Abstract
This work sheds light on the exceptional robustness of anatase TiO2 when it is downsized to an extreme value of 4 nm. Since at this size the surface contribution to the volume becomes predominant, it turns out that the material becomes significantly resistant against particles coarsening with temperature, entailing a significant delay in the anatase to rutile phase transition, prolonging up to 1000 °C in air. A noticeable alteration of the phase stability diagram with lithium insertion is also experienced. Lithium insertion in such nanocrystalline anatase TiO2 converts into a complete solid solution until almost Li1TiO2, a composition at which the tetragonal to orthorhombic transition takes place without the formation of the emblematic and unwished rock salt Li1TiO2 phase. Consequently, excellent reversibility in the electrochemical process is experienced in the whole portion of lithium content.
Highlights
From the five times lower surface enthalpy of the anatase which modifies significantly the total free Gibbs energy of the system when reducing the size[23,24]
The reduction of miscibility gap when decreasing the particle size was exposed. This translates into the extension of the α solid solution domains from 0.01 to 0.21 Li+ when going from 140 to 7 nm TiO230; a feature experienced in the case of the hematite α -Fe2O333 and the olivine LiFePO434 even though the defect chemistry associated to the reduction of size may contribute to this phenomenon
We are comparing two size of particles, 4 nm synthesized by a two steps room-temperature process[36] and 20 nm particles synthesized by a hydrothermal synthetic route originally adapted for dye-sensitized solar cells[37]
Summary
From the five times lower surface enthalpy of the anatase which modifies significantly the total free Gibbs energy of the system when reducing the size[23,24]. The complete filling of lithium cation into the octahedral interstitial site, leading to the end member Li1TiO2 composition, can be reached solely if the particle size is sufficiently reduced It occurs below 1.5 V through a second biphasic reaction separating the orthorhombic LiβTiO2 and Li1TiO2 for which the structure corresponds to a tetragonal lattice cell similar to the anatase with ca. We are exposing a complete vanishing of the miscibility gap upon lithium insertion which transforms the galvanostatic curve from a classic potential/composition plateau into a S-shape profile along the full range of lithium composition, without ending up to the formation of the rock-salt LiTiO2 The relevance of this new feature on the electrochemical performances of the particles and the fundamental and practical outcomes of such findings is discussed
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