Al-Doping-Induced Memory Effect of LiCrTiO₄ in Li-Ion Batteries

In Li-ion batteries (LIBs), the phenomenon of memory effect has only been found in a few two-phase electrode materials such as olivine LiFePO₄, anatase TiO₂, and Al-doped Li₄Ti₅O₁₂, which is attributed to the Li-ion exchange between adjacent vacant Li and Fe sites in LiFePO₄ and the Al-ion exchange between 8a and 16c sites in Al-doped Li₄Ti₅O₁₂. Here, the Al-doped LiCrTiO₄ was successfully synthesized by the spray-drying method and high-temperature sintering, which also exhibits the memory effect during only the charging process. The X-ray diffraction (XRD) and electrochemical results indicate that the saturated Al-doping level corresponds to the Al:Ti ratio of ca. 0.25 for LiCrTiO₄, and the memory effect increases with the Al-doping level evidently. According to the in situ XRD and magic-angle spinning nuclear magnetic resonance results, the Al³⁺ ions are doped at the 8a sites, the Li ions hop between 8a and 16c sites during the charging and discharging processes, and the Li ions further hop from the 16c site to the 16d site after deeply discharging and move back in the subsequent charging process. Therefore, the memory effect of Al-doped LiCrTiO₄ should originate from the reversible exchange of Li ions between 16c and 16d sites, and it is a common strategy to cause the memory effect in LIBs by inducing another reversible ionic migration in the electrode materials to affect the normal Li-ion extraction/insertion.