NASICON-Type MgTi(PO) Negative Electrode Material Exhibits Different Electrochemical Energy Storage Mechanisms in Na-Ion and Li-Ion Batteries.

A carbon-coated MgTi(PO) 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 in the voltage window of 0.01-3.0 V vs Na/Na. 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 at a current density of 5A g. Additionally, 99.1% capacity retention was achieved after 300 cycles, demonstrating excellent cycle stability. By comparison, MgTi(PO) delivered 629.2 mAh g in 0.01-3.0 V vs Li/Li, 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 LiPO matrix. Li ions were stored in the LiPO matrix and the SEI film formed/decomposed in subsequent cycles, contributing to the large Li capacity of MgTi(PO). 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.