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 Mg 0.5 Ti 2 (PO 4 ) 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 + /Na 0 . 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, Mg 0.5 Ti 2 (PO 4 ) 3 delivered 629.2 mAh g –1 in 0.01–3.0 V vs Li + /Li 0, 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 Li 3 PO 4 matrix. Li + ions were stored in the Li 3 PO 4 matrix and the SEI film formed/decomposed in subsequent cycles, contributing to the large Li + capacity of Mg 0.5 Ti 2 (PO 4 ) 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.