Characterization of Lithium Insertion into NASICON-Type Li[sub 1+x]Ti[sub 2−x]Al[sub x](PO[sub 4])[sub 3] and Its Electrochemical Behavior

Na super ionic conductor (NASICON) materials from the solid solution were prepared using a conventional solid-state reaction. A combination of powder X-ray diffraction analysis, , , and nuclear magnetic resonance (NMR) spectroscopy, and electrochemical techniques was used to fully characterize the lithium insertion into these materials. The electrochemical study showed the reversibility of the process with a hysteresis loop observed during the discharge–charge cycles and the existence of a large biphasic domain. At a discharge rate of C/20, the amount of lithium intercalated during the first cycle was about 0.9 Li/formula unit . Under equilibrium conditions, some differences were observed depending on , with the maximum quantity of intercalated lithium corresponding to 1.6 Li/formula unit . Profile refinement of X-ray powder diffraction patterns of pristine samples was achieved with , but that of intercalated samples was achieved with the space group. magic angle spinning–nuclear magnetic resonance (MAS–NMR) and susceptibility measurements allowed the estimation of the Ti(IV)-to-Ti(III) reduction extent. From the MAS-NMR spectra, the structural sites and the mobility of lithium were investigated in pristine and intercalated samples. In general, it was observed that exchange processes between structural sites increase with the amount of Li and Al.