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Abstract: (Li, Al)-co-doped manganese oxide spinel compounds, Li(Li0.1Al0.1Mn1.8)O4 are prepared at temperatures (T), 600, 650 and 750°C by polymer precursor method using polyvinyl pyrrolidone (PVP). The structure and morphology of the porous and sub-micron size particles of the compounds are characterized by the Rietveld refined X-ray diffraction (XRD), infrared and Raman spectroscopy, scanning electron microscopy (SEM), high resolution transmission electron microscopy (HR-TEM) and Brunauer, Emmett and Teller (BET) surface area techniques. Galvanostatic charge–discharge cycling in the voltage range of 3.5–4.3V vs. Li at various current (C) rates, 0.5–3C at 24°C up to 60 cycles and at 0.5C and at 55°C up to 50 cycles is carried out. The influence of the Li-excess, the type of Al3+ dopant cation and the amount of fuel (polyvinyl pyrrolidone) used in the synthesis on the electrochemical behavior of the spinel in a Li-cell at room (24°C) and at elevated temperature (55°C) has been studied. Results show that at 24°C and at 0.5C, the phases prepared at 600°C (I) and 650°C (II) showed reversible capacities of 79 and 105mAhg−1 and retained 99 (±1)% of the capacity after 50 cycles. At 1C, 2C and 3C rates, II showed stable capacities of 97, 83 and 57mAhg−1, respectively. At 55°C and at 0.5C, II showed a reversible capacity of 114mAhg−1 and retains 92% of it at the end of 50 cycles, signifying the best cathodic performance for a doped-LiMn2O4. Complementary cyclic voltammetry, impedance spectra and ex situ XRD data are described and discussed. [Copyright &y& Elsevier]

Abstract: Tavorite-structured LiFePO4F is synthesized by solid-state reaction and characterized by Rietveld refined X-ray diffraction, scanning electron microscopy, BET surface area, IR and Raman spectroscopy techniques. The ionic conductivity (σ ionic) of LiFePO4F estimated using impedance analysis at 27°C and at 50°C are 0.6(±0.1)×10−7 and 5.4(±0.1)×10−7 Scm−1, respectively, with an energy of activation (E a)=0.75eV. Its electrochemical behavior were examined by galvanostatic charge–discharge cycling up to 100 cycles, cyclic voltammetry (CV) and electrochemical impedance spectroscopy using Li-metal as the counter and reference electrode, at 0.1C rate in the voltage range, 1.5–4V. LiFePO4F delivers an initial discharge capacity of 115(±3)mAhg−1 which increases to 119(±3)mAhg−1 at the 20th cycle. The capacity degrades slowly thereafter over 100 cycles, with a capacity loss of 19%. CV data show a clear indication of the Fe3+/2+ redox couple at 3.3–2.6V that involves a two-phase reaction. Electrochemical impedance spectra measured at various voltages at selected cycles were fitted to an equivalent circuit and the variation of impedance parameters interpreted. [Copyright &y& Elsevier]