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.

The electrochemical signals are very important to analyze and regulate the electrochemical systems, and the electrochemical oscillation is a newly discovered electrochemical signal in Li-ion batteries (LIBs), including the voltage and current oscillations. In this work, the Li4Ti5O12 was prepared using a home-made spray-drying instrument and high-temperature sintering, and its electrochemical oscillation was studied in LIBs. The electrochemical oscillation arose when the as-prepared Li4Ti5O12 precursor was sintered in a powder (not pellet) form, and it became stronger by reducing the lithium content in Li4Ti5O12. There are two types of electrochemical oscillation as a single-period oscillation or a double-period oscillation, and they can be transformed through varying the operating temperature, the current rate, and the conductive agent ratio, which might be owing to the electrochemical kinetics of Li4Ti5O12 electrodes. Combining the sintered forms and the X-ray photoelectron spectroscopy results, whether there is some fresh surface (formed by grinding the sintered pellet) becomes a typical difference between the powder-sintered and pellet-sintered Li4Ti5O12, which would affect the nucleation step and the reaction kinetics, and we proposed a possible reaction process of the electrochemical oscillation during the galvanostatic charging process of Li4Ti5O12 in LIBs.

In the late 1960s, the establishment of Prigogine’s dissipative structure theory laid the foundation for the (electro)chemical oscillation phenomenon, which has been widely investigated in some electrochemical reactions, such as electro-catalysis and electro-deposition, while the electrochemical oscillation of Li-ion batteries has just been discovered in spinel Li4Ti5O12 a few years before. In this work, spinel LiCrTiO4 samples were synthesized by using a high-temperature solid-state method, characterized with SEM (Scanning electron microscope), XRD (X-ray diffraction), Raman and XPS (X-ray photoelectron spectroscopy) measurements, and electrochemically tested in Li-ion batteries to study the electrochemical oscillation. When sintering in a powder form at a temperature between 800 and 900 °C, we achieved the electrochemical oscillation of spinel LiCrTiO4 during charging, and it is suppressed in the non-stoichiometric LiCrTiO4 samples, especially for reducing the Li content or increasing the Cr content. Therefore, this work developed another two-phase material as the powder-sintered LiCrTiO4 exhibiting the electrochemical oscillation in Li-ion batteries, which would inspire us to explore more two-phase electrode materials in Li-ion batteries, Na-ion batteries, etc.

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