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.

To better understand the factors responsible for the poor electrochemical performances of the olivine-type LiMnPO 4 , various experiments such as chemical delithiation, galvanostatic charge and discharge, cyclic voltamperometry, and impedance conductivity, were carried out on both LiFePO 4 and LiMnPO 4 . Chemical delithiation experiments confirmed a topotactic two-phase electrochemical mechanism between LiMnPO 4 and the fully delithiated phase MnPO 4 ͑a = 5.909͑5͒ Å, b = 9.64͑1͒ Å, and c = 4.768͑6͒ Å͒. We conclude that the limiting factor in the MnPO 4 /LiMnPO 4 electrochemical reaction is nested mostly in the ionic and/or electronic transport within the LiMnPO 4 particles themselves rather than in charge transfer kinetics or structural instability of the MnPO 4 phase. For instance, the electrical conductivity of LiMnPO 4 ͑ ϳ 3.10 −9 S cm −1 at 573 K, ⌬E ϳ 1.1 eV͒ was found to be several orders of magnitude lower than that of LiFePO 4 ͑ ϳ 10 −9 S cm −1 at 298 K, ⌬E ϳ 0.6 eV͒.