Choosing the best molecular precursor to prepare Li4Ti5O12 by the sol–gel method using 1H NMR: evidence of [Ti3(OEt)13]− in solution

Abstract

¹H NMR spectroscopy at 400 MHz in toluene-d₈ of evaporated mixtures of lithium ethoxide and titanium(IV) isopropoxide in ethanol, used to prepare the spinel Li₄Ti₅O₁₂ by the sol–gel method, may help clarify why the atomic ratio 5Li : 5Ti and not 4Li : 5Ti is the right choice to obtain the pure phase when performing hydrolysis at room temperature. The mixtures xLiOEt/yTi(OPrⁱ)₄ in ethanol undergo alcohol exchange at room temperature, and the evaporated residues contain double lithium–titanium ethoxide [LiTi₃(OEt)₁₃] rather than simple mixtures of single metal alkoxides; this is of great relevance to truly understanding the chemistry and structural changes in the sol–gel process. Detailed inspection of the ¹H and ¹³C VT NMR spectra of mixtures with different Li/Ti atomic ratios unequivocally shows the formation of [LiTi₃(OEt)₁₃] in a solution at low temperature. The methylene signals of free lithium ethoxide and Li[Ti₃(OEt)₁₃] coalesce at 20 °C when the atomic ratio is 5 : 5; however, the same coalescence is only observed above 60 °C when the atomic ratio is 4 : 5. We suggest that the highest chemical equivalence observed by ¹H NMR spectroscopy achieved through chemical exchange of ethoxide groups involves the highest microscopic structural homogeneity of the sol precursor and will lead to the best gel after hydrolysis. Variable temperature ¹H NMR spectra at 400 MHz of variable molar ratios of LiOEt/Ti(OPrⁱ)₄ are discussed to understand the structural features of the sol precursor. While the precursor with the atomic ratio 5Li : 5Ti shows no signal of free LiOEt at 20 °C, both 4Li : 5Ti and 7Li : 5Ti show free LiOEt at 20 °C in their ¹H NMR spectra, indicating that the molar ratio 5Li : 5Ti gives the maximum rate of chemical exchange. DFT calculations have been performed to support the structure of the anion [Ti₃(OEt)₁₃]⁻ at room temperature.