The formation mechanism of NaLiTi3O7 and its effect on Li4Ti5O12 prepared by the NaCl molten salt flux method

Abstract

Octahedral single-crystal Li4Ti5O12 materials with {111} crystalline surfaces exposed are prepared in NaCl flux, but the presence of NaLiTi3O7 in the product deteriorates the electrochemical performance of Li4Ti5O12 materials. Herein, the formation mechanism of NaLiTi3O7 in the product was discussed, and it was suggested that the impurity content is affected by the diffusion of the raw materials in the NaCl molten salt flux. The more NaCl is present, the more NaLiTi3O7 is generated. Because Li2O and TiO2 are separated by the salt, and both reactants should diffuse a much longer distance to react. Once the ratio of Li2O and TiO2 reaches 1:3, NaLiTi3O7 will form immediately; otherwise, if the ratio reaches 2:5, Li4Ti5O12 forms. Based on this hypothesis, NaLiTi3O7 will form because there is not enough Li2O to react with TiO2 to form Li4Ti5O12. Therefore, the content of Li2CO3 in the material preparation process was increased, allowing more Li2O to come into contact with TiO2 in the NaCl flux, thereby increasing the probability of Li4Ti5O12 formation and reducing the generation of NaLiTi3O7. Two modification strategies have been proposed to inhibit the generation of impurities. It was found that increasing the amount of Li2CO3 to 15 wt % in excess can reduce impurities of NaLiTi3O7, and further increasing its amount will cause the occurrence of new impurities. Adding a small amount of LiCl to the precursor mixtures can also inhibit NaLiTi3O7 generation in the prepared LTO materials and improve their electrochemical performances.