Halides encapsulation in aluminum/boron phosphide nanoclusters: An effective strategy for high cell voltage in Na-ion battery

Design and synthesis of high energy density materials is an escalating area of research. Continuous strides are made to explore new and better materials for this purpose. Herein, we report a new and effective strategy to improve the cell voltage and performance of Na-ion batteries where phosphide nanoclusters are doped with endohedral halides. Bare and halide doped boron and aluminum phosphides (BP and AlP) are studied for their applications as electrodes (anodes) in Na-ion batteries. The geometric and electrochemical properties of Na/Na+ with both B12P12 and Al12P12 nanocages are investigated. The interaction energies (E-int.) and frontier molecular orbitals (FMOs) of Na/Na+ ion with B12P12 and Al12P12 nanocages are studied to investigate the effect of neutral or ionic Na on the geometric and electrochemical properties. Bare AlP and BP offer cell voltages of 0.56 and 0.61 V, mainly due to small differences in the interaction energies of Na and Na+ bound systems. The cell voltage is significantly improved by encapsulation of different halide ions into B12P12 and Al12P12 nanocages. Change in Gibbs free energy is significantly improved by the encapsulation of F- into B12P12 and Al12P12 among all considered halides, which results in increase in cell voltage up to 4.5 and 3.5 V for endo-F@B12P12 and endo-F@Al12P12, respectively. This value is higher as compare to cell voltage estimated for carbon nanotubes, functionalized BN nanosheets and B40 fullerenes. The cell voltage of 4.5 V is the highest for any such system. The results here are helpful in designing new materials with even better energy storage density.