First-principles study of Ti doping in FeF3·0.33H2O

•Ti-doping can broaden the hexagonal cavity in the FeF3·0.33H2O.•Fe-rich and Ti-rich growth conditions contribute to formation of TixFe1-xF3·0.33H2O.•Ti-doping can improve the conductivity of FeF3·0.33H2O.

The effect of Ti doping on the geometrical and electronic structures of FeF3·0.33H2O are systematically investigated by using the first principles calculations. We focused on TixFe1-xF3·0.33H2O systems, in which x is equal to 0, 0.08, 0.17 and 0.25, respectively. Different kinds of Ti dopant sites are checked and the most stable structure can be obtained by comparison of total energy. The crystal volume of TixFe1-xF3·0.33H2O expands gradually with increasing Ti doping concentration. Calculated formation energies indicate TixFe1-xF3 is easiest to fabricate and the difficulty of Ti doping FeF3 with hexagonal-tungsten-bronze(HTB) structure decreases with the increase of Ti doping concentration under the Fe-rich and Ti-rich growth conditions. Moreover, TixFe1-xF3·0.33H2O is thermodynamically stable, indicating that water molecule can preferentially occupy one-dimensional cavity in the TixFe1-xF3. The band gap of TixFe1-xF3·0.33H2O decreases with increasing Ti doping concentration and Ti0.25Fe0.75F3·0.33H2O exhibits character of half metal, indicating that the conductivity of FeF3·0.33H2O can be improved by Ti-doping. Besides, it can be confirmed that Ti-doping also can broaden the hexagonal cavity in the FeF3·0.33H2O by analyzing the crystal structure of FeF3·0.33H2O and TixFe1-xF3·0.33H2O. With excellent conductivity and larger hexagonal cavity, TixFe1-xF3·0.33H2O can afford open diffusion channels. Therefore, Li ions can remain unblocked, which is beneficial to fast charge and discharge.

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