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

The integration of Mn in Na₄Mn_1.5Fe_1.5(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 PO₄^3- with bulkier and less electronegative SiO₄^4- groups. The reinforced cathode exhibits enhanced rate performance, which is attributed to lattice expansion induced by the larger SiO₄^4- 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 SiO₄^4- to accommodate local charge redistribution without triggering structural collapse, as evidenced by reduced lattice volume fluctuations observed in in situ XRD. Consequently, Na₄Mn_1.5Fe_1.5(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.