Probing Capacity Trends in MLi$_2$Ti$_6$O$_{14}$ Lithium-Ion Battery Anodes Using Calorimetric Studies

Due to higher packing density, lower working potential, and area specific impedance, the MLi$_2$Ti$_6$O$_{14}$ (M = 2Na, Sr, Ba, and Pb) titanate family is a potential alternative to zero-strain Li$_4$Ti$_5$O$_{12}$ anodes used commercially in Li-ion batteries. However, the exact lithiation mechanism in these compounds remains unclear. Despite its structural similarity, MLi$_2$Ti$_6$O$_{14}$ behaves differently depending on charge and size of the metal ion, hosting 1.3, 2.7, 2.9, and 4.4 Li per formula unit, giving charge capacity values from 60 to 160 mAh/g in contrast to the theoretical capacity trend. However, high-temperature oxide melt solution calorimetry measurements confirm strong correlation between thermodynamic stability and the observed capacity. The main factors controlling energetics are strong acid–base interactions between basic oxides MO, Li$_2$O and acidic TiO$_2$, size of the cation, and compressive strain. Accordingly, the energetic stability diminishes in the order Na$_2$Li$_2$Ti$_6$O$_{14}$ > BaLi$_2$Ti$_6$O$_{14}$ > SrLi$_2$Ti$_6$O$_{14}$ > PbLi$_2$Ti$_6$O$_{14}$. This sequence is similar to that in many other oxide systems. ... mehrThis work exhibits that thermodynamic systematics can serve as guidelines for the choice of composition for building better batteries.