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 electrochemical reduction of hematite with various particle sizes by metallic lithium has been studied by means of X-ray diffraction ͑XRD͒ Mo ¨ssbauer and extended X-ray absorption fine structure ͑EXAFS͒ spectroscopy. Previous in situ XRD analysis coupled with electrochemical data showed that lithium can be inserted in the nanosized sample up to 1 Li per Fe 2 O 3 whereas bulk material undergoes an irreversible Li-driven transformation from an hexagonal anionic packing to a close cubic packed framework as soon as 0.03 Li is inserted in the corundum structure. The present data show that only 0.6 Li per formula unit are actually inserted in the structure of small particles. The remaining lithium ͑0.4͒ is engaged in irreversible reduction of surface groups, or capacitive behavior. Beyond the solid solution domains, both samples are multiphase, and consist of Li 2 Fe 2 O 3 , Fe 0 clusters ͑10-15 Å͒ and inserted ␣-Fe 2 O 3 , which proportions are used to calculate the mean iron oxidation state in the electrode as the reaction proceeds. From these data, we found that electrolyte decomposition can occur at very different steps of the reduction depending on the texture of the active materials. In addition, during the reduction process, we evidenced a reaction of disproportionation (3Fe 2ϩ → 2Fe 3ϩ ϩ Fe 0 ), an intense electrochemical grinding of the hematite particles and the formation of extremely fine metallic surface clusters. For the first time, the EXAFS/X-ray absorption near-edge structure signature of the divalent intermediate Li 2 Fe 2 O 3 phase is obtained.

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