Tuning Redox Potentials in NASICON Cathode via Covalent Lattice Modulation

NASICON-type Na₃MnTi(PO₄)₃ is a promising cathode for sodium-ion batteries (SIBs), yet its energy density remains limited by the incomplete activation of high-potential Mn redox couples. This study shows that the strategic incorporation of chromium effectively modulates Mn-O covalency, thereby lowering the energy of antibonding Mn (3d-e_g*) orbital and consequently elevating the redox potentials of the Mn^3+/2+ and Mn^4+/3+ couples. The resulting Na_3.5MnTi_0.5Cr_0.5(PO₄)₃ cathode achieves a high average discharge voltage of 3.40 V (vs. Na⁺/Na) and a competitive energy density of 586 Wh kg^{- 1}, which surpasses many recently reported NASICON cathodes. Mechanistic studies reveal this material exhibits a highly reversible single-phase solid-solution reaction within minimal volume expansion (2.5%), enabling exceptional cycling stability (87.6% retention over 5000 cycles at 20 C) and robust performance across a wide temperature range (-30 to 40 °C). The high level of cyclability exhibited by the Na_3.5MnTi_0.5Cr_0.5(PO₄)₃//hard carbon full‑cell (88.6% capacity retention after 1000 cycles at 2 C) further validates its practical viability. This work underscores the effectiveness of covalent modulation in tuning electronic structures, offering a generalizable strategy for designing high-voltage polyanionic frameworks for next-generation energy storage.