Anion Substitution to Suppress the Voltage Hysteresis of Na 3 MnTi(PO 4 ) 3 as a Cathode Material for Sodium-Ion Batteries.

The Mn-based polyanion compound Na 3 MnTi(PO 4 ) 3 (NMTP) with a Na superionic conductor (NASICON) structure has attracted incremental attention as a potential cathode material for sodium-ion batteries. However, the occupation of Mn 2+ on Na + vacancies usually leads to severe voltage hysteresis, which in turn results in significant capacity loss, slow Na + diffusion kinetics, and poor cycling stability. Herein, anion-substituted compounds Na 3 MnTi(PO 4 ) 3- x (SiO 4 ) x ( x = 0.1, 0.2, and 0.3) are synthesized. It reveals that the SiO 4 4- substitution could induce partial oxidation of Mn 2+ to Mn 3+ , and the latter has a lower occupancy preference on Na + vacancies. By the proposed charge compensation strategy, the Mn 2+ occupation on Na + vacancies can be significantly suppressed. As a result, the voltage hysteresis is substantially inhibited, and greatly improved electrochemical performance is achieved. This study offers an alternative strategy to address the voltage hysteresis associated with NMTP and other Mn-based NASICON cathode materials.