Entropy-driven mechanisms in P2-type layered oxide cathodes for sodium-ion batteries: new insights from first-principles and electrochemical analysis

P2-type Na$_x$MO$_2$ layered oxides (x < 1) are highly promising cathodes for Na-ion batteries (NIBs) but suffer from phase transitions, transition-metal (TM) migration, and structural distortions that limit cycling stability. Here, we combine first-principles modeling and electrochemical measurements to elucidate how configurational entropy governs their structural and electronic response. By comparing low-, medium-, and high-entropy compositions, we show that higher configurational entropy mitigates TM-centered octahedral distortions, suppresses shear-type deformations associated with P2 → O2 transitions via layer gliding, and distributes redox activity across multiple cations (Ni, Co, Fe), avoiding local over-oxidation. Defect-formation analyses reveal that high-entropy mixing significantly discourages out-of-layer TM migration, reducing TM/Na$_{vac}$ antisite formation and stabilizing the layered framework upon deep desodiation. Consistently, medium- and high-entropy materials exhibit superior capacity retention and structural reversibility compared to the low-entropy analogue, with further performance enhancement when using room-temperature ionic-liquid (RTIL)-based NaFSI-Pyr14 FSI electrolyte, which mitigates Mn dissolution and accounts for enhanced efficiency upon cycling. ... mehrThese findings demonstrate that configurational entropy is a powerful design parameter for achieving robust, high-performance P2-type layered cathodes and provide clear guidelines for entropy-assisted materials engineering in nextgeneration NIBs.