Substitutional Co and localized CoCr2O4 spinel domains in O3-type Na0.8Cr0.8Ti0.15Co0.05O2 cathodes for potassium-ion batteries

Layered oxide cathodes for potassium-ion batteries (PIBs) undergo substantial structural strain during cycling because of the large K + radius. Here, Co modification is introduced into O3-type Na 0 . 8 Cr 0 . 8 Ti 0 . 2 O 2 to regulate structural evolution and electrochemical reversibility. The resulting Na 0 . 8 Cr 0 . 8 Ti 0 . 15 Co 0 . 05 O 2 (NCTCO) contains substitutional Co in the layered host together with localized CoCr 2 O 4 spinel domains. NCTCO delivers 70.3 mA h g −1 at 1000 mA g −1 and retains 96.4% capacity after 500 cycles at 500 mA g −1 in K-metal half-cells. Electrochemical analysis shows a lower charge-transfer resistance after Co modification. In situ XRD reveals reduced lattice variation during the first charge, with a three-layer-equivalent c-axis expansion of 1.10% compared with 2.50% for NCTO, together with suppression of a high-voltage P1-type component. DFT calculations using idealized layered models indicate that substitutional Co lowers a representative K-ion migration barrier from ∼0.45 to 0.39 eV. These results show that Co modification improves the structural reversibility and electrochemical stability of the O3-type layered cathode while clarifying the distinct experimental and modeled roles of the two Co environments.