First-Principles Investigations of FeP@Graphene as Anode Material for Sodium-Ion Batteries

In recent years, numerous strategies have been proposed to tackle the challenges of large volume changes and capacity decay in conversion-type electrodes. Most studies have focused on experimental investigations, and only a few have centered on simulation studies. This research aims to apply density functional theory to study FeP and its modifications, which are one of the most widely recognized conversion-type electrodes in experimental studies due to its low cost and high theoretical capacity. Experiments have shown that combining FeP with graphene (FeP@graphene) can be a promising way to alleviate the volume variation issue. Furthermore, Cu doping in FeP has been reported to enhance conductivity and capacity.<br/>The FeP@graphene model was constructed based on experimental results of X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM), followed by a geometry optimization calculation using density functional theory (DFT). We conducted calculations on the adsorption energy of single sodium atom under various conditions, including distance from the graphene sheet and the nearest Fe and P of the FeP cluster. The results indicate that the adsorption energy is unaffected by whether Na is near Fe or P, while adsorption away from the FeP cluster results in weaker interaction, i.e., lower adsorption energy. The effect of Cu doping in FeP on material conductivity was studied by performing electronic structure analysis. The calculation results of the partial density of states (PDOS) did not exhibit substantial differences between models with and without Cu doping in FeP, but the adsorption energy of a single Na atom was significantly lower in the Cu doping system.<br/>Analyses of electron density difference maps (EDDM) and partial atomic charges are performed to further discern the distinctions between FeP@graphene with and without Cu doping. Climbing image-nudged elastic band (CI-NEB) calculations are conducted to investigate the diffusion behavior of Na in the FeP@graphene structure.