C.H. Shen

Structural transformation of LiVOPO4 to Li3V2(PO4)3 with enhanced capacity.

In the present investigation, we report the transformation of alpha-LiVOPO 4 to alpha-Li 3V 2(PO 4) 3, leading to an enhancement of capacity. The alpha-LiVOPO 4 sample was synthesized by a sol-gel method, followed by sintering at 550-650 degrees C in a flow of 5% H 2/Ar. The structural transformation of a triclinic alpha-LiVOPO 4 structure to a monoclinic alpha-Li 3V 2(PO 4) 3 structure was observed at higher sintering temperatures (700-800 degrees C in a flow of 5% H 2/Ar). The alpha-Li 3V 2(PO 4) 3 phase was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analysis, and X-ray absorption near edge spectrum (XANES) techniques. The valence shift of vanadium ions from +4 to +3 states was observed using in situ XANES experiments at V K-edge. The structural transformation is ascertained by the shape changes in pre-edge and near edge area of X-ray absorption spectrum. It was observed that the capacity was enhanced from 140 mAh/g to 164 mAh/g via structural transformation process of LiVOPO 4 to Li 3V 2(PO 4) 3.

Effect of Co2P on electrochemical performance of Li(Mn0.35Co0.2Fe0.45)PO4/C.

In this paper, we report the synthesis of carbon coated Li(Mn0.35Co 0.2Fe0.45)PO4 and discuss the effect of Co2P formation during the carbothermal reduction process, which enhances the electrochemical performance of cathode material for lithium ion batteries. It was observed that Co2P was favorably formed in 5% H2/Ar than in Ar atmosphere. The conductivity of Li(Mn0.35Co0.2Fe0.45)PO4/C sintered at 600-800 degrees C in 5% H2/Ar is increased as the temperature is increased. The O K-edge X-ray absorption near edge spectrum (XANES) demonstrates that content of hole carriers is increased in Li(Mn0.35Co0.2Fe0.45)PO4/C as the amount of Co2P increased. We also observed that the capacity of Li(Mn0.35Co0.2Fe0.45)PO4/C is increased with sintering temperature, and it exhibited a maximum capacity of 166 mAh/g at 700 degrees C. It was found that the enhancement in the discharge capacity of sintered Li(Mn0.35Co0.2Fe0.45)PO4/C was as a result of its higher electrical conductivity under 5% H2/Ar atmosphere as compared with Ar atmosphere.

Effect of LiI Amount to Enhance the Electrochemical Performance of Carbon-Coated LiFePO4

We used the wet chemistry method with different amounts of LiI to prepare LiFePO 4 and carbon-coated LiFePO 4 (LiFePO 4 /C) samples. The lithiation process of LiI for preparing the LiFePO 4 is proposed. We found that the amount of LiI greatly affected the purities of products. The LiFePO 4 /C sample showed constant values of current density during potential cycling up to 35 cycles as compared to LiFePO 4 , suggesting that the LiFePO 4 /C has better electrochemical performance. The obtained maximum capacity for LiFePO 4 /C can be approached to the theoretical capacity of 170 mAh/g. Moreover, Brunauer-Emmett-Teller measurements of LiFePO 4 and LiFePO 4 /C showed a surface area of 6.7 and 50 m 2 /g, respectively. It is reasonable to believe that the excellent performance of the compound developed in our work can be attributed to the smaller particle size coated with conductive carbon achieved by the controlling LiI in the sol-gel method.

Short-range magnetic correlations in spinel LiMn2O4

Neutron diffraction, inelastic neutron scattering, and ac magnetic susceptibility measurements were performed to study the magnetic correlations of the Mn spins in a fully oxygenated lithium manganospinel Li0.96Mn2O4. The compound crystallizes into a cubic structure of the Fd3m symmetry at 300 K and an orthorhombic Fddd symmetry at 150 K. Two separate types of magnetic phase transitions were found, signifying that the Mn3+ and Mn4+ spins order separately at two different temperatures. The magnetic correlations were antiferromagnetic in nature, and were observed to develop below 80 and 55 K, respectively, for the two components. Surprisingly, for both components the correlation lengths reached their maxima at 45 and 30 K, but were still short-range with correlation lengths at ∼120 and ∼40 A, respectively.

Chemical pressure control of Curie temperature in La0.6(Ba0.4-xCax)MnO3

Abstract The effects on structural and paramagnetic to ferromagnetic transitions by isovalent chemical substitution of smaller Ca2+ into the bigger Ba2+ sites in La0.6(Ba0.4−xCax)MnO3 (0≤x≤0.4) are investigated. With increasing x, a change from the rhombohedral cell (space group: R-3c) to an orthorhombic cell (space group: Pbnm) is observed. For x≥0.3, the structural transformation leads to an increase in the Mn–O(1) bond length along the c-axis which corresponds to the Jahn–Teller distortion of the distorted MnO6 octahedra in the orthorhombic cell. This effect gives rise to a decrease in TC (the temperature of the paramagnetic to ferromagnetic transition) with increasing Ca content.

Effect of Ca doping on the optical properties of La1.2Sr1.8Mn2O7

We present x-ray-diffraction, infrared reflectivity, and Raman-scattering measurements of La1.2(Sr1.8−xCax)Mn2O7 as a function of temperature and doping (x=0.0, 0.4, 0.6, and 0.8). At room temperature, x-ray-diffraction data show that the replacement of Sr ions with smaller Ca ions causes a shrinkage of the unit-cell volume. The far-infrared conductivity spectra of all doped samples are typical of an insulator, showing only phonons. When doped with Ca on Sr and a corresponding systematic reduction in Curie temperature (Tc), there is a suppression of the low-frequency optical spectral weight. For the x=0.0 compound, the Raman-active phonon associated with the internal stretching of the oxygen atoms in MnO6 octahedra shows a noticeable hardening below Tc. A polaron transport can be attributed to the observed frequency shifts. Interestingly, this hardening is reduced as the Ca content increases. These observations suggest that Ca doping disrupts the effects of the double-exchange interaction on the lattice d...

Optical studies of lattice and charge excitations in La1.2(Sr1.8-xCax)Mn2O7

Optical and Raman-scattering studies of layered La1.2(Sr1.8−xCax)Mn2O7 manganites are presented as a function of temperature and doping (x=0.0, 0.4, 0.6, and 0.8). The substitution of Ca2+ for Sr2+ leads to the suppression of the low-frequency optical spectral weight with decreasing Tc. Moreover, the observed anomalies of the Raman phonon parameters near Tc are most likely due to a strong spin-phonon coupling caused by the oxygen phonon modulation of the Mn–Mn spin exchange interaction. Our results provide important insight into the interplay of lattice, spin, and charge dynamics in these materials.