Characterization of Disordered Li(1+x)Ti2xFe(1-3x)O2 as Positive Electrode Materials in Li-Ion Batteries Using Percolation Theory

Recent theoretical and experimental works have shown that disordered positive electrode materials can function well in lithium cells. This work explores the solid solution series Li(1+x)Ti2xFe(1-3x)O2 (0 <= x <= 0.333) and compares the measured specific capacity variation with x to a recent theoretical model. The samples have varying degrees of cation disordering between lithium and transition metal layers that is dependent on x. The materials were characterized using induced coupled plasma optical emission spectroscopy, scanning electron microscopy, X-ray diffraction, and X-ray absorption spectroscopy (XAS) to quantify the degree of disorder and predict electrochemical performance. The specific capacities of lithium-limited samples (0 <= x <= 0.13) were found to agree very well with the recently proposed percolation theory model, whereas redox-limited samples (0.13 <= x <= 0.29) yielded slightly higher than expected capacities due to oxygen redox compensation characterized by oxygen K-edge XAS studies. Capacity retention was found to increase with lithium content. The voltage vs specific capacity relations for this set of materials do not suggest practicality, so this work is primarily of academic interest, but it suggests that more disordered materials should be explored.