Structurally Compatible Anion Substitution for the Enhanced NASICON-Na4Mn1.5Fe1.5(PO4)2P2O7 Cathode

The integration of Mn in Na4Mn1.5Fe1.5(PO4)2P2O7 (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 PO43– with bulkier and less electronegative SiO44– groups. The reinforced cathode exhibits enhanced rate performance, which is attributed to lattice expansion induced by the larger SiO44– 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 SiO44– to accommodate local charge redistribution without triggering structural collapse, as evidenced by reduced lattice volume fluctuations observed in in situ XRD. Consequently, Na4Mn1.5Fe1.5(PO4)1.95(SiO4)0.05P2O7 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.