Anion Substitution to Suppress the Voltage Hysteresis of Na₃MnTi(PO₄)₃ as a Cathode Material for Sodium-Ion Batteries

The Mn-based polyanion compound Na₃MnTi(PO₄)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 Mn²⁺ 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 Na₃MnTi(PO₄)_3-x(SiO₄)_x (x = 0.1, 0.2, and 0.3) are synthesized. It reveals that the SiO₄^4- substitution could induce partial oxidation of Mn²⁺ to Mn³⁺, and the latter has a lower occupancy preference on Na⁺ vacancies. By the proposed charge compensation strategy, the Mn²⁺ 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.