Computational investigation of NaKFePO4F fluorophosphate as a high-performance cathode material for Na/K-ion batteries

Abstract

Recently, NaKFePO₄F, a layered iron-based fluorophosphate, has been proposed as a promising cathode material for both sodium-ion (SIBs) and potassium-ion batteries (KIBs), with an ion-exchange strategy significantly enhancing its capacity and addressing its low electronic conductivity. However, the atomic-scale mechanisms driving these improvements have yet to be fully explained. For this reason, density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations were systematically employed to assess the electrochemical feasibility of NaKFePO₄F as a novel cathode material for these batteries. Analysis of energetically stable configurations reveals that a 50% exchange of Na with K stabilizes and activates the previously inert sites in the pristine Na₂FePO₄F material. Notably, NaKFePO₄F exhibits enhanced thermodynamic stability and electronic conductivity, with a reduced band gap of 2.40 eV compared to 3.18 eV in the pristine material. Moreover, NaKFePO₄F was found to exhibit a low activation energy barrier of 0.42 eV for K ions, as determined by climbing image nudged elastic band (CI-NEB) computations. AIMD predictions also indicate that this material can sustain elevated temperatures from 300 K to 800 K, with ion diffusivity described accordingly. Ultimately, NaKFePO₄F achieved an average discharge voltage of 3.67 V and an energy density of 426 Wh/kg for KIBs, surpassing the 3.49 V discharge voltage and 405 Wh/kg energy density of SIBs. Given these predicted results, NaKFePO₄F is expected to be a promising cathode material for post-lithium-ion battery technology.