No abstract

The electrochemical reaction of lithium with crystallized α­Fe2O3 (hematite) has been studied by means of in situ X-ray diffraction. When reacting large particles false(∼0.5 μnormalmfalse), we observed the well-known transformation of the close-packed anionic array from hexagonal (hc) to cubic (ccp) stacking. At the early stage of the reduction, a very small amount of lithium false(xnormalc<0.1 normalLi/Fe2O3false) can be inserted before this structural transformation occurs. Nanosize α­Fe2O3 made of fine monolithic particles (200 Å) behaves very different, since up to one Li per formula unit false(α­Li1Fe2O3,xnormalc=1false) can be inserted in the corundum structure without phase transformation. To our knowledge, this is the first time this phase is maintained for such large xnormalc values. This cationic insertion was found to come with a small cell volume expansion evaluated to 1%. Unsuccessful attempts to increase the xnormalc values on large particles by decreasing the applied discharge current density suggest that the particle size is the only parameter involved. The better structural reversibility of this monophasic process compared to the biphasic one was confirmed by electrochemical cycling tests conducted with hematite samples of various particle sizes. Therefore, by using nanosize particles, we can drastically increase the critical Li concentration required to observe the normalhc→normalccp transition. This work demonstrates that a careful control of the texture/particle size of electrochemically active oxide particles is likely an important variable that has been largely disregarded for such properties. © 2002 The Electrochemical Society. All rights reserved.

The potential distribution through plastic Li-ion cells during electrochemical testing was monitored by means of three-or four-electrode measurements in order to determine the origin of the poor electrochemical performance (namely, premature cell failure, poor storage performance in the discharged state) of LiMn,O,/C Li-ion cells encountered at 55°C. Several approaches to insert reliably one or two reference electrodes that can be either metallic lithium or an insertion compound such as Li,Ti,O,, into plastic Li-ion batteries are reported. Using a reference electrode, information regarding the evolution of (i) the state of charge of each electrode within a Li-ion cell, (ii) their polarization, and (iii) their rate capability can be obtained. From these three-electrode electrochemical measurements, coupled with chemical analyses, X-ray diffraction, and microscopy studies, one unambiguously concludes that the poor 55°C performance is mainly due to the instability of the LiMn,O, phase toward Mn dissolution in LiPF,-type electrolytes. A mechanism, based on Mn dissolution, is proposed to account for the poor storage performance of LiMn,O,/C Li-ion cells.