Layered high-entropy oxides with optimized microstructure as highly stable cathodes for sodium-ion batteries

So far, the secondary battery market has been dominated by lithium-ion batteries (LlBs) featuring high gravimetrie and volumctric cncrgy dcnsitics. Jlowcvcr, thc uncvcn distribution on Earth and thc increasing demand for lithium resources make LlB technology unsustainable. Recenily, sodium ion baUeries (SIBs) bave atlracted mucb atlention as a one of the mosl promising "beyond lithium" energy storage technologies, due to the abundance and unifonn distribution of sodium resources and similar working rnechanisrn to LlBs. Neve1iheless, SlB technology is not as mature and needs to be improved for implementaLion in practical devices. Presently, the main challenge lies in designing electrode (particularly cathode) materials with sufficient reversibility and high capacity. Research is currently focused on sodium layered oxides (NaxM02), which are among the most promising candidates tor SIB cathodc matcrials. Thc common strategy to improve their electrochemi.cal performance i.s to replace various elements in the M sublattice to inhibit distortion and limit irreversible pbase transitions. The project "OPHELIA" (OPerando studies of High-Entropy oxide-based active material for Na­ Ton battery cathodes) mainly aims at designing and studying layered high-entropy oxides (LHEOs) that show great potential as advanced STB cathode mate1ials [l]. lnitial studies conducted withi11 this project have demonstrated the pivotal role played by the LHEO microstructure [2], with materials consisting of larger and better-crystallized grains outperfoming those with similar composition and ditfcrcnt microstructurc both in tcrms of ratc capability and long-tcrm stability. Actually, LHEOs prepared by sol-gel method and calcination under optimized conditions are found to be highly stable, with a capacity retention of about 85% over 200 cycles at l C rate. Studies on these materials using X-ray absorption spectroscopy (XAS) are currently underway. The Authors gratefully acknowledge the European Union for the fin ancia! support to this pr ject through Next-Generation EU funds (National Recovery and Resilience Pian, PNRR - Mission 4 "Education and Resem·ch"- C2 component -lnvestment 1.1, Fund for the National Research Program and Projccts ofRclcvanl National Tntcrcst, PRIN).