Correlation Between Li-Fe Anti-Site and Memory Effect of LiFePO₄ in Li-Ion Batteries

In Li-ion batteries, the origin of memory effect in Al-doped Li₄Ti₅O₁₂ has been revealed as the reversible Al-ion switching between 8a and 16c sites in the spinel structure, but it is still not clear about that for olivine LiFePO₄, which is one of the most important cathode materials. In this work, a series of Na-doped and Ti-doped LiFePO₄ are prepared in a high-temperature solid-state method, electrochemically investigated in Li-ion batteries and characterized by X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Magic-Angle-Spinning Nuclear Magnetic Resonance (MAS NMR). Compared with non-doped LiFePO₄, the Ti doping can simultaneously suppress the memory effect and the Li-Fe anti-site, while they are simultaneously enhanced by the Na doping. Meanwhile, the Ti doping improves the electrochemical performance of LiFePO₄, opposite to the Na doping. Accordingly, a schematic diagram of phase transition is proposed to interpret the memory effect of LiFePO₄, in which the memory effect is attributed to the defect of Li-Fe anti-site.