Understanding the Structural Evolution and Storage Mechanism of NASICON-Structure Mg0.5Ti2(PO4)(3) for Li-Ion and Na-Ion Batteries

NASICON-structural materials, which belong to a family of alkali metal ion conductors or storers in alkali metal ion batteries, can provide a stable three-dimensional framework, fast alkali metal ions diffusion paths, and abundant deintercalation sites. Herein, a NASICON-structure, Mg0.5Ti2(PO4)(3), prepared by a solid-state reaction shows high reversible specific capacities, excellent capacity retentions, and excellent rate performances in Li-ion and Na-ion batteries (LIBs and SIBs). The structural evolution and ionic occupation of Mg0.5Ti2(PO4)(3) during Li or Na intercalation/deintercalation are systematically studied by in situ/ex situ X-ray diffraction (XRD) and ex situ neutron diffraction (ND). Reversible multiple-stage reactions in LIBs and a single two-phase reaction in SIBs are observed, which is different than the storage mechanisms of LiTi2(PO4)(3) and NaTi2(PO4)(3). Moreover, the results of element analysis and ND and density functional theory (DFT) calculations verify that Mg ions are not replaced by ions from the electrolyte and remain in the M1 positions during charge/discharge. These results demonstrate that, besides transition metal, ion type and distribution in the M1 site can also significantly influence the electrochemical behavior of NASICON materials. The clarifications of structural evolution and storage mechanism will be of great importance for further designs on the NASICON-structure electrodes.