Structurally Compatible Anion Substitution for the Enhanced NASICON-Na₄Mn_1.5Fe_1.5(PO₄)₂P₂O₇ Cathode

Abstract

The integration of Mn in Na 4 Mn 1.5 Fe 1.5 (PO 4 ) 2 P 2 O 7 (NMFPP) enhances the energy density but compromises the Na + mobility and structural stability due to limited electron hopping and pronounced Jahn–Teller effects. To address this, a structurally compatible anionic substitution strategy is implemented by partially replacing PO 4 3– with bulkier and less electronegative SiO 4 4– groups. The reinforced cathode exhibits enhanced rate performance, which is attributed to lattice expansion induced by the larger SiO 4 4– units, thereby facilitating Na + diffusion and reducing impedance during charge–discharge processes, as supported by GITT and DRT analyses. In addition, the improved cycling stability results from the lower electronegativity of Si, which enables SiO 4 4– to accommodate local charge redistribution without triggering structural collapse, as evidenced by reduced lattice volume fluctuations observed in in situ XRD. Consequently, Na 4 Mn 1.5 Fe 1.5 (PO 4 ) 1.95 (SiO 4 ) 0.05 P 2 O 7 achieves a capacity retention of up to 85.42% for 500 cycles at 1 C and 80.54% over 1500 cycles at 5 C. These findings highlight anion substitution as a promising strategy for optimizing polyanionic frameworks toward high-performance sodium-ion batteries.