The formation mechanism of Li4Ti5O12-y solid solutions prepared by carbothermal reduction and the effect of Ti3+ on electrochemical performance.

Samples of Li4Ti5O12-y solid solutions are synthesized by one-step solid-state carbothermal reduction reaction using Li2CO3, anatase, and carbon black under a nitrogen atmosphere. The underlying formation mechanism that leads to Li4Ti5O12-y solid solutions is proposed. The formation mechanism of the Li4Ti5O12-y solid solution is investigated by in situ variable temperature X-Ray diffraction (VT-XRD) and thermogravimetric analysis/differential scanning calorimetry (TGA-DSC). First, some Ti4+ centers are converted to Ti3+ (TiO2-TiO2-x) because of the presence of carbon black. Secondly, Li2CO3 reacts with TiO2-x (anatase) to form Li2TiO3. Thirdly, Li2TiO3 reacts with TiO2-x to form the Li4Ti5O12-y solid solution, while anatase starts to transform into rutile at the same time. Rutile reacts with Li2TiO3 to form Li4Ti5O12-y at higher temperatures. The presence of Ti3+ not only improves the electrical conductivity but also improves the ionic conductivity. As a result, the as-prepared material exhibits good rate capability and cycling stability with 99.3% capacity retention after 200 cycles.