Boosting stability and rate performance in sodium-ion batteries: first-principles insights into K$^+$ /NH$_4$$^+$ doped NaV$_3$O$_8$ cathodes

Sodium vanadate (NaV$_3$O$_8$) has emerged as a promising and cost-effective cathode candidate for next-generation sodium-ion batteries (SIBs); however, its practical application is hindered by its structural instability and limited rate performance. Heterogeneous ion doping strategy has been proposed as a potential solution to these challenges, but the underlying reaction mechanisms and the specific effects of different dopants on NaV$_3$O$_8$ remain poorly understood. In this study, we systematically examined the effects of K$^+$ and NH$_4$$^+$ ion doping on the structure, electronic properties, and electrochemical performance of NaV$_3$O$_8$ using first-principles calculations. We elucidated the energy storage mechanism of NaV$_3$O$_8$ through formation energy calculations and energy convex hull diagrams. Our findings reveal that doping with K$^+$ and NH$_4$$^+$ significantly increases the initial discharge voltage, raising it from 2.75 V to 3.08 V. Additionally, this doping reduces the sodium ion diffusion energy barrier (from 0.53 eV to 0.25 eV), effectively alleviating volume changes in the electrode material during charge/discharge cycles and enhancing its cycling stability. ... mehrFurthermore, crystal orbital Hamilton population (COHP) calculations indicated that K$^+$ and NH$_4$$^+$ doping markedly improves the stability of the V–O bond, effectively inhibiting vanadium dissolution and further enhancing the practical performance of the electrode materials. These findings underscore the dual role of K+ and NH4+ dopants in optimizing both thermodynamic and kinetic properties of NaV$_3$O$_8$, offering practical design principles for high-performance, durable SIB cathodes.