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

The Mn-based polyanion compound NaMnTi(PO) (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 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 NaMnTi(PO)(SiO) ( = 0.1, 0.2, and 0.3) are synthesized. It reveals that the SiO substitution could induce partial oxidation of Mn to Mn, and the latter has a lower occupancy preference on Na vacancies. By the proposed charge compensation strategy, the Mn 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.