Guo Xiangxin

Variation of Ti Valence with Lithium Content in Lithiated/Delithiated Li4Ti5O12 Studied by X-Ray Absorption near the Edge Structure

Synchrotron-based x-ray absorption near-edge structure (XANES) spectroscopy is applied to investigate the variation of Ti valence in the discharge and charge processes of Li4Ti5O12. Through analysis of the XANES data, the average valence of Ti can be calculated, which allows quantitative determination of the ratios of Ti3+/Ti4+ as a function of Li content in the electrodes. It is found that the ratios in the composites of Li4Ti5O12/acetylene black (AB)-carbon are larger than those in Li4Ti5O12/Ag/AB-carbon after the discharge to 1.1 V, indicating a larger amount of Li inserted into the former than that into the latter. This finding provides a good explanation for the fact that the Li4Ti5O12/AB-carbon samples exhibit larger storage capacities than the Li4Ti5O12/Ag/AB-carbon ones prepared in this work, concerning the larger Ti3+/Ti4+ ratio and the larger amount of Li+ inserted in the electrode for satisfying the charge neutralization requirement.

A superhigh discharge capacity induced by a synergetic effect between high-surface-area carbons and a carbon paper current collector in a lithium–oxygen battery*

This paper invesitages the synergetic effect between high-surface-area carbons, such as Ketjan Black (KB) or Super P (SP) carbon materials, and low-surface-area carbon paper (CP) current collectors and it also examines their influence on the discharge performance of nonaqueous Li–O2 cells. Ultra-large specific discharge capacities are found in the KB/CP cathodes, which are much greater than those observed in the individual KB or CP cathodes. Detailed analysis indicates that such unexpectedly large capacities result from the synergetic effect between the two components. During the initial discharges of KB or SP materials, a large number of superoxide radical species in the electrolytes and Li2O2 nuclei at the CP surfaces are formed, which activate the CP current collectors to contribute considerable capacities. These results imply that CP could be a superior material for current collectors in terms of its contribution to the overall discharge capacity. On the other hand, we should be careful to calculate the specific capacities of the oxygen cathodes when using CP as a current collector; i.e., ignoring the contribution from the CP may cause overstated discharge capacities.

Evolutions of Crystal Structure, Stoichiometry and Electrochemical Behavior with Co Substitution in LiNi1_y CoyO2 Positive Electrodes

LiNi1-y CoyO2 (0.1 ≤ y ≤ 0.4) positive electrode materials are synthesized by a chemical method with stoichiometric acetates of related cations. Their crystal structure, stoichiometry and electrochemical behaviors versus Co concentration are investigated by x-ray diffraction, synchrotron-based x-ray absorption fine structure and galvanostatic cycling measurements. The results reveal that the non-stoichiometric Ni2+, Li/Ni cation mixing and polarization are reduced as the amount of Co substitution increases, clearly indicating that the Co element is a medium for easily oxidizing Ni2+ to Ni3+ during the synthesis process.