On the strong impact of doping in the triangular antiferromagnet CuCrO2

Electronic band structure calculations using the augmented spherical wave method have been performed for CuCrO2. For this antiferromagnetic (T_N = 24 K) semiconductor crystallizing in the delafossite structure, it is found that the valence band maximum is mainly due to the t_2g orbitals of Cr^3+ and that spin polarization is predicted with 3 mu_B per Cr^3+. The structural characterizations of CuCr1-xMgxO2 reveal a very limited range of Mg^2+ substitution for Cr^3+ in this series. As soon as x = 0.02, a maximum of 1% Cr ions substituted by Mg site is measured in the sample. This result is also consistent with the detection of Mg spinel impurities from X-ray diffraction for x = 0.01. This explains the saturation of the Mg^2+ effect upon the electrical resistivity and thermoelectric power observed for x > 0.01. Such a very weak solubility limit could also be responsible for the discrepancies found in the literature. Furthermore, the measurements made under magnetic field (magnetic susceptibility, electrical resistivity and Seebeck coefficient) support that the Cr^4+ "holes", created by the Mg^2+ substitution, in the matrix of high spin Cr^3+ (S = 3/2) are responsible for the transport properties of these compounds.Comment: 9 pages, 11 figures, more information at http://www.physik.uni-augsburg.de/~eyert

In-situ Raman spectroscopic investigation of LiMn1.45Ni0.45M0.1O4 (M = Cr, Co) 5 V cathode materials

International audienceIn-situ Raman spectroscopy is employed to investigate the valence state variations of nickel and manganese,as well as the local structure change of LiMn1.45Ni0.45M0.1O4 (M ¼ Cr, Co) cathodes (LMN) duringgalvanostatic chargeedischarge. Raman spectra are collected between 3.5 and 4.9 V in the wave numberrange of 100e800 cm1. The Raman observations showed that the pristine cathodes of Cr- and Co-dopedLMN have essentially the same spectra, and they also have similar evolution patterns during cyclingshowing their reversible behaviour in the de-lithiation and lithiation processes. The Raman spectra of thepristine cathodes have eleven bands, located at 162, 220, 378, 408, 486, 498, 528, 593, 613, 639 and672 cm1. The bands with wave number <300 cm1 are attributed to the translation mode of molecularvibration; the 486, 593 and 639 cm1 bands are assigned to the stretching mode of MneO bond; and thevibration modes at 408, 498, 528 and 613 cm1 originated from the NieO bond; The band at 672 cm1 isattributed to A1g mode of Cr3þeO/Co3þeO. During cycling, several new bands are detected near the endof charge, among which the T2g(T) band at 170 cm1 is attributed to the translation mode of lattice vibrationin which the lithium concentration is low, and the T2g band at 538 cm1 is due to the presence ofNi4þeO bond in the crystal structure. The T2g(T) and T2g(Ni4þeO) bands are clearly evident at V 4.78(x ~ 0.32) and V 4.82 (x ~ 0.28) for Cr- and Co-doped LMN, respectively

Effect of conducting additives on the properties of composite cathodes for lithium-ion batteries

In an attempt to achieve lithium-ion batteries with high rate capability, the effect of conducting additives with various shapes and contents on the physical and electrochemical performances was evaluated. Although the density of the cathode decreased upon the addition of the additives, the electrical conductivity and electrochemical performance were greatly improved. The composite cathodes with well-dispersed multi-walled carbon nanotubes (MWCNTs) exhibited excellent high rate capabilities and cyclabilities. In the case of cathode with 8 wt.% of MWCNTs (low density-LD), the highest discharge capacity of 136 mAh/g was obtained at 5 C-rate and capacity retention of 97% for 50 cycles was observed at 1 C-rate of discharge. The cathode with a mixture of 2 wt.% of Super P and 4 wt.% of MWCNTs (LD) also exhibits improved cycle performances. The volume changes in the charge and discharge processes were successfully controlled by the bundles distributed between the host particles.close8