Exploring Multi-Transition-Metal NASICON Frameworks as High-Performance Cathodes for Sodium-Ion Batteries

The search for sustainable, high-performance cathodes has driven a growing interest in sodium superionic conductor (NASICON)-type phosphates for sodium-ion batteries (SIBs). To identify promising NASICONs containing earth-abundant transition metals (TMs) and to systematically examine the role of multiple TMs in influencing the various properties of NASICON cathodes, we employ density functional theory calculations to investigate nine NASICON compositions containing Mn, Cr, and/or Fe, and spanning unary, binary, and ternary combinations. Our calculations reveal that unary systems, in terms of their Na intercalation phase behavior, exhibit well-defined stabilization at intermediate Na contents ($x$ in Na$_x$TM$_2$(PO$_4$)$_3$), while binary and ternary systems display more complex phase behavior, with some systems showing a shift of thermodynamic minima from $x$ = 3 to 2. Intercalation voltages highlight the dominant role of Fe$^{4+}$/Fe$^{3+}$ redox activity in elevating average voltages ($\sim$4.0 V), while Mn and Cr introduce intermediate-to-low voltage redox activity. Electronic structure data demonstrate non-systematic changes in the band gap, especially in systems containing multiple TMs. Na$^+$ mobility results identify mixed-TM frameworks as favorable, achieving Na$^+$ migration barriers in the 0.3-0.4 eV range. Importantly, we identify Na$_x$MnFe$_{0.5}$Cr$_{0.5}$(PO$_4$)$_3$ to be a promising ternary composition for subsequent experimental validation, offering an optimal intersection of phase stability, voltages, thermodynamic (meta)stability, and Na$^+$ migration barriers. Together, our study provides fundamental insights into the interplay between compositional complexity, thermodynamic stability, electronic structure, and ionic transport in NASICONs, and offers actionable design principles for utilising multi-TM NASICONs as high performance SIB cathodes.