NASICON-Type Mg0.5Ti2(PO4)3 Negative Electrode Material Exhibits Different Electrochemical Energy Storage Mechanisms in Na-Ion and Li-Ion Batteries.

A carbon-coated Mg0.5Ti2(PO4)3 polyanion material was prepared by the sol-gel method and then studied as the negative electrode materials for lithium-ion and sodium-ion batteries. The material showed a specific capacity of 268.6 mAh g-1 in the voltage window of 0.01-3.0 V vs Na+/Na0. Due to the fast diffusion of Na+ in the NASICON framework, the material exhibited superior rate capability with a specific capacity of 94.4 mAh g-1 at a current density of 5A g-1. Additionally, 99.1% capacity retention was achieved after 300 cycles, demonstrating excellent cycle stability. By comparison, Mg0.5Ti2(PO4)3 delivered 629.2 mAh g-1 in 0.01-3.0 V vs Li+/Li0, much higher than that of the sodium-ion cells. During the first discharge, the material decomposed to Ti/Mg nanoparticles, which were encapsulated in an amorphous SEI and Li3PO4 matrix. Li+ ions were stored in the Li3PO4 matrix and the SEI film formed/decomposed in subsequent cycles, contributing to the large Li+ capacity of Mg0.5Ti2(PO4)3. However, the lithium-ion cells exhibited inferior rate capability and cycle stability compared to the sodium-ion cells due to the sluggish electrochemical kinetics of the electrode.