in the oxidation state of Co occurs upon the complete removal of Li.