RESUMO
We present Si nanotubes prepared by reductive decomposition of a silicon precursor in an alumina template and etching. These nanotubes show impressive results, which shows very high reversible charge capacity of 3247 mA h/g with Coulombic efficiency of 89%, and also demonstrate superior capacity retention even at 5C rate (=15 A/g). Furthermore, the capacity in a Li-ion full cell consisting of a cathode of LiCoO2 and anode of Si nanotubes demonstrates a 10 times higher capacity than commercially available graphite even after 200 cycles.
RESUMO
It is a common observation that when ionic liquids are added to electrolytes the performances of lithium ion cells become poor, while the thermal safeties of the electrolytes might be improved. In this study, this behavior is investigated based on the kinetics of ionic diffusion. As a model ionic liquid, we chose butyldimethylimidazolium hexafluorophosphate (BDMIPF(6)). The common solvent was propylene carbonate (PC), and lithium hexafluorophosphate (LiPF(6)) was selected as the lithium conducting salt. Ionic diffusion coefficients are estimated by using a pulsed field gradient NMR technique. From a basic study on the model electrolytes (BDMIPF(6) in PC, LiPF(6) in PC, and BDMIPF(6) + LiPF(6) in PC), it was found that the BDMI(+) from BDMIPF(6) shows larger diffusion coefficients than the Li(+) from LiPF(6). However, the anionic (PF(6)(-)) diffusion coefficients present little difference between the model electrolytes. The higher diffusion coefficient of BDMI(+) than that of Li(+) suggests that the poor C-rate performance of lithium ion cells containing ionic liquids as an electrolyte component can be attributed to the two-cation competition between Li(+) and BDMI(+).