Effect of particle size on lithium intercalation into α - Fe2O3

The electrochemical reaction of lithium with crystallized α-Fe2O3 (hematite) has been studied by means of in situ X-ray diffraction. When reacting large particles (∼0.5 μm), 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 (xc < 0.1 Li/Fe2O3) can be inserted before this structural transformation occurs. Nanosize α-Fe2O3 made of fine monolithic particles (200 Å) behaves very different, since up to one Li per formula unit (α-Li1Fe2O3 ,xc = 1) 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 xc values. This cationic insertion was found to come with a small cell volume expansion evaluated to 1%. Unsuccessful attempts to increase the xc 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 hc → ccp 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