Reversible Al-Site Switching and Consequent Memory Effect of Al-Doped Li₄Ti₅O₁₂ in Li-Ion Batteries

Among many electrode materials, only a small amount of two-phase electrode materials were found to possess the memory effect, for instance, olivine LiFePO₄, anatase TiO₂, and Al-doped Li₄Ti₅O₁₂, in which the underlying mechanism is still not clear beyond the electrochemical kinetics. Here, we further studied the memory effect of Al-doped Li₄Ti₅O₁₂ to reveal the microstructure and the microprocess. By controlling the potentiostatic step after discharging, we found that the memory effect of Al-doped Li₄Ti₅O₁₂ was closely related to the discharged lattice parameters and the subsequent charge capacity. According to the ex situ magic-angle spinning (MAS) NMR results, we first revealed that the Al ions would move from 8a to 16c sites, when the electrode was discharged and potentiostatic at a low potential, and then move back through charging in the spinel structure of Al-doped Li₄Ti₅O₁₂, which would contribute to the capacity as the Li ions. Therefore, the reversible Al-ion switching between 8a and 16c sites should be the origin of memory effect in Al-doped Li₄Ti₅O₁₂, which would inspire us to explore the memory effect of other electrode materials in Li-ion batteries (LIBs), as well as optimize the performance of electrode materials by controlling the ionic switching.