Search

Abstract: In this work, we studied the local environment of a series of LiNi y Mn y Co1−2y O2 materials synthesized by wet chemistry via oxalate route. Measurements included the temperature dependence of the magnetic susceptibility χ m (T), the magnetization M(H), and the electron spin resonance. The influence of the synthesis conditions on the magnetic and electronic properties is presented and discussed. Magnetic properties and electron spin resonance spectroscopy provides further information on Ni2+, Mn4+ ion distribution in LiNi y Mn y Co1−2y O2. The magnetic data allows the determination of the cationic disorder, i.e. the concentration of Ni2+ ions in the 3a lithium sites, as a function of the composition, in quantitative agreement with the results of XRD experiments. We find that the lithium extraction proceeds by different steps. In a first step, Ni2+ in converted first in Ni3+, then in Ni4+ as x decreases from x =1, while Co remains in the Co3+ valence state. The ESR spectra show a resonance due to Mn4+, which is analyzed self-consistently with magnetic experiments. [Copyright &y& Elsevier]

This paper addresses the synthesis structural and electrochemical properties of LiFe0.5Mn0.5PO4 electrode materials for Li-ion batteries. The charge–discharge reaction of Li/LiPF6-EC–DEC/LiFe0.5Mn0.5PO4 cell carried out at the 1-C rate shows a capacity retention of 128 mAh/g. The local structure of the delithiated Li x Fe0.5Mn0.5PO4 phases have been studied by Fourier transform infrared spectroscopy and magnetometry. Spectral features indicate that the structure of the delithiated phase remains in the orthorhombic system. The compositional dependence of the magnetic moment is found to be in quantitative agreement with the theoretical value predicted for oxidation of M 2+ ions in the high spin state. [ABSTRACT FROM AUTHOR]

A series of LiNi x Mn y Co z O2 ( x = y, z = 1 − 2 y) oxides have been synthesized by “chimie douce” and investigated as positive electrodes in rechargeable lithium batteries. Layered LiNi y Mn y Co1 − 2 y O2 materials with high homogeneity and crystallinity were synthesized using the wet-chemical method assisted by carboxylic acid as the polymeric agent. The long range and local structural properties are investigated with experiments including X-ray diffraction, Fourier transform infrared spectroscopy, and electron paramagnetic resonance spectroscopy. The evolution of the structure is discussed as a function of the cobalt content that confers layer-like behavior on the framework. Electrochemical performance of LiNi y Mn y Co1 − 2 y O2 oxides is tested in cells using nonaqueous 1 M LiPF6 dissolved in ethylene carbonate–diethyl carbonate. Charge–discharge profiles are investigated as a function of the rate capability and the voltage window. A relation is found between the gravimetric capacity and the cation disorder of the positive electrode as indicated by structural analysis. Fast lithium extraction attributed to the larger interslab space has been observed in the cobalt-rich oxides. [ABSTRACT FROM AUTHOR]

We present a review of the structural properties of LiFePO4. Depending on the mode of preparation, different impurities can poison this material. These impurities are identified and a quantitative estimate of their concentrations is deduced from the combination of X-ray diffraction analysis, Fourier transform infrared spectroscopy, Raman spectroscopy, and magnetic measurements. An optimized preparation provides samples with carbon-coated particles free of any impurity phase, insuring structural stability and electrochemical performance that justify the use of this material as a cathode element a new generation of lithium secondary batteries. [ABSTRACT FROM AUTHOR]

Abstract: A series of LiMn y Fe1−y PO4 samples have been prepared in the whole range 0≤ y ≤1. Chemical delithiation could be achieved to obtain Mn y Fe1−y PO4 in the range 0≤ y ≤0.8, keeping the same crystal phase (olivine structure, space group Pnma). The composition y =0.8 is the limit where the delithiated phase is still crystallized, but abruptly suffers strains at the molecular scale evidenced by both optical spectroscopy and X-ray diffraction. The analysis of the magnetic properties shows that in all the samples the concentration of impurities is negligible. The concentration of polarons, either holes associated to Li vacancies in LiMn y Fe1−y PO4 or electrons associated to the existence of Li left in the matrix of Mn y Fe1−y PO4, is found to be small (≤1%) in all the samples. For y ≤0.6, all the Mn3+ ions Mn y Fe1−y PO4 are in the high-spin state (S =2). At larger manganese concentration, however, the Mn3+ ions in excess of the critical concentration y c =0.6 undergo a transition to the low-spin state (S =1). As a consequence, and in contrast with prior works, we find that Mn0.8Fe0.2PO4 has magnetic interactions that are much smaller, and no antiferromagnetic ordering in this compound is detected, at least above 20K. Antiferromagnetic ordering that had been reported so far for Mn y Fe1−y PO4 at large y-composition might come from incomplete delithiation. The spin-transition of Mn3+ in concentration (y–y c) to the low-spin state is at the origin of the strain fields at the molecular scale that increase with y for y >0.6, and ultimately prevents the full delithiation for y >0.8. This result sheds light on the reason for the degradation of cathode properties in Mn-rich compounds of the heterosite–purpurite series, while the electrochemical properties are good in the range y ≤0.6 but only at slow rates, due to the very small hopping mobility of the small polaron. [Copyright &y& Elsevier]

Abstract: An experimental relationship between local structure and electrochemical properties is established for lithium metal phosphates LiMePO4 (Me = Fe, Co, Mn), LiFeP2O7 and Li3Fe2(PO4)3 which crystallise with either the olivine-like or the Nasicon-like structure. Structural features of several lithium metal phosphates are investigated by a local probe such as the FTIR spectroscopy in order to explain the influence of either or covalent bonds on redox energies. The local cationic arrangement is also discussed with the aid of a molecular vibration model to figure out the strong covalent bonds within a complex for glassy Nasicon compounds. [Copyright &y& Elsevier]