Memory-effect-induced electrochemical oscillation of an Al-doped Li4Ti5O12 composite in Li-ion batteries

Electrochemical oscillation during galvanostatic charging and discharging of Zr-modified Li4Ti5O12 in Li-ion batteries

The electrochemical oscillation in Li-ion batteries has been reported for two-phase electrode materials of Li4Ti5O12 and LiCrTiO4, which is originated from the group-by-group phase transition in a multi-particle electrode. For both Li4Ti5O12 and LiCrTiO4, the electrochemical oscillation exhibits usually during charging, while rarely for discharging. Herein, a series of Zr-modified Li4Ti5O12 samples are prepared by using the spray-drying combined with high-temperature sintering method, and the electrochemical oscillation is observed during not only the charging process, but also the discharging process, which gradually grows up and then disappears by increasing the Li content. Compared with Li4Ti5O12, the specific capacity of Zr-modified Li4Ti5O12 decreases gradually by increasing the Zr/Ti ratio, owing to the impurity phases. According to the XRD, XPS and STEM results, the Zr element tends to accumulate on the surface to form ZrO2 nanoparticles, rather than dope into the bulk phase of Li4Ti5O12, which makes Li4Ti5O12 particles well dispersive. In contrast to the Li deficiency for only charging, the electrochemical oscillation during both charging and discharging should be attributed to the Li excess, but too much Li2TiO3 phase will suppress the electrochemical oscillation. Therefore, the Li excess can induce the electrochemical oscillation during both charging and discharging of Zr-modified Li4Ti5O12, which can be adopted to investigate the electrochemical oscillation of other materials in LIBs.

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.