A first-principles investigation of the influence of polyanionic boron doping on the stability and electrochemical behavior of NaV(PO).

NaV(PO) (NVP) is one of the most promising candidates for use as cathodes in room-temperature sodium ion batteries owing to its high structural stability and rapid Na transportation kinetics. The cationic doping of foreign ions at Na or V sites in the NVP lattice has proven to be an effective approach for enhancing the electrochemical performance of NVP. In this work, we present a first-principles density functional theory investigation of the impact of polyanionic boron doping at P sites on the structural and electrochemical behavior of NVP. Our simulation results suggest that B doping considerably increases the structural stability of NVP while shrinking its lattice size to some extent. Since B donates far fewer electrons to connected O atoms, the surrounding V atoms become more positive, causing the operating voltage to increase with B content. However, the reduction in lattice size is not beneficial for the Na transportation kinetics. As demonstrated by a search for the transition state, a concerted ion-exchange mechanism is preferred for Na transportation, and increased B doping leads to a higher Na diffusion barrier. Improvements in electrochemical performance due to B doping see (Hu et al. Adv Sci 3(12):1600112, 2016) appear to originate mainly from the resulting increased electrical conductivity.