Atomic‐Scale Understanding of K ⁺ Reversibility in δ‐K _0.5 V ₂ O ₅ : Synergistic Experimental and First‐Principles Study

Abstract Potassium‐ion batteries (KIBs) are emerging as a viable alternative to lithium‐based systems, yet demand structurally robust and electronically efficient cathode materials. Herein, δ‐K _0.5 V ₂ O ₅ is presented as a high‐performing, stable, and multifunctional cathode for KIBs. The material exhibits minimal formation energy and stable lattice distortion across varying K‐contents (0.4–0.9) during cycling, synergistically confirmed by operando XRD and spin‐polarized DFT+U calculations. Electrochemical analyses reveal a multi‐step K ⁺ diffusion, enhanced redox capacity (97 mAh·g ⁻¹ at C/20), and 71% capacity retention over 300 cycles. Notably, the diffusion coefficient during charging is significantly higher, indicating facile K ⁺ extraction dynamics. DFT+U calculations uncover half‐metallicity, tunable bandgaps, and strong V–O covalency. A molecular orbital theory approach is implemented to illustrate the band diagrams and Jahn‐Teller‐induced orbital splitting. Burstein‐Moss shift is observed in the bandgap during potassiation, hitherto, unreported for this material indicating Fermi level movement and partial band filling. Further, differential charge density maps confirm potassium‐induced electron transfer to vanadium intervening apical oxygen atoms protruding toward the interlayer gap. In this work, it is uncovered that δ‐K _0.5 V ₂ O ₅ not only demonstrates promising aspects for KIBs, but also exhibits exotic material behavior, such as half metallicity, enabling potential integration into low‐voltage electronic devices.