Tuning Redox Potentials in NASICON Cathode via Covalent Lattice Modulation

Abstract

ABSTRACT NASICON‐type Na 3 MnTi(PO 4 ) 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 (3 d ‐ e g *) orbital and consequently elevating the redox potentials of the Mn 3+/2+ and Mn 4+/3+ couples. The resulting Na 3.5 MnTi 0.5 Cr 0.5 (PO 4 ) 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 3.5 MnTi 0.5 Cr 0.5 (PO 4 ) 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.