Na/K Diffusion in FeP as an Anode Material for Ion Batteries

Potassium- and sodium-ion batteries (KIBs and SIBs) have attracted wide attention due to abundant potassium and sodium resources, low cost, and high safety. Therefore, in this work, FeP as an anode material is selected to study diffusions of Na/K using a density functional theory method in order to explain our experimental result that the rate performance of the FeP/C anode material in KIBs is much better than that in SIBs. The rate performance of the FeP/C anode material in KIBs and SIBs is related to diffusion activation energies including interaction energies between Na/K and FeP and migration energy barriers. Our calculations find that diffusion of Na/K in FeP is a multistep one. The most favorable one among various possible diffusion paths is obtained. A migration energy barrier is a key factor to distinguish between K and Na diffusions in FeP. Na–P/K–P bond lengths, consequent binding energies between Na/K and FeP, and local structures of transition states account for migration energy barriers. The calculated result of the larger migration energy barrier for Na in FeP than K in FeP can well explain our experimental phenomenon that the rate performance of FeP/C in KIBs is better than that in SIBs. This work provides a theoretical basis that iron phosphide can be used as a better, low-cost, and higher-energy-density anode material for KIBs.