Selection Rules of Transition Metal Dopants for Prussian Blue Analogs Enabling Highly Reversible Sodium Storage

Rational element doping is demonstrated as an effective strategy to optimize crystal stability and enhance the electronic conductivity of Prussian blue analogs (PBAs) to achieve a satisfactory sodium storage performance. However, unraveling the dopant selection principles is still a big challenge. Herein, the integrated crystal orbital Hamilton population (ICOHP) function is adopted to evaluate the strength of chemical bonds of N-transition metals (N-TM) and guide the dopant selection. Among the series of ICOHP values for N-TM (TM = Mn, Fe, Co, Ni, Cu, Zn), the Cu & horbar;N bond exhibits the lowest ICOHP value, which indicates that Cu doping can improve the stability of PBAs compared to other dopants. Experimentally, among TM-substituted Fe-based PBAs (TMFeHCFs), the as-prepared sample with 20 at.% Cu doping (CuFeHCF-2) exhibits the best cycling performance, with a capacity retention of 83.5% after 400 cycles at 1 C, which is consistent with the theoretical calculation results. In addition, in situ XRD and in situ, Raman reveal a highly reversible monoclinic-cubic two-phase conversion and redox-active pairs, respectively. This study provides valuable guidelines for dopant selection to enhance the performance of PBAs cathodes. Systematically theoretical calculations are performed to seek the optimal transition metals dopant to improve the stability of PBAs. Based on theoretical results, the as-prepared sample with 20 at.% Cu doping (CuFeHCF-2) exhibits superior electrochemical performance. image