Abstract
AbstractThe LiTiMO4 (spinel) ↔ LiTiMO4 (ramsdellite) transformation has been investigated by combining computational and experimental techniques, for M = Ti, V, Cr, Mn, and Fe, in order to understand the characteristics of this transformation and the influence of the metal M on the relative stability of the ramsdellite polymorph. The calculations predict that all the aforementioned LiTiMO4 spinels are thermodynamically stable with respect to the ramsdellite polymorph, with the calculated enthalpy variation of the transformation being less than 40 kJ mol–1. In the case of normal spinels [Li]t[TiM]oO4 we estimated a transformation temperature in the range 900–1600 °C, whereas the polymorphic transformation for inverse spinels [Li0.5M0.5]t[TiM0.5Li0.5]oO4 is accompanied by a lower entropy gain, hence a much higher temperature would be needed to overcome the enthalpy of the transformation. The transformation is thus entropically controlled. Accordingly, experimental results show that normal [Li]t[TiM]oO4 spinels (M = Ti, V, Cr) readily transform into ramsdellites at temperatures between 900 and 1400 °C, whereas this transition is hindered for spinels LiTiMO4 (M = Fe, Mn), which possess a high degree of inversion. Based on a possible mechanism for the transformation, we infer that the impossibility of transforming the Mn and Fe inverse‐spinels into their ramsdellite forms is also due to the high energy barriers that must be overcome during the transformation. The obtained LiTiMO4 (M = Ti, V, and Cr) ramsdellites, which contain TiIV and MIII, display a twofold electrochemical application, namely that one lithium ion per formula unit can be inserted into these compounds at about 1.4 V vs. Li (reduction of TiIV to TiIII). The oxidation of MIII ions to MIV enables lithium deinsertion from LiTiMO4 (M = Ti, V, and Cr) at potentials of 1.9 V (M = Ti), 3.1 V (M = V), and 4.2 (M = Cr) vs. Li. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)
Published Version
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