The spinel LiMn1.5Ni0.5O4 has been prepared at low temperature using a new sol‐gel process. X‐ray diffraction analysis reveals that the obtained material presents a cubic spinel structure with a unit cell parameter lower than that of stoichiometric LiMn2O4 . X‐ray photoelectron spectroscopy confirms the divalent state of doped nickel in the material. Furthermore, LiMn1.5Ni0.5O4 compound can intercalate a second lithium both chemically and electrochemically, leading to the formation of a new Li2Mn1.5Ni0.5O4 material with the same cubic spinel structure. This material presents only one plateau at around 3 V vs. normalLi/Li+ with a large discharge capacity of 160 mAh/g and fairly good cyclability. This result is in contrast to the undoped Li2Mn2O4 , where two plateaus at 4 and 3 V are observed because of the occurrence of a structural transition caused by the Jahn‐Teller effect. The structure and the electrochemical behavior of Li1+xMn1.5Ni0.5O4 are investigated and compared to those of LiMn1.5Fe0.5O4 material prepared by the same sol‐gel process.

No abstract

Microvoltammograms of a single particle of lithium titanium oxide (LTO, normalLi4∕3normalTi5∕3normalO4 ) spinel were interpreted using a core-shell model in the spinel∕rock-salt two-phase transition process. Lithium insertion into the particle was controlled by the diffusion in LTO rock-salt structure outlayer. The quick charging was effectively enhanced by reducing the LTO particle size. Lithium extraction from the particle was significantly affected by the charge transfer on the LTO spinel outlayer rather than by the diffusion of lithium ions. The discharge speed during the internal short-circuit abuse test connecting with the LTO anode was 3 orders of magnitude slower than that with graphite anode. At the internal short-circuit point, transformation to the low electron-conductive LTO spinel phase will work to suppress the rapid extension of internal short circuit reaction. A 2V class lithium-ion battery system using the LTO anode had a high output and input power density of 4000W∕kg for a 10s pulse condition. Quick-charging performance showed 80% of the full capacity in 1min . It was demonstrated that the 2V class lithium-ion battery system exhibited excellent high power and quick charging with outstanding safety characteristics by using the LTO anode.

No abstract

The electrochromism of amorphous V2O5 thin films prepared by the vacuum evaporation of V2O5 powder was studied through optical absorption, XPS and SIMS measurements. The films were colored and bleached with the aid of an electrolyte consisting of lithium perchlorate (LiCLO4·3H2O) in propylene carbonate (CH3CHOCOOCH2). The as-grown films were yellow, like V2O5 powder, while the colored films were greenish-gray, like VO2 powder. The XPS spectra of the O1s and V2p3/2 core levels of the as-grown films matched those of V2O5 powder, while those of the colored films were similar to those of VO2 powder. SIMS measurement revealed that Li+ ions are introduced into the films by coloration and expelled by bleaching. From these results, it is concluded that the coloration of a-V2O5 films is due to a change in the molecular state from V2O5 to VO2. This change is caused by the double injection of electrons and Li+ ions into the films.

No abstract

No abstract

Flow boiling in micro- and mini-channels has attracted much attention in recent years. But the phenomena is such confined channels have not been fully understood and explained. Some conclusions reached by different authors are even contradictory. The present research is trying to study some aspects of flow boiling in mini- and micro-channels. In the present paper boiling heat transfer and two-phase flow patterns in rectangular narrow channels were studied. The gap size of the channel was varied as 2, 1, 0.5 and 0.2 mm with the channel width and length being kept at 20 mm and 100 mm, respectively. In the present mini- and micro-channels, four flow patterns were identified; bubbly, intermittent, wavy and annular flow. They can be also divided into several sub-flow patterns. Flow patterns showed strong channel gap size dependence. Smaller gap size deleted bubbly flow, thus induced simpler flow patterns to shift the annular flow at lower vapor quality. The channels can be divided into two groups depending on the gap size; the larger gap group of 2 and 1 mm, and the smaller gap group of 0.5 and 0.2 mm. The larger gap group showed similar heat transfer behavior as conventional size of tubes. The smaller gap group indicated some peculiar phenomena. Heat transfer coefficient in the smaller gap group was relatively high in the low quality region. Then heat transfer coefficient decreased monotonously with increasing vapor quality. This behavior was considered attributable to the micro-bubble generation in the channel corners and an early partial dryout of thin liquid film. Thus the relationship between heat transfer coefficient and flow pattern should be carefully pursued in micro- and mini-channels to develop heat transfer correlations based on flow patterns.