Theoretical Study on High-Entropy Oxyfluoride Cathodes for Sodium-Ion Batteries

High-entropy (HE) materials comprise a family of emerging solid-state materials, where multiple elements can occupy the same lattice positions and therefore enhance the configurational entropy. HE oxides (HEOs) can mitigate challenges facing layered cathode materials, such as capacity fading, and facilitate long-term cyclability. However, the mechanism behind the effect of HE on electrochemical properties is still poorly understood. In the current work, we employed classical force field and first-principles density functional theory (DFT) calculations to gain atomistic-level understanding of the thermodynamic and electrochemical features of a family of recently developed high-entropy oxyfluoride (HEO-F) cathode materials with the general formula Na_xLi_1-xMO_1.9F_0.1 (M ∈ Ni, Fe, Mn, Ti, Mg; x = 1.0, 0.9, 0.8). We used Monte Carlo simulated annealing (MCSA) in conjunction with classical force fields to determine the most favorable atomic arrangement within these high-entropy oxyfluorides. Subsequently, we conducted DFT calculations at different sodium concentrations during charging, analyzing the oxidation states, Bader charges, and partial density of states of the transition metal (TM) atoms, to elucidate their participation in the redox processes. Crystal orbital Hamilton population (COHP) calculations were performed to assess the strength of the metal-oxygen bonds, which are crucial for the cathode stability. Furthermore, we investigated the potential occurrence of antisite defects, involving cation exchange between Li and TM atoms. Analyses of all three compositions of Na_xLi_1-xMO_1.9F_0.1 (x = 1.0, 0.9, 0.8) suggest that the Na_0.9 system exhibits superior electrochemical properties, in agreement with experiments. We identified key factors that can contribute to this superior performance, including (1) low crystal lattice variation during cycling, (2) enhanced electronic conductivity, (3) optimal charge balancing among transition metal atoms at high desodiation, (4) strong metal-oxygen bonding, and (5) limited occurrence of energetically unfavorable antisite defects.